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Accurate control of force on the environment is mechanically necessary for many tasks involving the lower extremities. We investigated drifts in the horizontal (shear) active force produced by right-footed seated subjects and the effects of force matching by the other foot. Subjects generated constant shear force at 15% and 30% of maximal voluntary contraction (MVC) using one foot. Visual feedback of shear force magnitude was provided for the first 5s, then turned off for 30s. During the 30% MVC task, we observed parallel drops in active shear and vertical force magnitudes leading to consistent drifts in the resultant force magnitude, not in its direction. Force matching by the other foot resulted in significantly lower forces when feedback was available throughout the trial. No feedback was provided for the matching foot. When the matching foot began exerting force, the task foot experienced a notable drop in all force components, with a change in force direction only for the task foot. After this initial drop, the downward drift in the task foot stopped or reversed. Subjects were unaware of these drifts and errors. Our findings suggest that shear force production involves setting a referent coordinate vector, which shows drifts and matching errors, while its direction remains stable. Involvement of the matching foot appears to perturb the neural commands to the task foot, with minor differences observed between feet. The discrepancy between the consistent force drifts and lack of awareness of the drifts indicates a difference between force perception-to-act and perception-to-report. [ABSTRACT FROM AUTHOR]

We explored two types of anticipatory synergy adjustments (ASA) during accurate four-finger total force production task. The first type is a change in the index of force-stabilizing synergy during a steady state when a person is expecting a signal to produce a quick force change, which is seen even when the signal does not come (steady-state ASA). The other type is the drop in in the synergy index prior to a planned force change starting at a known time (transient ASA). The subjects performed a task of steady force production at 10% of maximal voluntary contraction (MVC) followed by a ramp to 20% MVC over 1 s, 3 s, and as a step function (0 s). In another task, in 50% of the trials during the steady-state phase, an unexpected signal could come requiring a quick force pulse to 20% MVC (0-surprise). Inter-trial variance in the finger force space was used to quantify the index of force-stabilizing synergy within the uncontrolled manifold hypothesis. We observed significantly lower synergy index values during the steady state in the 0-ramp trials compared to the 1-ramp and 3-ramp trials. There was also larger transient ASA during the 0-ramp trials. In the 0-surprise condition, the synergy index was significantly higher compared to the 0-ramp condition whereas the transient ASA was significantly larger. The finding of transient ASA scaling is of importance for clinical studies, which commonly involve populations with slower actions, which can by itself be associated with smaller ASAs. The participants varied the sharing pattern of total force across the fingers more in the task with "surprises". This was coupled to more attention to precision of performance, i.e., inter-trial deviations from the target as reflected in smaller variance affecting total force, possibly reflecting higher concentration on the task, which the participants perceived as more challenging compared to a similar task without surprise targets. [ABSTRACT FROM AUTHOR]

Purpose: Reduced spinal excitability during the transcranial magnetic stimulation (TMS) silent period (SP) has recently been shown to last longer than previously thought in the upper limbs, as assessed via spinal electrical stimulation. Further, there is reason to expect that contraction intensity affects the duration of the reduced spinal excitability. Methods: This study investigated spinal excitability at different time delays within the TMS-evoked SP in m.rectus femoris. Fifteen participants performed non-fatiguing isometric knee extensions at 25%, 50% and 75% of maximum voluntary contraction (MVC). Lumbar stimulation (LS) induced a lumbar-evoked potential (LEP) of 50% resting M-max. TMS stimulator output induced a SP lasting ~ 200 ms. In each contraction, a LEP (unconditioned) was delivered ~ 2–3 s prior to TMS, which was followed by a second LEP (conditioned) 60, 90, 120 or 150 ms into the silent period. Five contractions were performed at each contraction intensity and for each time delay in random order. Results: Compared to the unconditioned LEP, the conditioned LEP amplitude was reduced (− 28 ± 34%, p = 0.007) only at 60 ms during 25% of MVC. Conditioned LEP amplitudes during 50% and 75% of MVC were reduced at 60 ms (− 37 ± 47%, p = 0.009 and − 37 ± 42%, p = 0.005, respectively) and 150 ms (− 30% ± 37%, p = 0.0083 and − 37 ± 43%, p = 0.005, respectively). LEP amplitude at 90 ms during 50% of MVC also reduced (− 25 ± 35%, p = 0.013). Conclusion: Reduced spinal excitability is extended during 50% and 75% of MVC. In future, paired TMS-LS could be a potential method to understand changes in spinal excitability during SP (at different contraction intensities) when testing various neurophysiological phenomena. [ABSTRACT FROM AUTHOR]

The concept of synergies has been used to address the grouping of motor elements contributing to a task with the covariation of these elements reflecting task stability. This concept has recently been extended to groups of motor units with parallel scaling of the firing frequencies with possible contributions of intermittent recruitment (MU-modes) in compartmentalized flexor and extensor muscles of the forearm stabilizing force magnitude in finger pressing tasks. Here, we directly test for the presence and behavior of MU-modes in the tibialis anterior, a non-compartmentalized muscle. Ten participants performed an isometric cyclical dorsiflexion force production task at 1 Hz between 20 and 40% of maximal voluntary contraction and electromyographic (EMG) data were collected from two high-density wireless sensors placed on the skin over the right tibialis anterior. EMG data were decomposed into individual motor unit frequencies and resolved into sets of MU-modes. Inter-cycle analysis of MU-mode magnitudes within the framework of the uncontrolled manifold (UCM) hypothesis was used to quantify force-stabilizing synergies. Two or three MU-modes were identified in all participants and trials accounting, on average, for 69% of variance and were robust to cross-validation measurements. Strong dorsiflexion force-stabilizing synergies in the space of MU-modes were present in all participants and for both electrode locations as reflected in variance within the UCM (median 954, IQR 511–1924) exceeding variance orthogonal to the UCM (median 5.82, IQR 2.9–17.4) by two orders of magnitude. In contrast, MU-mode-stabilizing synergies in the space of motor unit frequencies were not present. This study offers strong evidence for the existence of synergic control mechanisms at the level of motor units independent of muscle compartmentalization, likely organized within spinal cord circuitry. [ABSTRACT FROM AUTHOR]

