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Introduction: The aim of the study was to compare maximal force, force steadiness, and the discharge characteristics of motor units in the tibialis anterior (TA) muscle during submaximal isometric contractions for ankle dorsiflexion and adduction of the foot., Methods: Nineteen active young adults performed maximal and submaximal isometric dorsiflexion and adduction contractions at five target forces (5%, 10%, 20%, 40%, and 60% maximal voluntary contraction [MVC]). The activity of motor units in TA was recorded by high-density EMG., Results: The maximal force was similar between dorsiflexion and adduction, despite EMG amplitude for TA being greater ( P < 0.05) during dorsiflexion than adduction. Τhe coefficient of variation (CV) for force (force steadiness) during dorsiflexion was always less ( P < 0.05) than during adduction, except of 5% MVC force. No differences were observed for mean discharge rate; however, the regression between the changes in discharge rate relative to the change of force was significant for dorsiflexion ( R2 = 0.25, P < 0.05) but not for adduction. Discharge variability, however, was usually less during dorsiflexion. The CV for interspike interval was less ( P < 0.05) at 10%, 20%, and 40% MVC but greater at 60% MVC during dorsiflexion than adduction. Similarly, the SD values of the filtered cumulative spike train of the motor units in TA were less ( P < 0.05) at 5%, 10%, 20%, and 40% MVC during dorsiflexion than adduction., Conclusions: Although the mean discharge rate of motor units in TA was similar during foot adduction and ankle dorsiflexion, discharge variability was less during dorsiflexion resulting in less accurate performance of the steady adduction contractions. The neural drive to bifunctional muscles differs during their accessory function, which must be considered for training and rehabilitation interventions., (Copyright © 2024 by the American College of Sports Medicine.)

Tendon properties impact human locomotion, influencing sports performance, and injury prevention. Hamstrings play a crucial role in sprinting, particularly the biceps femoris long head (BFlh), which is prone to frequent injuries. It remains uncertain if BFlh exhibits distinct mechanical properties compared to other hamstring muscles. This study utilized free-hand three-dimensional ultrasound to assess morphological and mechanical properties of distal hamstrings tendons in 15 men. Scans were taken in prone position, with hip and knee extended, at rest and during 20%, 40%, 60%, and 80% of maximal voluntary isometric contraction of the knee flexors. Tendon length, volume, cross-sectional area (CSA), and anteroposterior (AP) and mediolateral (ML) widths were quantified at three locations. Longitudinal and transverse deformations, stiffness, strain, and stress were estimated. The ST had the greatest tendon strain and the lowest stiffness as well as the highest CSA and AP and ML width strain compared to other tendons. Biceps femoris short head (BFsh) exhibited the least strain, AP and ML deformation. Further, BFlh displayed the highest stiffness and stress, and BFsh had the lowest stress. Additionally, deformation varied by region, with the proximal site showing generally the lowest CSA strain. Distal tendon mechanical properties differed among the hamstring muscles during isometric knee flexions. In contrast to other bi-articular hamstrings, the BFlh high stiffness and stress may result in greater energy absorption by its muscle fascicles, rather than the distal tendon, during late swing in sprinting. This could partly account for the increased incidence of hamstring injuries in this muscle., (© 2024 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

The aim of the study was to assess the potential influence of footedness and dominance on maximal force, force fluctuations and neural drive during dorsiflexion. Fifteen left-footed (LF) and fifteen right-footed (RF) young adults performed 2 maximal voluntary contractions (MVC) and 3 steady submaximal isometric contractions at five target forces (5, 10, 20, 40 and 60% MVC) with the dorsiflexors of both legs. High-density electromyography (EMG) was used to record the discharge characteristics of motor units (MUs) of Tibialis Anterior. MVC force and EMG amplitude (root mean square) were similar between the two legs and groups (p > 0.05). Force fluctuations (Coefficient of Variation, CoV for force), mean discharge rate of MUs, discharge variability (CoV of interspike interval), and variability in neural drive (standard deviation of filtered cumulative spike train) were greater (p < 0.05) and the input-output gain of the MUs (ΔDR/ΔF) was lower (p < 0.05) for the LF relative to the RF group. The differences in force fluctuations during steady contractions with the dorsiflexors were associated with footedness but not with dominance. They reflect greater variability in motor neuron output, as suggested by coefficient of variation for interspike interval (independent input) and the standard deviation of the smoothed discharge times (common input)., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

The aim of the study was to assess the influence of habitual training history on force steadiness and the discharge characteristics of motor units in tibialis anterior during submaximal isometric contractions. Fifteen athletes whose training emphasized alternating actions (11 runners and 4 cyclists) and fifteen athletes who relied on bilateral actions with leg muscles (7 volleyball players, 8 weight-lifters) performed 2 maximal voluntary contractions (MVC) with the dorsiflexors, and 3 steady contractions at 8 target forces (2.5%, 5%, 10%, 20%, 30%, 40%, 50% and 60% MVC). The discharge characteristics of motor units in tibialis anterior were recorded using high-density electromyography grids. The MVC force and the absolute (standard deviation) and normalized (coefficient of variation) amplitudes of the force fluctuations at all target forces were similar between groups. The coefficient of variation for force decreased progressively from 2.5% to 20% MVC force, then it plateaued until 60% MVC force. Mean discharge rate of the motor units in tibialis anterior was similar at all target forces between groups. The variability in discharge times (coefficient of variation for interspike interval) and the variability in neural drive (coefficient of variation of filtered cumulative spike train) was also similar for the two groups. These results indicate that athletes who have trained with either alternating or bilateral actions with leg muscles has similar effects on maximal force, force control, and variability in the independent and common synaptic input during a single-limb isometric task with the dorsiflexors., (© Journal of Sports Science and Medicine.)