We explored the phenomenon of unintentional force drift seen in the absence of visual feedback during knee extension contractions in isometric conditions. Based on the importance of knee extensors for the anti-gravity function, we hypothesized that such force drifts would be slower and smaller compared to those reported for the upper extremities. We also explored possible effects of foot dominance and gender on the force drifts. Young healthy persons produced isometric knee extension contractions to different levels, ranging from 15 to 25% of maximal voluntary contraction force, with the help of visual feedback, and then, the visual feedback was turned off. Force change over the time interval without visual feedback was quantified. In the absence of visual feedback, force drifted to smaller magnitudes. The drift magnitude expressed in percent of the initial force magnitude was smaller for smaller initial force levels, ranging between 8 and 15% of the initial force for the initial force magnitude of 15% and 25% of maximal voluntary contraction force. The time exponent of the force drift was independent of the initial force magnitude and was, on average, 6.45 s. There were no significant effects of foot dominance or gender, although the male subjects tended to show stronger scaling of the drift magnitude with the initial force level compared to the female subjects. The results show that unintentional force drift is a common phenomenon across limbs and muscle groups. This conclusion fits the theory of control with spatial referent coordinates and the general tendency of all natural systems to drift to states with lower potential energy. [ABSTRACT FROM AUTHOR]

Titin is a filamentous protein spanning the half-sarcomere, with spring-like properties in the I-band region. Various structural, signaling, and mechanical functions have been associated with titin, but not all of these are fully elucidated and accepted in the scientific community. Here, I discuss the primary mechanical functions of titin, including its accepted role in passive force production, stabilization of half-sarcomeres and sarcomeres, and its controversial contribution to residual force enhancement, passive force enhancement, energetics, and work production in shortening muscle. Finally, I provide evidence that titin is a molecular spring whose stiffness changes with muscle activation and actin-myosin-based force production, suggesting a novel model of force production that, aside from actin and myosin, includes titin as a “third contractile” filament. Using this three-filament model of sarcomeres, the stability of (half-) sarcomeres, passive force enhancement, residual force enhancement, and the decrease in metabolic energy during and following eccentric contractions can be explained readily. [ABSTRACT FROM AUTHOR]

We investigated the ability of two persons to produce force-stabilizing synergies in accurate multi-finger force production tasks under visual feedback on the total force only. The subjects produced a time profile of total force (the sum of two hand forces in one-person tasks and the sum of two subject forces in two-person tasks) consisting of a ramp-up, steady-state, and ramp-down segments; the steady-state segment was interrupted in the middle by a quick force pulse. Analyses of the structure of inter-trial finger force variance, motor equivalence, anticipatory synergy adjustments (ASAs), and the unintentional drift of the sharing pattern were performed. The two-person performance was characterized by a dramatically higher amount of inter-trial variance that did not affect total force, higher finger force deviations that did not affect total force (motor equivalent deviations), shorter ASAs, and larger drift of the sharing pattern. The rate of sharing pattern drift correlated with the initial disparity between the forces produced by the two persons (or two hands). The drift accelerated following the quick force pulse. Our observations show that sensory information on the task-specific performance variable is sufficient for the organization of performance-stabilizing synergies. They suggest, however, that two actors are less likely to follow a single optimization criterion as compared to a single performer. The presence of ASAs in the two-person condition might reflect fidgeting by one or both of the subjects. We discuss the characteristics of the drift in the sharing pattern as reflections of different characteristic times of motion within the subspaces that affect and do not affect salient performance variables. [ABSTRACT FROM AUTHOR]

This study examined the effects of elevated buffer capacity [~32 mM HCO] through administration of sodium bicarbonate (NaHCO) on maximally stimulated isolated mouse soleus (SOL) and extensor digitorum longus (EDL) muscles undergoing cyclical length changes at 37 °C. The elevated buffering capacity was of an equivalent level to that achieved in humans with acute oral supplementation. We evaluated the acute effects of elevated [HCO] on (1) maximal acute power output (PO) and (2) time to fatigue to 60 % of maximum control PO (T), the level of decline in muscle PO observed in humans undertaking similar exercise, using the work loop technique. Acute PO was on average 7.0 ± 4.8 % greater for NaHCO-treated EDL muscles ( P < 0.001; ES = 2.0) and 3.6 ± 1.8 % greater for NaHCO-treated SOL muscles ( P < 0.001; ES = 2.3) compared to CON. Increases in PO were likely due to greater force production throughout shortening. The acute effects of NaHCO on EDL were significantly greater ( P < 0.001; ES = 0.9) than on SOL. Treatment of EDL ( P = 0.22; ES = 0.6) and SOL ( P = 0.19; ES = 0.9) with NaHCO did not alter the pattern of fatigue. Although significant differences were not observed in whole group data, the fatigability of muscle performance was variable, suggesting that there might be inter-individual differences in response to NaHCO supplementation. These results present the best indication to date that NaHCO has direct peripheral effects on mammalian skeletal muscle resulting in increased acute power output. [ABSTRACT FROM AUTHOR]

Heat shock proteins play a key regulatory role in cellular defense. To investigate the role of the inducible 70-kDa heat shock protein (HSP70) in skeletal muscle atrophy and subsequent recovery, soleus (SOL) and extensor digitorum longus (EDL) muscles from overexpressing HSP70 transgenic mice were immobilized for 7 days and subsequently released from immobilization and evaluated after 7 days. Histological analysis showed that there was a decrease in cross-sectional area of type II myofiber from EDL and types I and II myofiber from SOL muscles at 7-day immobilization in both wild-type and HSP70 mice. At 7-day recovery, EDL and SOL myofibers from HSP70 mice, but not from wild-type mice, recovered their size. Muscle tetanic contraction decreased only in SOL muscles from wild-type mice at both 7-day immobilization and 7-day recovery; however, it was unaltered in the respective groups from HSP70 mice. Although no effect in a fatigue protocol was observed among groups, we noticed a better contractile performance of EDL muscles from overexpressing HSP70 groups as compared to their matched wild-type groups. The number of NCAM positive-satellite cells reduced after immobilization and recovery in both EDL and SOL muscles from wild-type mice, but it was unchanged in the muscles from HSP70 mice. These results suggest that HSP70 improves structural and functional recovery of skeletal muscle after disuse atrophy, and this effect might be associated with preservation of satellite cell amount. [ABSTRACT FROM AUTHOR]

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Muscle weakness following constant load exercise under heat stress has been associated with hyperthermia-induced central fatigue. However, evidence of central fatigue influencing intense self-paced exercise in the heat is lacking. The purpose of this investigation was to evaluate force production capacity and central nervous system drive in skeletal muscle pre- and post-cycle ergometer exercise in hot and cool conditions. Nine trained male cyclists performed a 20-s maximal voluntary isometric contraction (MVC) prior to (control) and following a 40-km time trial in hot (35°C) and cool (20°C) conditions. MVC force production and voluntary activation of the knee extensors was evaluated via percutaneous tetanic stimulation. In the cool condition, rectal temperature increased to 39.0°C and reached 39.8°C in the heat (P < 0.01). Following exercise in the hot and cool conditions, peak force declined by ~90 and ~99 N, respectively, compared with control (P < 0.01). Mean force decreased by 15% (hot) and 14% (cool) (P < 0.01 vs. control). Voluntary activation during the post-exercise MVC declined to 93.7% (hot) and 93.9% (cool) (P < 0.05 vs. control). The post-exercise decline in voluntary activation represented ~20% of the decrease in mean force production in both conditions. Therefore, the additional increase in rectal temperature did not exacerbate the loss of force production following self-paced exercise in the heat. The impairment in force production indicates that the fatigue exhibited by the quadriceps is mainly of peripheral origin and a consequence of the prolonged contractile activity associated with exercise. [ABSTRACT FROM AUTHOR]

While much is known about sequential effects in motor timing, less is understood about whether movement parameters such as force show sequential dependencies. In this study, we examined the effect of timing constraints on repetitive unimanual force production sequences. Ten healthy participants produced a series of pinch grip forces in time to a metronome and to visually specified force amplitudes. Either visual feedback of force produced or the auditory metronome removed 10 s into the experimental trial, with participants performing continued responses for the remaining 20 s. In the continuation trials, a negative lag-1 autocorrelation in the inter-response intervals (IRIs) was observed as is commonly seen in motor timing tasks. However, removal of visual feedback resulted in a systematic increase in mean force output through the course of the trial, resulting in positive lag-1 autocorrelation values. An interaction was found between mean IRI and peak force (PF) magnitude, with greater force variability seen for the larger intervals. However, the imposition of dual force and timing constraints had no effect either on the underlying variability of the PF or on the IRIs. The results are discussed in the context of force and time being independently specified components of a generalized motor program. [ABSTRACT FROM AUTHOR]

The purpose of the study was to explore the ability of the central nervous system (CNS) to organize synergies at two levels of a hypothetical control hierarchy involved in two-hand, multi-finger tasks. We investigated indices (Δ V) of finger force co-variation across trials as reflections of synergies stabilizing the total force ( F TOT). Subjects produced constant force with one or two finger-pairs (from one hand or two hands). In trials starting with one finger-pair, subjects added another finger-pair without changing F TOT. In trials starting with two finger-pairs, subjects removed one of the finger-pairs without changing F TOT. Adding or removing a finger-pair resulted in a transient drop in Δ V computed for the finger-pair that remained active throughout the trial. This drop in Δ V was seen simultaneously with force changes. Compared to the original steady-state, addition of a finger-pair led to a significant drop in Δ V at the newly established steady-state. This drop eliminated negative co-variation among finger forces that had stabilized F TOT. In contrast, in trials starting with two finger-pairs, no negative co-variation between finger forces within-a-pair was seen. Removing a finger-pair led to the emergence of negative co-variation between finger forces at the new steady-state. The Δ V index computed across two finger-pairs confirmed the existence of negative force co-variation. The emergence and disappearance of force stabilizing synergies within a finger-pair may signal limitations in the ability of the CNS in forming synergies at two different hierarchical levels. [ABSTRACT FROM AUTHOR]