Context: Bridge exercises are extensively used in trunk-strengthening programs. The aim of this study was to investigate the effect of bridging duration on lateral abdominal muscle thickness and gluteus maximus activation., Design: Cross-sectional., Methods: Twenty-five young males participated in this study. Transversus abdominal (TrA), external and internal oblique ultrasound thickness, gluteus maximus electromyographic activation, and sacral tilt angle were simultaneously measured for every second during 30-second bridging exercise. The contraction thickness ratio and root mean squared signal (normalized to maximum isometric contraction signal) during 6 exercise durations (from 0 to 5, 10, 15, 20, 25, and 30 s) were also calculated and compared using analysis of variance designs., Results: TrA and internal oblique contraction thickness ratio and gluteus maximus root mean squared increased during the first 8 to 10 seconds and remained elevated until the end of the 30-second exercise (P < .05). External oblique contraction thickness ratio declined during exercise (P < .05). Five-second bridging showed less TrA thickness and anteroposterior and mediolateral sacral tilt angle and a lower anteroposterior tilt variability compared with bridges, which lasted more than 10 seconds (P < .05)., Conclusions: Bridge exercises longer than 10 seconds may be better for promoting TrA recruitment than bridges of shorter duration. Clinicians and exercise specialists could adjust the duration of bridge exercise based on the aims of the exercise program.

The purpose of the study was to assess the influence of short, intermediate, and long muscle lengths on dorsiflexor force steadiness and the discharge characteristics of motor units in tibialis anterior during submaximal isometric contractions. Steady contractions were performed at 5 target forces (5, 10, 20, 40, and 60% maximal voluntary contraction, MVC) for 3 ankle angles (75°, 90°, and 105°). MVC force was less ( p  = 0.043) at the smallest joint angle compared with the other two angles. The absolute (standard deviation) and normalised amplitudes (coefficient of variation) of the force fluctuations were similar for all 3 ankle angles at each target force. The coefficient of variation for force decreased progressively from 5% to 20% MVC force and then it plateaued at 40% and 60% MVC force. At all target forces, the mean discharge rate (MDR) of the motor units at 75° was greater than at 90° ( p  = 0.006) and 105° ( p  = 0.034). Moreover, the MDR was similar for 5% and 10% MVC forces and then increased gradually until 60% MVC force ( p  < 0.005). The variability in discharge times (coefficient of variation for interspike interval) and variability in neural drive (coefficient of variation of filtered cumulative spike train) were similar at all ankle angles. Variability in neural drive had a greater influence on force steadiness than did the variability in discharge times. Changes in ankle-joint angle did not influence either the normalised amplitude force fluctuations during steady submaximal contractions or the underlying modulation of the discharge characteristics of motor units in tibialis anterior.

Introduction: Examination of the mechanical behavior of the hamstrings when acting as antagonists provides information about loading of this muscle group and its role for joint stability during forceful quadriceps contractions. The aim of this study was to quantify biceps femoris long head fascicle length (FL), angle of pennation (PA) and distal tendon/aponeurosis strain during maximum voluntary contraction efforts of the knee extensors using real-time ultrasound., Methods: Fourteen participants performed passive joint movements and maximum voluntary knee extension and flexion efforts of the knee flexors at 0°, 45° and 90° of knee flexion. An ultrasound probe was used to visualize FL, PA and tendon/aponeurosis strain from the distal part of the muscle., Results: Two-way analysis of variance designs indicated that: (a) antagonist BFlh tendon/aponeurosis strain increased significantly up to 2.77 ± 1.25% relative to rest (p < 0.05). The FL increased non-significantly (2.86 ± 6.81%) while the PA was unaltered during isometric MVC efforts of the knee extensors (p > 0.05) (b) FL, PA and tendon/aponeurosis strain of the BFlh when acting as antagonist were not significantly affected by knee joint angular position (p > 0.05)., Conclusions: Antagonist hamstring function takes the form of a lengthened tendon/aponeurosis, no fascicle shortening and submaximal neural activation. Future research could examine whether exercise interventions that aim to alter tendon/aponeurosis mechanical properties of the hamstrings when acting as antagonists are beneficial for injury prevention and rehabilitation.