The organization of thumb and index finger forces in a pinch formation was investigated under conditions where kinetic constraints on interdigit force coupling were removed. Two visually guided isometric force tasks at submaximal levels were used to characterize the spatial and temporal aspects of interdigit force coupling. Task 1 provided an initial characterization of interdigit force coordination when the force relationship between the digits was not specified. Task 2 probed the extent to which a preferred coordination of the thumb and index finger could be decoupled, both temporally and with respect to force magnitude, by specifying the coordination between the digit forces. Digit forces were measured using a pinch apparatus that was instrumented to record the magnitude and direction of the thumb ( F t) and index finger ( F i) forces, independently. Two apparatus conditions allowed further examination of interdigit force coordination when the relationship between digit forces was mechanically constrained (pivot condition), and when the relationship between digit forces was not constrained, allowing the neuromotor system to select a preferred pattern of interdigit coordination (fixed condition). Sixteen right-handed adults exerted a pinch force against the apparatus to match a single-cycle sine wave that varied between 15 and 35% of each participant’s maximal voluntary pinch force. The target was presented with positive or negative target sense, to vary the order of force level and direction of force change across the trials. When the mechanical constraints allowed selection of a preferred coordination pattern, F t = F i was a robust result. In contrast, when the coordination between the digit forces was specified by the requirement to simultaneously produce and control independent thumb and index finger forces while acting on a stable object, subjects were able to produce forces that markedly deviated from the F t = F i coordination. The organization of pinch is characterized by a preferred, tight coupling of digit forces, which can be modified based on task demands. [ABSTRACT FROM AUTHOR]

We explored the ability of the central nervous system (CNS) to assemble synergies stabilizing the output of sets of effectors at two levels of a control hierarchy. Specifically, we asked a question: can the CNS organize both two-hand and within-a-hand force stabilizing synergies in a simple two-hand force production task that involves two fingers per hand? Intuitively, one could expect a positive answer; that is, forces produced by each hand are expected to co-vary negatively across trials to bring down the total force variability, while forces produced by each finger within-a-hand are expected to co-vary negatively to reduce the variability of that hand’s contribution to the total force. The subjects were instructed to follow a trapezoidal time profile with the signal corresponding to the force produced by a set of instructed fingers in one-hand tasks with two-finger force production and in two-hand tasks with involvement of both symmetrical and asymmetrical finger pairs in the two hands. Finger force co-variation across trials was quantified and used as an index of stabilization of the force produced by all the instructed fingers, and of the force produced by finger pairs within-a-hand. No major differences were seen between the dominant and the non-dominant hand and between the two-hand tasks with symmetrical and asymmetrical finger involvement. Stronger synergies were seen in the index–middle finger pair as compared to the ring-little finger pair. The main result of the study is the significantly weaker or even lacking two-finger force stabilizing synergies within-a-hand during two-hand tasks while such synergies were present in one-hand tasks. This observation points at a potential limitation in the ability of the CNS to organize synergies at two levels of a control hierarchy simultaneously. It also allows suggesting a hypothesis on two types of synergies in the human motor repertoire, well-practiced synergies that form a library serving as the foundation for all novel actions, and freshly assembled synergies. [ABSTRACT FROM AUTHOR]

We investigated multi-finger synergies stabilizing the total moment of force and the total force when the subjects produced a quick cyclic change in the total moment of force. The seated subjects performed the task with the fingers of the dominant arm while paced by the metronome at 1.33 Hz. They were required to produce a rhythmic, sine-like change in the total pronation–supination moment of force computed with respect to the midpoint between the middle and ring fingers. The framework of the uncontrolled manifold hypothesis was used to compute indices of stabilization of the total moment and of the total force across 20 cycles. Variance of the total moment showed a cyclic pattern with peaks close to the peak rate of the moment change. Variance of the total force was maximal close to peak moment into supination. Higher magnitudes of the moment directed against the required moment direction (antagonist moment) were produced by individual fingers during supination efforts as compared to pronation efforts. Indices of multi-finger synergies showed across-trials stabilization of the total moment over the whole cycle but not of the total force. These indices were smaller during supination efforts. We conclude that the central nervous system facilitates multi-finger synergies stabilizing the total rotational action across a variety of tasks. Synergies stabilizing the total force are not seen in tasks that do not explicitly require accurate force control. Pronation efforts are performed more efficiently and with better stabilization of the action. [ABSTRACT FROM AUTHOR]

The present study addresses the influence of aging on the ability to regulate mechanical effects arising during arm movements due to the multi-joint structure of the arm. Two mechanical factors were considered, interaction torque (IT) and inertial resistance (IR). Regulation of these two factors can be demanding in terms of the timing and magnitude of the required muscle torque (MT), specifically during fast movements. We hypothesized that aging exacerbates the challenge regarding the regulation of these effects with muscular control due to declines in the motor system. This hypothesis was tested by comparing performance of a cyclic line-drawing task in two age groups, young and older adults. Only two joints, the shoulder and elbow, participated in motion. Four orientations of the lines were used to provide variations in the requirements for regulation of IT and IR. Cyclic frequency was manipulated to emphasize the dependence of the mechanical factors on movement speed. Various characteristics of fingertip motion showed that there were no age-related deteriorations in accuracy of line drawing. However, older adults were systematically slower, particularly in the directions of high IR. A detailed analysis of the magnitude of MT and the contribution of this torque to production of net torque at each joint demonstrated that older adults modified joint control and decreased the demands for MT by skillful exploitation of IT in a way specific for each particular line orientation. The results point to a tendency in older adults to decrease the production of muscle force. Nevertheless, older adults also demonstrated an ability to partially compensate for declines in the force production by developing sophisticated strategies of joint control that exploit the multi-joint mechanical structure of the arm. This ability suggests that the internal representation of inter-segmental dynamics and the capability to use it for movement control does not decay with age. The study emphasizes the importance of analysis of joint motion and control characteristics for the investigation of arm movements and for comparison of these movements between different subject populations. [ABSTRACT FROM AUTHOR]