The purpose of this study was to quantify strain and elongation of the long head of the biceps femoris (BFlh) and the semitendinosus (ST) tendon/aponeurosis. Forty participants performed passive knee extension trials from 90° of knee flexion to full extension (0°) followed by ramp isometric contractions of the knee flexors at 0°, 45° and 90° of knee flexion. Two ultrasound probes were used to visualize the displacement of BFlh and ST tendon/aponeurosis. Three-way analysis of variance designs indicated that: (a) Tendon/aponeurosis (passive) elongation and strain were higher for the BFlh than the ST as the knee was passively extended (p<0.05), (b) contraction at each angular position was accompanied by a smaller BFlh tendon/aponeurosis (active) strain and elongation than the ST at higher levels of effort (p<0.05) and (c) combined (passive and active) strain was significantly higher for the BFlh than ST during ramp contraction at 0° but the opposite was observed for the 45° and 90° flexion angle tests (p<0.05). Passive elongation of tendon/aponeurosis has an important effect on the tendon/aponeurosis behavior of the hamstrings and may contribute to a different loading of muscle fibers and tendinous tissue between BFlh and ST., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Introduction: Our knowledge of the neurophysiology of post-activation potentiation (PAP) is limited. The purpose of this study was to examine the effect of PAP on twitch torque and H-reflex amplitude after a 10-s maximal voluntary contraction (MVC)., Methods: PAP measurements were assessed with the plantarflexors in a relaxed state and during a tonic contraction at 10% MVC., Results: The H-reflex/maximum M-wave ratio (H/M) decreased significantly (P<0.05) and returned to baseline levels after 1 min. The decrement in H/M was depressed when the plantarflexors were active at 10% MVC, and the depression was more obvious in the lateral gastrocnemius than in the soleus muscle., Conclusions: The inhibition induced immediately after contraction could be attributed to post-activation depression. We conclude that PAP after a 10-s MVC cannot be attributed to increased motor neuron excitability through the reflex pathway as assessed by the H-reflex technique., (© 2014 Wiley Periodicals, Inc.)

Backround: The purpose of this study was to compare joint kinematics, knee and trunk muscle activation and co-activation patterns during a sit-to-stand movement in women with knee osteoarthritis and age-matched controls., Methods: Eleven women with knee osteoarthritis (mean and standard deviation, age: 66.90, 4.51 years, height: 1.63, 0.02 m, mass: 77.63, 5.4 kg) and eleven healthy women (mean and standard deviation, age: 61.90, 3.12 years, height: 1.63 m, 0.03, mass: 78.30, 4.91 kg) performed a Sit to Stand movement at a self-selected slow, normal and fast speed. Three-dimensional joint kinematics of the lower limb, vertical ground reaction forces and electromyographic activity of the biceps femoris vastus lateralis and erectus spinae were recorded bilaterally., Findings: A two-way ANOVA showed that the osteoarhtitis group performed the sit to stand task using a smaller knee and hip range of motion compared with the control group while no differences in temporal kinematics and ground reaction force-related parameters were observed. In addition, women with osteoarhtritis displayed significantly lower vastus lateralis coupled with a higher biceps feomoris electromyographic activity and higher agonist-antagonist co-contraction and co-activation than asymptomatic women. The activation of erectus spinae was not different between groups., Interpretation: Results indicate that patients with moderate knee osteoarthritis rise from the chair using greater knee muscle co-contraction, earlier and greater activation of the hamstrings which results in reduced hip and knee range of motion. This may be a way to overcome the pain and potential muscle atrophy of knee extensor muscles without compromising overall task duration., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Background: Although the straight leg raise (SLR) test frequently is used to assess hamstring extensibility in individuals with low back pain (LBP), evidence relating LBP, SLR, and hamstring extensibility remains unclear. The SLR measures the angle between the lifted leg and the horizontal, however, and, as such, it is not a direct measure of the elongation capacity of the hamstrings., Objective: To examine the differences in hamstring elongation (quantified via ultrasonography) and SLR score between individuals with LBP and asymptomatic controls and to determine the relationship between hamstring elongation, SLR, and functional disability scores., Design: Cross-sectional study., Setting: University laboratory., Participants: Forty men and women with chronic LBP (mean ± SD, age 43.51 ± 3.71 years and 40 control subjects (age 45.11 ± 4.01 years) participated in this study., Assessment: Passive SLR, elongation assessed via ultrasonography, and functional disability., Assessment and Outcome Measurements: SLR score, elongation of tendinous tissue within the semitendinosus muscle, and Oswestry Disability Index., Results: Two-way analysis of variance tests indicated a significantly lower SLR score and a greater Oswestry score in LBP group compared with control subjects (P < .05). In contrast, there were no significant group differences in hamstring elongation (P > .05). Gender did not have an effect on all dependent measures (P > .05). Hamstring elongation showed a low correlation with SLR score and a minimal correlation with Oswestry score., Conclusions: These results indicate that the SLR score is not determined by hamstring elongation (quantified via ultrasonography)., (Copyright © 2015 American Academy of Physical Medicine and Rehabilitation. Published by Elsevier Inc. All rights reserved.)

Background and Purpose: People with knee osteoarthritis (OA) display limitations in daily activities and a lower quality of life. The purpose of this study was to investigate the differences in strength balance and activation during maximum strength efforts between women with knee OA and asymptomatic women., Methods: Twelve women with knee OA (age 60.33 ± 6.66 years) and 11 controls (age 56.54 ± 5.46 years) performed maximum isokinetic eccentric and concentric knee extension and flexion tests at 60°/s, 120°/s, and 150°/s. Surface electromyography (EMG) was recorded from the biceps femoris (BF), vastus lateralis (VL), and vastus medialis (VM). Hamstrings-to-quadriceps moment ratios (H/Q), the synergetic (VL/VM), and co-contraction (BF/[VM + VL]) EMG ratios were calculated., Results: Analysis-of-variance designs showed that women experiencing knee OA had significantly higher H/Q moment ratios and VM/VL EMG ratios than controls (P < 0.05). The co-contraction index was significantly lower in the OA group only during knee flexion (P < 0.05)., Conclusions: Women with knee OA showed a higher H/Q moment ratios probably because of the need for better joint stability or a lower quadriceps capacity. This deficiency was accompanied by a higher VM activation, which probably serves to stabilize the patella upon maximum contraction as well as a higher activation of antagonist muscles.