The organisation of the human neuromuscular-skeletal system allows an extremely wide variety of actions to be performed, often with great dexterity. Adaptations associated with skill acquisition occur at all levels of the neuromuscular-skeletal system although all neural adaptations are inevitably constrained by the organisation of the actuating apparatus (muscles and bones). We quantified the extent to which skill acquisition in an isometric task set is influenced by the mechanical properties of the muscles used to produce the required actions. Initial performance was greatly dependent upon the specific combination of torques required in each variant of the experimental task. Five consecutive days of practice improved the performance to a similar degree across eight actions despite differences in the torques required about the elbow and forearm. The proportional improvement in performance was also similar when the actions were performed at either 20 or 40% of participants’ maximum voluntary torque capacity. The skill acquired during practice was successfully extrapolated to variants of the task requiring more torque than that required during practice. We conclude that while the extent to which skill can be acquired in isometric actions is independent of the specific combination of joint torques required for target acquisition, the nature of the kinetic adaptations leading to the performance improvement in isometric actions is influenced by the neural and mechanical properties of the actuating muscles. [ABSTRACT FROM AUTHOR]

We investigated the production of time profiles of the total moment of force produced in isometric conditions by the four fingers of a hand. We hypothesized that these tasks would be associated with multi-finger synergies stabilizing the time profile of the total moment across trials but not necessarily stabilizing the time profile of the total force produced by the fingers. We also expected the multi-finger synergies to prevent an increase in the moment variability with its magnitude. Seated subjects pressed on force sensors with the four fingers of the right hand and produced two time profiles of the total moment of force, starting from a certain pronation effort, leading to a similar supination effort, and back to the initial pronation effort. One of the profiles was a sequence of straight lines (M-Ramp) while the other was a smooth curve (M-Sine). The subjects showed an increase in the total force during each task. This was accompanied by an increase in the force produced by the fingers opposing the required direction of the total moment—antagonist fingers. Variability of the total force and of the total moment showed complex, non-monotonic changes with the magnitude of the force and moment, respectively. In both tasks, the subjects showed patterns of co-variation of commands to fingers that stabilized the required moment profile over trials. The time profile of the total force was stabilized to a lesser degree or not stabilized at all. The share of fingers with larger moment arms (index finger for pronation efforts and little finger for supination efforts) was higher when the fingers acted to produce moments in a required direction but not necessarily when they acted as antagonists. The results demonstrate the existence of multi-finger synergies stabilizing the combined rotational action. They fit a hypothesis that stabilization of rotational actions may be a default strategy conditioned by everyday experience. The data also suggest that the mechanical advantage hypothesis is valid for sets of effectors that act in the required direction but not for sets of effectors that act as antagonists. [ABSTRACT FROM AUTHOR]

We studied changes in multi-finger synergies associated with predictable and unpredictable force perturbations applied to a finger during a multi-finger constant total force production task. The main hypothesis was that indices of multi-finger synergies can show anticipatory changes in preparation for a predictable perturbation. Subjects sat in a chair and pressed on force sensors with the four fingers of the right hand. The task was to produce a constant level of total force. The fingers acted against loads that produced upward directed forces. The loads (applied either to the index or to the ring finger) could be disengaged either by the subject or by the experimenter. An index of finger co-variation, Δ V was computed across sets of 12 trials at each time sample and for all tasks separately. During steady-state force production, all subjects showed positive Δ V values corresponding to strong negative covariation among finger forces interpreted as a force-stabilizing synergy. Prior to self-triggered unloading, subjects showed an anticipatory drop in Δ V that started 100-125 ms prior to the unloading time. Such early changes were absent in trials with experimenter-triggered unloading. After an unloading, subjects changed forces of both perturbed and unperturbed fingers and reached a new sharing pattern of the total force. In experimenter-triggered conditions, changes in the forces of unperturbed fingers could be seen as early as 120 ms following an unloading. The index Δ V dropped following a perturbation and then recovered; the recovery occurred faster in self-triggered conditions. We conclude that humans can use feed-forward changes in multi-finger synergies (anticipatory synergy adjustments) in anticipation of a predictable perturbation. These changes may help avoid prolonged weakening of a multi-digit force-stabilizing synergy. We discuss a possibility that anticipatory postural adjustments may represent a particular case of the phenomenon of anticipatory synergy adjustments and suggest a hierarchical control scheme that incorporates a possibility of independent control over the output of a multi-element system and covariation patterns among outputs of its elements. [ABSTRACT FROM AUTHOR]

The purpose of the study was to develop a model of force variability for a fast action performed by a multi-effector system and to verify it for multi-finger quick force production. The experiments involved quick isometric contractions to different target force levels using different finger combinations. Force variance calculated over sets of trials for a multi-finger force production task showed non-monotonic single-peak profiles of force variance with a peak at a time between the times of the maxima of the force rate and of the total force. When analyzed in the four-dimensional space of finger forces, the variance peak was mostly expressed in the direction of the force rate, and was absent in the directions orthogonal to it. The non-monotonic time profile of the force variance could be reproduced by a model of force production, which assumes that each finger force profile is based on a template function scaled in duration and magnitude with two parameters assigned prior to each trial with some variability. The model allows decomposition of the force variance into two fractions related to variability in setting the magnitude and duration scaling parameters. The former fraction changes monotonically with time, while the latter shows a transient peak in the middle of the action. The model was able to reproduce experimental variance time profiles across conditions with the total error of under 8%. The results demonstrate, in particular, that fast multi-finger actions may show transient changes in motor variability in certain directions of the finger force space, particularly in the direction of the first force derivative, without any task-specific coordinating action by the controller. These findings require a reconsideration of some of the conclusions drawn in recent studies on the structure of motor variability in redundant multi-effector systems. [ABSTRACT FROM AUTHOR]

We used the uncontrolled manifold (UCM) approach to study the synergy formation during learning an unusual multi-finger task. The subjects produced accurate force ramps with challenging sets of four fingers (two per hand). We tested hypotheses on stabilization of the contributions of subsets of effectors to the task force (FTASK) and to the moment in the frontal plane (force-stabilization and moment-stabilization, respectively). Force signals were used to compute magnitudes of hypothetical independent signals, modes. The variance of the mode magnitudes across repetitions of the task was partitioned into two components, within the UCM (VUCM), which did not affect the average value of a selected performance variable (force or moment), and orthogonal to the UCM (VORT), which affected the variable. Prior to practice, subjects showed high error indices and failed to show stabilization of each hand’s contribution to FTASK (VORT≥VUCM), while the pronation-supination moment was stabilized by the fingers of each hand (VORTVUCM). The total forces produced by each of the two hands showed negative covariation across trials, which supported the force-stabilization hypothesis but not moment-stabilization hypothesis. Both force-stabilization and moment-stabilization hypotheses were supported by analysis of mode magnitudes to all eight fingers. Over 2 days of practice, the performance of the subjects improved considerably. This was accompanied by the emergence of within-a-hand force-stabilization for each of the two hands without deterioration of moment-stabilization. Quantitatively better within-a-hand force-stabilization was seen in male subjects as compared to females throughout the course of the experiment. Force-stabilization by all eight fingers improved quantitatively with practice. Practice also resulted in higher finger forces in maximal force production (MVC) trials and higher forces produced by unintended fingers in single-finger MVC trials (higher enslaving). We conclude that the UCM approach allows quantifying changes in the coordination of effectors during practice, and offers insights into the microstructure of this coordination with respect to different performance variables and different subsets of effectors. The approach can be used to test whether new synergies emerge in the process of practice. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate changes in finger forces and their interactions in one-hand multi-finger force production tasks involving finger addition and removal. Eight male subjects were instructed to produce maximal (MVC) forces with explicitly instructed ('master') fingers. After reaching maximal force with a set of master fingers, the subjects added/removed one master finger while continuing to produce the MVC with the new group of master fingers. The non-instructed ('slave') fingers also produced forces (enslaving). Finger addition/removal led to changes in the forces of individual master fingers expected from earlier studies of maximal force production by different finger groups acting synchronously. A significant increase in the forces of remaining master fingers was observed after finger removal and a close-to-significant drop in the forces of previously recruited master fingers was observed after finger addition. These effects were larger when subjects started the task with a smaller number of explicitly involved fingers. The enslaving effects increased after finger addition while they did not change after finger removal. Forces produced by the same group of master fingers acting in different tasks showed no history-dependent effects. However, significant effects of history were seen in enslaving. These observations speak against hypotheses of more independent behaviour of effectors during their asynchronous involvement. They show that finger interaction may show effects of the history of finger involvement in a task. [ABSTRACT FROM AUTHOR]

This study examined the force production characteristics, activation/coactivation and endurance capacity of the neck extension and flexion muscles in healthy men (n=29) and women (n=28) divided into three age groups (18–26 years, 30–37 years and 45–55 years). Force and electromyography (EMG) measurements were performed during the maximal voluntary isometric extension and flexion actions. This was followed by an endurance test (ET; 60% force level of maximal voluntary contraction sustained until exhaustion), after which the force and EMG recordings were repeated. Men were both stronger and had higher values (P<0.001) for explosive force (rate of force development, RFD) than women in both actions. Younger subjects of both genders exhibited larger (P<0.05 in women) RFD values than older subjects in extension. The coactivation of the antagonist muscles during the maximal extension or flexion did not differ significantly between men and women, but the coactivation of the antagonists was larger (P<0.05) in the older age groups than in the youngest group. Women maintained the 60% force level longer than men in both actions (extension, P<0.001; flexion, not significant). The fatiguing loading led to significant decreases in maximal isometric force (P<0.001) and RFD (P<0.01–0.001), but these relative decreases did not differ between the groups. In conclusion, large gender differences in the voluntary extension and flexion force production characteristics of the neck muscles did exist, as reported earlier for other muscles of the body. No age-related differences were observed in maximal force of the extension and flexion actions within the age ranges of the subject groups studied here, but the older subjects exhibited greater coactivation and produced lower force values in the early portions of the force/time curve of the extension than the youngest group. The data indicate that explosive force production may be sensitive to aging earlier than maximal strength in the case of the neck extensor muscles. [ABSTRACT FROM AUTHOR]

Movement-related cortical d.c. shifts accompanying the execution of four different prehensile tasks were investigated using six normal adult subjects. The goal was to identify patterns of brain electrical activity that differentiated a precision grip configuration (thumb and index finger or 2f) from a full precision grip configuration (thumb and all fingers or 5f) at different total force levels. As such, this was the first study to systematically manipulate both grip configuration and force level while also measuring movement-related potentials (MRP) during the control phase of an isometric prehensile task. This investigation focused on assessing the sustained, performance-related negativity (N-P) associated with the execution of particular grip configurations at different total force levels (percentage maximum voluntary force, MVF). The results from this study demonstrated significant interactions between grip configuration, force level and amplitude of the N-P. First, an overall increase in force output does not correspond to larger N-P amplitudes under these task conditions. Second, force level and grip configuration interact significantly in determining the peak N-P, especially in low-force conditions. Overall, the findings reveal a task-specific sensitivity of movement-related potentials associated with the control phase of a prehensile force task while humans execute different grip configurations and force levels. [ABSTRACT FROM AUTHOR]