A tendinous inscription divides the semitendinosus muscle in two parts and it may have an effect on its function. The purpose of this study was to determine the effects of joint position and gender on the tendinous inscription morphology. Ultrasonography scans were taken from 76 young males and females at rest, in nine combinations of hip and knee joint angles. The length of the tendinous inscription arms and the angles formed by the two arms (apex angle), the tendinous inscription with the superficial (surface angle), and deep (deep angle) aponeurosis were determined. The tendinous inscription was clearly visible in 70 (out of 76) subjects. Analysis of variance designs showed that increasing hip flexion angle from 0 to 90° increased the long arm and muscle thickness but decreased the short tendinous inscription arm (P < 0.05). Changing knee flexion angle from 0 to 90° was accompanied by a longer tendinous inscription arm and an increased apex angle (P < 0.05). Long arm length and muscle thickness significantly increased from the shortest (hip 0° - knee 90°) to the longest muscle lengths (hip 0° - knee 90°). Males had a significantly higher surface, apex, and deep angle and a lower normalized tendinous inscription long arm than females (P < 0.05). These results indicate that the effect of the tendinous inscription (if any) on semitendinous muscle function depends on hip and knee joint angle while it may be gender dependent., (Copyright © 2013 Wiley Periodicals, Inc.)

The semitendinosus (ST) consists of a long distal tendon and it is divided in two parts by a tendinous inscription (TI). The purpose of this study was to quantify strain and elongation of the TI and the distal tendon of ST. Fourteen subjects performed ramp isometric contractions of the knee flexors at 0°, 45° and 90° of knee flexion. Two ultrasound probes were used to visualize the displacement of the distal tendon and selected points across the TI and aponeuroses. Three-way analysis of variance designs indicated that: (a) strain and elongation of the ST distal muscle-tendon junction were higher than that of the aponeurosis - TI junction points (p < 0.05) (b) the long arm of the TI reach strain of 49.86 ± 7.77% which was significantly (p < 0.05) higher than that displayed by the short arm (28.35 ± 0.59%) (c) Strain of tendinous and TI-aponeuroses segments significantly increased from 90° to 0° of knee flexion while the inverse was observed for the TI arm length (p < 0.05). (d) Tendon strain was significantly higher than strain of the TI-aponeuroses segments at 45° and 90° of knee flexion while the opposite was observed at 0° of knee flexion. The arrangement of TI along ST length results in differential local strains, indicating that the mechanical properties of the ST muscle are affected by tendon, aponeuroses and tendinous inscription interactions., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Goal scoring represents the ultimate purpose of soccer and this is achieved when players perform accurate kicks. The purpose of the present study was to compare accurate and inaccurate soccer kicks aiming to top and bottom targets. Twenty-one soccer players performed consecutive kicks against top and bottom targets (0.5m(2)) placed in the center of the goal. The kicking trials were categorized as accurate or inaccurate. The activation of tibialis anterior (TA), rectus femoris (RF), biceps femoris (BF) and gastrocnemius muscle (GAS) of the swinging leg and the ground reaction forces (GRFs) of the support leg were analyzed. The GRFs did not differ between kicking conditions (P > 0.05). There was significantly higher TA and BF and lower GAS EMG activity during accurate kicks to the top target (P < 0.05) compared with inaccurate kicks. Furthermore, there was a significantly lower TA and RF activation during accurate kicks against the bottom target (P < 0.05) compared with inaccurate kicks. Enhancing muscle activation of the TA and BF and reducing GAS activation may assist players to kick accurately against top targets. In contrast, players who display higher TA and RF activation may be less accurate against a bottom target. It was concluded that muscle activation of the kicking leg represents a significant mechanism which largely contributes to soccer kick accuracy., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The purpose of this study was to understand the detailed architectural properties of the human hamstring muscles. The long (BFlh) and short (BFsh) head of biceps femoris, semimembranosus (SM) and semitendinosus (ST) muscles were dissected and removed from their origins in eight cadaveric specimens (age 67.8±4.3 years). Mean fiber length, sarcomere length, physiological cross-section area and pennation angle were measured. These data were then used to calculate a similarity index (δ) between pairs of muscles. The results indicated moderate similarity between BFlh and BFsh (δ=0.54) and between BFlh and SM (δ=0.35). In contrast, similarity was low between SM and ST (δ=0.98) and between BFlh and SM (δ=1.17). The fascicle length/muscle length ratio was higher for the ST (0.58) and BFsh (0.50) compared with the BFlh (0.27) and SM (0.22). There were, however, high inter-correlations between individual muscle architecture values, especially for muscle thickness and fascicle length data sets. Prediction of the whole hamstring architecture was achieved by combining data from all four muscles. These data show different designs of the hamstring muscles, especially between the SM and ST (medial) and BFlh and BFsh (lateral) muscles. Modeling the hamstrings as one muscle group by assuming uniform inter-muscular architecture yields less accurate representation of human hamstring muscle function., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The aim of this study was to examine the effects of incline plyometrics training on muscle activation and architecture during vertical jumping and maximum strength. Twenty male participants were divided in two training groups which followed after a 4 week training program. The incline plyometrics group (n = 10) trained by performing consecutive jumps on an inclined surface (15°) while the plane plyometrics (PP) group (n = 10) performed the same jumps on a plane surface. Both groups trained four times per week and performed 8 sets of 10 jumps in each session. Subjects performed squat jumps, counter movement jumps and drop jumps (DJ) prior to and immediately after the training period, while the electromyographic activity of the medial gastrocnemius (MGAS) and tibialis anterior muscles and the architecture of MGAS were recorded. Maximal isokinetic and isometric strength of the plantar flexors were performed. Analysis of variance showed that only the IP group improved fast DJ height performance by 17.4 and 14.4% (20 and 40 cm, p < 0.05). This was accompanied by a significantly higher MGAS activity during the propulsion phase (24% from 20 cm and 50% from 40 cm, p < 0.05) of the DJ and a longer working fascicle length (5.08%, p < 0.05) compared with the PP group. There were no significant changes in isokinetic and isometric strength of the plantar flexors after training for both groups. The increase of jumping performance, after incline plyometrics should be taken into consideration by coaches, when they apply hopping exercise to improve explosiveness of the plantar flexors.

The aim of this study was to examine the tendinous inscription (TI) of the human semitendinosus (ST) muscle using dissection (cadavers) and ultrasound (in vivo). Ultrasonography (US) scans were taken in 18 young males at rest and at maximum voluntary contraction (MVC). Further, the ST was dissected and removed from its origins in 10 cadaveric specimens (5 cadavers). The cadaveric long arm of the TI was 6.67 ± 0.64 cm (6.45 ± 1.21 in US) while the shorter arm was 2.39 ± 0.38 cm (1.99 ± 0.75 in US). The angle formed by the two TI arms ranged from 53.19 (US) to 56.05° (cadavers) while more superficial fascicles intersected the inscription at significantly higher angles (range 31.98 ± 6.15 to 34.69 ± 7.71°) compared with deeper fascicles (p < 0.05). Fascicle length did not differ between compartments, but it was significantly smaller in superficial compared with deeper layers (p < 0.05). With the exception of the angle between the TI arm and the deep aponeurosis, all measured angles as well as the length of the long arm of the TI increased significantly from rest to MVC (p < 0.05). The role of the TI probably lies in the local interconnections with the fascicles of either compartment, which upon contraction is such that the ST muscle contracts as one muscle. However, the TI arm morphology changes from rest to MVC, indicating a nonuniform displacement of the TI, mainly between the superficial and deeper layers of the muscle., (Copyright © 2011 S. Karger AG, Basel.)

Kannas, TM, Kellis, E, and Amiridis, IG. Biomechanical differences between incline and plane hopping. J Strength Cond Res 25(12): 3334-3341, 2011-The need for the generation of higher joint power output during performance of dynamic activities led us to investigate the force-length relationship of the plantar flexors during consecutive stretch-shortening cycles of hopping. The hypothesis of this study was that hopping (consecutive jumps with the knee as straight as possible) on an inclined (15°) surface might lead to a better jumping performance compared with hopping on a plane surface (0°). Twelve active men performed 3 sets of 10 consecutive hops on both an incline and plane surface. Ground reaction forces; ankle and knee joint kinematics; electromyographic (EMG) activity from the medial gastrocnemius (MG), soleus (Sol) and tibialis anterior (TA); and architectural data from the MG were recorded. The results showed that participants jumped significantly higher (p < 0.05) when hopping on an inclined surface (30.32 ± 8.18 cm) compared with hopping on a plane surface (27.52 ± 4.97 cm). No differences in temporal characteristics between the 2 types of jumps were observed. Incline hopping induced significantly greater ankle dorsiflexion and knee extension at takeoff compared with plane hopping (p < 0.05). The fascicle length of the MG was greater at initial contact with the ground during incline hopping (p < 0.05). Moreover, the EMG activities of Sol and TA during the propulsion phase were significantly higher during incline compared with that during plane hopping (p < 0.05). It does not seem unreasonable to suggest that, if the aim of hopping plyometrics is to improve plantar flexor explosivity, incline hopping might be a more effective exercise than hopping on a plane surface.