We investigated force-sharing among three fingers which acted in parallel and produced ramp profiles of total force from zero to the maximal voluntary force. The feedback to the subject was provided by a visual signal on the monitor and could correspond to the sum of forces of all the fingers or to the sum of forces of two fingers, while the force of the third finger was added with a coefficient 2 or 0.5. If the subjects did not know about the distorted feedback, they showed a template-sharing pattern within the whole range of total force values. This pattern did not depend on which finger force was multiplied and by which coefficient. If the subjects knew in advance how the feedback signal would be calculated, they tried to perform the task using either only the finger whose force was multiplied by 2 or two fingers when the force of the third one was multiplied by 0.5. Further into the trial, however, they were unable to track the ramp pattern using only one or two fingers and demonstrated a search activity that could continue until the end of the trial or lead eventually to a three-finger sharing pattern similar to the template pattern used in cases of undistorted feedback. We conclude that the limited number of preferred sharing pattern within the studied task reflects an organization of the fingers into a structural unit (involving one, two, or all three fingers) by the central nervous system. The availability of structural units defines the presence of stable solutions available for the system. [ABSTRACT FROM AUTHOR]

The mechanical and energy characteristics of isolated fast-twitch muscle fibres (type 1) of Xenopus laevis in isometric- and isovelocity contractions were measured at 20°C. The fibres were stimulated at either 60 Hz or 20 Hz to produce contractions at different levels of activation. The high stimulation frequency gave fused contractions, while at the low stimulation frequency tension fluctuated. When maximum isometric force had been reached, the fibres were shortened by 10% of the fibre length at different velocities. At 60 Hz stimulation during shortening the rate of heat production increased above the isometric rate of heat production. At 20 Hz stimulation during shortening, however, the rate of heat production was not different from the isometric rate of heat production. Mechanical efficiency was the same at the high and low level of activation. The actomyosin efficiency (i.e. the mechanical efficiency corrected for ”activation heat”) was highest at the low level of activation. We conclude that in fast-twitch muscle fibres from X. laevis, actomyosin efficiency is highest for partially activated muscle. From a comparison of the present results with those obtained from a study of slow-twitch muscle fibres presented earlier, it is concluded that fast-twitch muscle fibres are less efficient than slow-twitch muscle fibres. [ABSTRACT FROM AUTHOR]

We investigated the effect of shortening velocity on the efficiency of single intact slow-twitch muscle fibres (type 3) of Xenopus laevis, at different levels of activation (10, 15, 20 and 40 Hz). Fused contractions were obtained at 40 Hz stimulation. When maximal isometric force had been reached, the fibres were shortened by 10% of the fibre length ( L) at 0.4, 1 and 2 L/s. To investigate whether the oscillating force at low stimulation frequencies influenced power output and the rate of heat production, we also performed these experiments with the fibre bathed in dantrolene. The results with fused contractions in the presence of dantrolene were the same as with unfused contractions. At 40 Hz stimulation during shortening the rate of heat production increased above that measured during isometric contractions, while at the lower stimulation frequencies the rate of heat production was less than that during isometric contractions. Mechanical efficiency was highest at low activation, and increased more with shortening velocity than at high activation. The actomyosin efficiency (i.e. the efficiency corrected for 'activation heat') was also highest at 10 Hz stimulation. We conclude that in slow-twitch muscle fibres of X. laevis, near the optimum shortening velocity, cross-bridge efficiency is highest for partially activated muscle. [ABSTRACT FROM AUTHOR]

Single intact slow-twitch (type 3) muscle fibres from the iliofibularis muscle of Xenopus laevis were shortened at a constant velocity (0.4 L/s, where L is the initial length at different levels of activation (40, 15, 12.5, and 10 Hz). A stimulation frequency of 40 Hz gave fused tetanic records. At this frequency the mean heat production rate during shortening (0.38±0.05 W/g dry weight) was slightly higher than the isometric heat production rate (0.33±0.03 W/g dry weight). The lower stimulation frequencies gave unfused tetanic contractions, the average isometric force of which was 40±3% of the isometric force at 40 Hz. In these unfused tetani during shortening the heat production rate (0.18±0.02 W/g dry weight) significantly decreased below the isometric heat production rate (0.25±0.02 W/g dry weight). At full activation the rate of total energy production (mechanical power plus heat production rate) during shortening was 1.88±0.32 times the isometric total energy production rate. This effect, i.e. an increase in energy turnover with shortening, is known as the Fenn effect. At sub-maximal stimulation the energy output during shortening was only 1.07±0.08 times the isometric value. These results show that the Fenn effect is dependent on the level of activation. The efficiency (ratio of mechanical power to total energy output) was independent of the stimulation frequency (0.37±0.06). [ABSTRACT FROM AUTHOR]

We compared matching of finger forces under isometric conditions with matching of forces produced against a spring load (anisometric conditions) in twenty normal subjects. The instruction was to generate the same force in both hands holding a grip between thumb and index finger in each hand. Visual feedback indicating the target force and the actual force applied were presented for one (reference) hand only. The forces produced in each hand were measured continuously during matching trials. A special device provided the opportunity to change from isometric to anisometric force production. Matching was required under symmetric conditions, in which force was generated in both hands either isometrically or anisometrically, as well as under asymmetric conditions in which isometric force has to be matched to anisometric force or the reverse. Under symmetric conditions matching error was consistently smaller in anisometric than in isometric force production. However, the striking feature was a severe mismatch between hands when forces had to be produced differently. For most subjects, a force generated against the spring load in the reference hand was greatly overestimated by the matching hand working isometrically. For the reverse condition consistent underestimations were observed. This effect cannot be attributed to left/right differences or a simple confusion of subjects in the asymmetric tasks. Some of the factors confounded with the conditions of force production were ruled out as an explanation by additional experimental controls. The mismatch neither depends on signals related to different finger positions associated with target forces nor is it alleviated when differently produced forces are matched sequentially. The finding that perceived muscular force depends on conditions of force production requires a reevaluation of the afferent and centrally generated signals (corollary discharge) assumed to contribute to sensations of force. [ABSTRACT FROM AUTHOR]