The purpose of this study was to investigate the neuromuscular efficiency of women with knee osteoarthritis (OA) when performing a sit-to-stand movement and during maximum strength efforts. Twelve women with unilateral knee OA (age 60.33±6.66 years, height 1.61±0.05 m, mass 77.08±9.2 kg) and 11 controls (age 56.54±5.46 years, height 1.64±0.05 m, mass 77.36±13.34 kg) participated in this study. Subjects performed a sit-to-stand movement from a chair while position of center of pressure and knee angular speed were recorded. Furthermore, maximal isokinetic knee extension and flexion strength at 60°/s, 120°/s and 150°/s was measured. Surface, electromyography (EMG) from the biceps femoris (BF), vastus lateralis (VL) and vastus medialis (VM) was recorded during all tests. Analysis of variance (ANOVA) showed that during the sit-to-stand OA group demonstrated significantly lower knee angular speed (44.49±9.61°/s vs. 71.68±19.86°/s), a more posterior position of the center of pressure (39.20±7.02% vs. 41.95±2.49%) and a higher antagonist BF activation (57.13±20.55% vs. 32.01±19.5%) compared with controls (p<0.05). Further, women with knee OA demonstrated a lower Moment-to-EMG ratio than controls in extension and eccentric flexion at 60°/s and 150°/s, while the opposite was found for concentric flexion at 60°/s (p<0.05). Among other factors, the slower performance of the sit-to-stand movement in women with OA is due to a less efficient use of the knee extensor muscles (less force per unit of EMG) and, perhaps, a higher BF antagonist co-activation. This may lead subjects with OA to adopt a different movement strategy compared with controls., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Context: The effects of fatigue on impact loading during running are unclear, with some authors reporting increased impact forces and others reporting decreased forces., Objective: To examine the effects of isokinetic fatigue on muscle cocontraction ratios about the knee and ankle during running., Design: Cross-sectional study., Setting: Neuromechanics laboratory., Patients or Other Participants: Female middle-distance runners (age  =  21.3 ± 1.93 years) with at least 5 years of training experience., Intervention(s): Participants ran on the treadmill at 3.61 m/s before and immediately after the fatigue protocol, which consisted of consecutive, concentric knee extension-flexion at 120°/s until they could no longer produce 30% of the maximum knee-extension moment achieved in the familiarization session for 3 consecutive repetitions., Main Outcome Measure(s): Electromyographic (EMG) amplitude of the vastus medialis (VM), biceps femoris (BF), gastrocnemius (GAS), and tibialis anterior (TA) was recorded using surface electrodes. Agonist∶antagonist EMG ratios for the knee (VM∶BF) and ankle (GAS∶TA) were calculated for the preactivation (PR), initial loading response (LR(1)), and late loading response (LR(2)) phases of running. Hip-, knee-, and ankle-joint angular displacements at initial foot contact were obtained from 3-dimensional kinematic tracings., Results: Fatigue did not alter the VM∶BF EMG ratio during the PR phase (P > .05), but it increased the ratio during the LR(1) phase (P < .05). The GAS∶TA EMG ratio increased during the LR(1) phase after fatigue (P < .05) but remained unchanged during the PR and LR(2) phrases (P > .05)., Conclusions: The increased agonist EMG activation, coupled with reduced antagonist EMG activation after impact, indicates that the acute decrease in muscle strength capacity of the knee extensors and flexors results in altered muscle-activation patterns about the knee and ankle before and after foot impact.

The purpose of this study was to examine whether muscle architecture of the long head of biceps femoris (BF) and semitendinosus (ST) muscles varies along their length. The ST and BF muscles were dissected and removed from their origins in eight cadaveric specimens (age range 67.8-73.4 years). One-way analysis of variance designs were used to compare fascicle length (FL), pennation angle (PA) and muscle thickness (MT) between proximal, mid-belly and distal positions. Tendon and muscle length properties were also quantified. For the BF muscle, one-way analysis of variance tests showed a higher PA (23.96±3.82°) and FL (7.12±0.48 cm) proximally than distal positions (PA=17.78±1.95° and FL=6.35±0.89 cm, respectively). For the ST, there was a significantly (p<0.05) lower PA (8.81±1.22°) and FL (13.10±1.54 cm) proximally than distally (PA=14.69±1.09° and FL=15.49±2.30 cm, respectively). Muscle thickness significantly increased from distal to more proximal positions (p<0.05). These data suggest that the ST and BF architecture is not uniform and that measurement of these parameters largely depends on the measurement site. Modeling these muscles by assuming a uniform architecture along muscle length may yield less accurate representation of human hamstring muscle function., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

The aim of this study was to examine the differences in muscle architecture during isometric tests between children and adults. Eight boys (age= 11.2 +/- 0.26 years) and eight men (age= 22.3 +/- 2.01 years) performed plantar flexion isometric efforts at angles of -15 degrees, 0 degrees, 15 degrees at 0%, 40%, 60%, 80% of MVC. Analysis of variance tests indicated that adults showed greater fascicle length from rest to 80% of MVC (p < .05), greater pennation angle at 80% and 100% of MVC (p < .05) and greater aponeuroses displacement at levels of effort greater than 60% of MVC (p < .05). These differences observed in MG would appear to favor better utilization of the force-length and the force-velocity relationships, of the muscle in adults compared with children.

The purpose of this study was to compare the architectural parameters of the long head of biceps femoris (BFlh) and semitendinosus (ST) muscles by comparing measurements from ultrasound (US) with those obtained from direct dissection. The BFlh and ST architectures were examined bilaterally in 6 legs from 3 male cadavers. The fascicle length, pennation angle, muscle thickness and muscle and tendon length were obtained from direct measurement and US scans along each muscle. Intraclass correlation coefficients between the two methods ranged from 0.905 to 0.913 for the BFlh variables and from 0.774 to 0.974 for the ST parameters. Compared with the direct measurements, the US method showed a mean typical error of 0.09-0.14 cm for muscle thickness, 1.01-1.31 degrees for the pennation angle, 0.92-1.71 cm for fascicle length and muscle-tendon length measurements. The US method is a valid alternative tool for assessing basic architectural parameters of ST and BFlh components of the hamstring muscles.

Study Design: Controlled laboratory study., Objectives: To compare the changes in lower limb sagittal kinematics in running after a knee fatigue protocol with those observed after an ankle fatigue protocol., Background: Impaired force-generating ability of specific muscles may affect running mechanics, with negative implications for injury occurrence and performance. Identifying the strategies used to compensate for fatigue of selected muscles may assist in the design of more effective exercise programs for injury prevention and performance enhancement in running., Methods and Measures: Sagittal plane kinematic data and the electromyographic (EMG) signal of the vastus medialis (VM), gastrocnemius (GAS), and biceps femoris (BF) muscles were collected from 15 females running at 3.61 m/s on a treadmill prior to, and following, an isokinetic knee extension/flexion and, on a separate day, an ankle plantar flexion/dorsiflexion fatigue protocol performed at 120 degrees/s., Results: Ankle muscle fatigue caused decreased ankle dorsiflexion, while knee fatigue caused increased knee flexion at initial contact (P<.05). Both protocols increased knee flexion angle at toe-off, as well as the amplitude of GAS and VM EMG signal, with the hip more extended after knee fatigue and the ankle more plantar flexed after ankle muscle fatigue. Ankle muscle fatigue caused a significant increase in hip extensionand ankle plantar flexion angular velocity, and a decline of BF EMG signal during the swing phase (P<.05). Knee muscle fatigue decreased hip and knee flexion angular velocity and increased BF EMG signal during the swing phase (P<.05)., Conclusion: Localized muscle fatigue effects on sagittal kinematics differed between the 2 protocols. However, the strategy used to compensate for fatigue was similar for both protocols: to protect the joints at initial impact and to prevent impairments in performance during toeoff and swing phase.

Background: Agonist and antagonist co-activation plays an important role for stabilizing the knee joint, especially after fatigue. However, whether selective fatigue of agonists or antagonist muscles would cause different changes in muscle activation patterns is unknown., Hypothesis: Knee extension fatigue would have a higher influence on landing biomechanics compared with a knee flexion protocol., Study Design: Repeated-measures design., Methods: Twenty healthy subjects (10 males and 10 females) performed two sets of repeated maximal isokinetic concentric efforts of the knee extensors (KE) at 120 degrees s(-1) until they could no longer consistently produce 30% of maximum torque. On a separate day, a similar knee flexion (KF) fatigue protocol was also performed. Single leg landings from 30cm drop height were performed before, in the middle and after the end of the fatigue test. The mean normalized electromyographic (EMG) signal of the vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF) and gastrocnemius (GAS) at selected landing phases were determined before, during and after fatigue. Quadriceps:hamstrings (Q:H) EMG ratio as well as sagittal hip and knee angles and vertical ground reaction force (GRF) were also recorded., Results: Two-way analysis of variance designs showed that KE fatigue resulted in significantly lower GRF and higher knee flexion angles at initial contact while maximum hip and knee flexion also increased (p<0.05). This was accompanied by a significant decline of BF EMG, unaltered EMG of vastii and GAS muscles and increased Q:H ratio. In contrast, KF fatigue had no effects on vGRFs but it was accompanied by increased activation of VM, BF and GAS while the Q:H increased during before landing and decreased after impact., Conclusion: Fatigue responses during landing are highly dependent on the muscle which is fatigued.

Appropriate reliability is a necessary condition for the use of surface EMG for evaluation of hamstring muscle function in cases of knee joint pathologies or ligament injuries. The aim of the study was to investigate the test-retest reliability of power spectrum and amplitude of surface electromyographic (EMG) measurements of semitendinosus (ST) and biceps femoris (BF) during ramp isometric contractions. Eleven males performed maximum isometric contractions (MVC) of the knee flexors in two sessions, a week apart with simultaneous recording of surface EMG of the BF and ST. Intra class correlation (ICC) and standard error measurements (SEM) were applied to assess test-retest reliability of the averaged EMG (aEMG) and the median frequency (MF) over 10 levels of force, from 0% to 100% of the maximum. The ICC values ranged from 0.38 to 0.96 for the aEMG with SEM values reaching 11.37% of MVC. For the MF, the ICCs ranged from 0.44 to 0.98 (SEM range 4.49-18.19Hz). In our set up, ramp contractions can be used to examine hamstring EMG patterns with acceptable reliability.

The purpose of this study was to examine whether prediction of antagonist moment (M(flx)) of the hamstrings using clinically applicable models depends on the muscles examined. Nine healthy males performed maximal isometric knee extension and flexion contractions at 0 degrees , 45 degrees and 90 degrees angles. Calibration knee flexion efforts were also performed at different levels of intensity. The resulting electromyographic (EMG) - moment curves were fitted using polynomial equations which were then used to estimate M(flx) from the antagonist EMG. Analysis of variance designs showed that the M(flx) predicted using the biceps femoris EMG was not significantly different compared with those predicted using the semitendinosus EMG data (p>0.05). Further, prediction of M(flx) using the EMG of both muscles or a combination of EMGs and architectural properties reduced estimation error but did not provide significantly different predicted values compared with the simpler single-muscle EMG - moment models (p<0.05). It appears that M(flx) prediction using the present EMG - moment model is not muscle specific. Prediction using models which combine EMG data and anatomic parameters of the hamstring muscle components yielded more accurate estimates and therefore their use to examine co-contraction levels is recommended.