Recent evidence for the use of visual cues in the programming of the precision grip has been given by Gordon et al. (1991). Visually invoked size-related information influenced the physical forces used to produce a lift, even when it was not consistent with other sensory information. In the present study, blind-folded subjects were required to feel the size of an object by haptic exploration prior to lifting it. Two boxes of equal weight and unequal size were used for the lift objects and were attached to an instrumented (grip) handle. Grip force and load force, their rates, and the vertical move ment of the object were measured. Most subjects report that the small box was heavier, which is consistent with size-weight illusion predictions. However, peak grip force, grip force rate, peak load force, and load force rate were greater for the large box when the boxes were randomly presented, but not when the same boxes were lifted consecutively. If subjects did not feel the box prior to a lift, these parameters were scaled in between those normally employed for the large and small box. Most subjects apparently programmed the parallel increase of the grip and load force during the loading phase as one force rate pulse. This represented a 'target strategy' in which an internal neural representation of the objects weight determined the actual target parameter (i.e. just enough force required to overcome gravity). The other subjects exhibited a slower stepwise increase in grip and load force rate. The subjects choosing this 'probing strategy' did not scale the force parameters differently for the two boxes. Furthermore, they did not perceive any difference between the objects' weight. Together, these results suggest that haptic exploration may be used to convey size information and further support the hypoth esis that size-related information may be combined with other sensory information in the programming of the precision grip. [ABSTRACT FROM AUTHOR]

This study examined whether the torques and EMG activity that precede and accompany bilateral arm pulls made by standing humans demonstrate a pulse height form of organization. Nine adults made abrupt bilateral pulls in the sagittal plane against a handle, to force targets equal to 5, 10, 20, 40, 60, 80 and 95% of their maximal pulling force (%MPF). The force applied at the handle, ground reaction forces, the center of pressure (CP), and EMG activity in gastrocnemius (GS), biceps femoris (BF), tibialis anterior (TA) and quadriceps (QD) muscles were recorded. Our analysis divided the action into a pre-pull phase (events prior to the increase of handle force) and a pulling phase (while handle force was greater than zero). We evaluated the effects of %MPF on the durations and peak amplitudes of the pre-pull and pulling angular impulses about the ankle joint and on pre-pull EMG patterns. The results showed that the angular impulse associated with the pulling torque (due to the reactive force on the body during the pull) had a pulse height organization: peak torque increased linearly with %MPF, and the durations of the pulling torque were relatively constant. In contrast, a pulse height organization did not characterize the pre-pull period for either the angular impulse associated with ankle torque (due to net ground reaction force) or EMG activity in the leg muscles. Rather, peak ankle torque typically increased up to some submaximal %MPF and then plateaued, perhaps due to a constraining effect of foot length on CP. The durations of pre-pull ankle torques increased over the whole range of %MPF, thereby compensating for the limit on ankle torque. Depending on the subject, the muscles were recruited in two different orders: GS-BF-TA-QD, or GS-TA-BF-QD. As the %MPF increased, the EMG onset times of all four muscles occurred earlier, and there was a greater likelihood that the BF, TA and QD muscles would be recruited on a given trial. The changes in the ankle torque and EMG patterns were gradual, suggesting that the pre-pull phase could have one underlying form of organization, with parameters that are tuned to task goals and anatomical constraints. [ABSTRACT FROM AUTHOR]

This experiment examined the preparation and the production of isometric force in Parkinson's disease (PD). PD patients, elderly, and young subjects generated force levels that were a percentage of their maximum (15, 30, 45, and 60%). Subjects were cued on the upcoming target force level and they were asked to produce the required response as fast as possible. PD patients showed a similar progression of force variability and dispersion of peak forces to that of control subjects, implying they have an accurate 'internal model' of the required forces. Force production impairments were seen, however, at the within-trial level. PD patients had more irregular force-time curves that were characterized by changes in the rate of force production. The results suggest a more 'noisy' output from the motor system and an inability to produce smooth forces. PD patients were also substantially slower in initiating a force production and the delay was localized in the pre-motor reaction time. [ABSTRACT FROM AUTHOR]

We studied the relation between force normalized by dry mass per unit length and the myosin fraction of muscle dry mass. The two tibialis anterior muscles were dissected from 12 frogs ( Rana temporaria). Then, from one muscle, two single fast-twitch fibres were isolated. Each fibre was mounted isometrically in Ringer’s solution, and electrically stimulated using a standardized protocol. Peak force production, normalized by the fibre’s dry mass per unit length, varied by a factor of 1.4. Little variation in normalized force was measured between fibres from the same animal, whereas between animals a significant difference was found ( P<0.05). The contralateral muscle was used to determine the myosin fraction of the dry mass. The relationship between the fraction myosin of the dry mass and force normalized by dry mass per unit length showed a high correlation ( r = 0.81; n = 12). From this we conclude that variation in normalized tetanic force is determined greatly (65%) by variations in myosin content. [ABSTRACT FROM AUTHOR]