Context: Evaluating moment balance around the knee helps athletic trainers set appropriate targets for injury prevention and rehabilitation programs., Objective: To examine the knee flexor (KF) to knee extensor (KE) moment ratios using the moments when each muscle group acts as an agonist and using the moments when the KE acts as an agonist and the KF acts as an antagonist., Design: Cross-sectional., Setting: University research laboratory., Patients or Other Participants: Seventeen pubertal males (age = 13.7 +/- 0.2 years, height = 1.61 +/- 0.04 m, mass = 51.3 +/- 2.7 kg)., Intervention(s): The subjects performed maximal isokinetic concentric KE (KE(CON)) and eccentric KF (KF(ECC)) efforts and performed eccentric KE (KE(ECC)) and concentric KF efforts at 60 degrees /s and 180 degrees /s while we recorded the bipolar surface electromyographic (EMG) signal of the involved muscles. The KF antagonist moment was estimated from EMG-moment relationships determined during calibration KF efforts. Maximal moments were used to estimate the KF:KE ratios, and EMG-based moments were used to estimate the antagonist to agonist ratios., Main Outcome Measure(s): We calculated KF:KE ratios for various angular positions, velocities, and movement directions., Results: The KF(ECC):KE(CON) ratio significantly increased as the knee extended (P < .05) at increased angular velocity (P < .05), reaching a value of 3.14 +/- 1.95 at full extension. The estimated knee flexor antagonist to knee extensor agonist ratio also increased near full extension (0.32 +/- 0.21)., Conclusions: Although the KFs have a higher capacity to produce maximal moment near knee extension and at increased angular velocities, knee joint movement is achieved through a balanced coactivation of the 2 antagonistic muscle groups to maintain joint stability and movement efficiency. The combined use of moment and EMG data can provide additional useful information regarding muscle balance around the knee.

Although the use of practicable EMG-moment models for knee joint moment prediction appears promising, the repeatability of the estimated forces remains unclear. The purpose of this study was to apply an EMG-moment model to predict the antagonist moment of the knee flexors (Mflx) during maximal isometric knee extension efforts. Nine healthy males performed maximal isometric knee extension and flexion contractions at 0 degrees , 45 degrees and 90 degrees angles with recordings of the net moment and EMG of thigh muscles. Calibration knee flexion efforts were performed at different levels of intensity and the resulting EMG-moment curves were fitted using second-order polynomials. The polynomials were then used to predict Mflx. This procedure was repeated a week after. The results indicated non-significant differences in test-retest Mflx. Intraclass correlation coefficients ranged from 0.852 to 0.912 indicating high test-retest reliability of the estimated Mflx. For isometric contractions, the present model is suitable as a method to estimate antagonist muscle moments.

This study evaluated and compared the effectiveness of an aerobics-calisthenics (A-CAL) and an aerobics/weight training (A-WT) programs on lower limb strength and body fat (%). Thirty-five adult women (age 42.1 +/- 5.2 years) were randomly assigned to A-CAL (n = 14), A-WT (n = 14), or a control group (n = 7). The A-CAL and A-WT trained 3 days per week for 10 weeks. Maximal bilateral isometric and isokinetic knee extension (KEXT) and flexion (KFLEX) torque, squat jump (SJ), and body fat (%) were measured before and immediately after training. The results revealed nonsignificant differences between A-CAL and A-WT (p > 0.05). Both A-CAL and A-WT improved SJ (p < 0.001). A-WT increased isometric torque of KEXT and KFLEX (p < 0.05), isokinetic torque of KFLEX (p < 0.05), and decreased body fat (%) (p < 0.05) when compared with controls. In summary, the application of a 10-week light-weight training program improved selected strength parameters of healthy women, compared with controls, but the effectiveness of the calisthenics exercises as an independent form of strength training is dubious.

The examination of the moment exerted by the hamstrings during maximum isokinetic knee extensor tests is useful when comparing isokinetic strength and muscle activity patterns between children and adults. The purpose of this study was to examine the effect of antagonist moment of the hamstrings on the isokinetic moment of the knee extensors in pubertal children and to determine whether this effect is altered following a fatigue task. Eighteen healthy pubertal males [age 14.3 (0.5) years] performed 34 maximal isokinetic concentric efforts of the knee extensors at 60 degrees.s(-1). The average moment of force and electromyographic (aEMG) signal of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) at 11-30 degrees, 31-50 degrees, 51-70 degrees and 71-90 degrees of knee flexion were calculated for each repetition. The hamstrings antagonist moment was determined before and after the fatigue task by fitting the aEMG-moment relationship at different levels of muscle effort using second-degree polynomials. The percentage contribution of the antagonist moment to the resultant joint moment ranged from 7.1 % to 60.4 % throughout the range of motion, with the highest percentage observed close to full knee extension (11-30 degrees). The antagonist effect was significantly greater during concentric tests of the knee extensors compared to the corresponding eccentric tests ( p<0.05). Following the fatigue test, there was an overall decline of the resultant joint moment, but no changes in the predicted hamstrings moment were observed. These results indicate that when testing maximal knee extensor isokinetic strength in pubertal boys, activity of the hamstrings implies a reduction of the net extensor moment as compared to the isolated capacity of the knee extensors. However, this antagonist effect is not altered following the performance of an isokinetic fatigue knee extension task.