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The dexterity of hands and fingers is related to the strength of control by cortico-motoneuronal connections which exclusively exist in primates. The cortical command is associated with a task-specific, rapid proprioceptive adaptation of forces applied by hands and fingers to an object. This neural control differs between "power grip" movements (e.g., reach and grasp of a cup) where hand and fingers act as a unity and "precision grip" movements (e.g., picking up a raspberry) where fingers move independently from the hand. In motor tasks requiring hands and fingers of both sides a "neural coupling" (reflected in bilateral reflex responses to unilateral stimulations) coordinates power grip movements (e.g., opening a bottle). In contrast, during bilateral precision movements, such as playing piano, the fingers of both hands move independently, due to a direct cortico-motoneuronal control, while the hands are coupled (e.g., to maintain the rhythm between the two sides). While most studies on prehension concern unilateral hand movements, many activities of daily life are tackled by bilateral power grips where a neural coupling serves for an automatic movement performance. In primates this mode of motor control is supplemented by a system that enables the uni- or bilateral performance of skilled individual finger movements.

The aim of this study was to explore the neural basis of adaptation, memory and skill transfer during human stepping over obstacles. Whilst walking on a treadmill, subjects had to perform uni- and bilateral obstacle steps. Acoustic feedback information about foot clearance was provided. Non-noxious electrical stimuli were applied to the right tibial nerve during the mid-stance phase of the right leg, i.e. 'prior' to the right or 'during' the left leg swing over the obstacle. The electromyogram (EMG) responses evoked by these stimuli in arm and leg muscles are known to reflect the neural coordination during normal and obstacle steps. The leading and trailing legs rapidly adapted foot clearance during obstacle steps with small further changes when the same obstacle condition was repeated. This adaptation was associated with a corresponding decrease in arm and leg muscle reflex EMG responses. Arm (but not leg) muscle EMG responses were greater when the stimulus was applied 'during' obstacle crossing by the left leg leading compared with stimulation 'prior' to right leg swing over the obstacle. A corresponding difference existed in arm muscle background EMG. The results indicate that, firstly, the somatosensory information gained by the performance and adaptation of uni- and bilateral obstacle stepping becomes transferred to the trailing leg in a context-specific manner. Secondly, EMG activity in arm and leg muscles parallels biomechanical adaptation of foot clearance. Thirdly, a consistently high EMG activity in the arm muscles during swing over the obstacle is required for equilibrium control. Thus, such a precision locomotor task is achieved by a context-specific, coordinated activation of arm and leg muscles for performance and equilibrium control that includes adaptation, memory and skill transfer.

This study investigated the interactions of supraspinal with spinal neuronal circuits during obstacle steps by recordings of electroencephalography (EEG), reflex activity and limb muscle electromyography (EMG). Subjects walking with reduced vision on a treadmill were acoustically informed about an approaching obstacle and received feedback about task performance. Only following a task-relevant acoustic signal, spinal reflex responses, evoked by tibial nerve stimulation during mid-stance, were enhanced in proximal arm and leg flexor muscles prior to obstacle compared to normal swing, reflecting the neuronal preparation of the task. During swing over the obstacle, limb muscle EMG activity was greater than in normal swing. Both the preparation and the performance (i.e. ascending movement slope of the obstacle-crossing leg) were associated with an enhanced EEG signal mainly in the prefrontal cortex of the right hemisphere. Adaptational changes in performance, reflex activity and muscle activation during repetitive obstacle stepping were not reflected in the EEG activity, probably due to an insufficient resolution of the EEG. The observations suggest that drive from supraspinal centers initiates and maintains spinal neuronal activity underlying obstacle task preparation and performance.

The aim of this study was to investigate spinal reflex (SR) modulation during the performance and learning of a precision locomotor task. Healthy subjects had to minimize foot clearance when repeatedly stepping on a treadmill over a randomly approaching obstacle. The subjects walked with reduced vision and were informed about the approaching obstacle and task performance by acoustic warning and feedback signals, respectively. SRs were randomly evoked by tibial nerve stimulation (with non-nociceptive and nociceptive stimulus intensity) during the mid-stance phase in both normal and pre-obstacle stepping. Foot clearance and electromyographic activity of the tibialis anterior and biceps femoris muscles of the right leg were analysed. Only if a delay was introduced between warning signal and nerve stimulation, was the SR amplitude in both muscles enhanced prior to obstacle steps compared with normal steps for both stimulus intensities. Thus, the reflex enhancement depended on the subject's awareness of the approaching obstacle. Improved performance was reflected in a decreased foot clearance, but did not correlate with the course of SR amplitude. It is concluded that obstacle stepping is associated with a facilitation of SR pathways, probably by supraspinal drive. This facilitation might provide assistance in safe obstacle stepping, e.g. to compensate quickly if resistance is encountered.

This investigation was designed to study the spontaneous functional recovery of adult rats with incomplete spinal cord injury (SCI) at thoracic level during a time course of 2 weeks. Daily testing sessions included open field locomotor examination and electromyographic (EMG) recordings from a knee extensor (vastus lateralis, VL) and an ankle flexor muscle (tibialis anterior, TA) in the hindlimbs of treadmill walking rats. The BBB score (a locomotor score named after Basso et al., 1995, J. Neurotrauma, 12, 1-21) and various measures from EMG recordings were analysed (i.e. step cycle duration, rhythmicity of limb movements, flexor and extensor burst duration, EMG amplitude, root-mean-square, activity overlap between flexor and extensor muscles and hindlimb coupling). Directly after SCI, a marked drop in locomotor ability occurred in all rats with subsequent partial recovery over 14 days. The recovery was most pronounced during the first week. Significant changes were noted in the recovery of almost all analysed EMG measures. Within the 14 days of recovery, many of these measures approached control levels. Persistent abnormalities included a prolonged flexor burst and increased activity overlap between flexor and extensor muscles. Activity overlap between flexor and extensor muscles might be directly caused by altered descending input or by maladaptation of central pattern generating networks and/or sensory feedback.

We aimed to study the neuronal coordination of lower and upper limb muscles. We therefore evaluated the effect of small leg displacements during gait on leg and arm muscle electromyographic (EMG) activity in walking humans. During walking on a split-belt treadmill (velocity 3.5 km/h), short accelerations or decelerations were randomly applied to the right belt during the mid or end stance phase. Alternatively, trains of electrical stimuli were delivered to the right distal tibial nerve. The EMG activity of the tibialis anterior (TA), gastrocnemius medialis (GM), deltoideus (Delt), triceps (Tric) and biceps brachii (Bic) of both sides was analysed. For comparison, impulses were also applied during standing and sitting. The displacements were followed by specific patterns of right leg and bilateral arm muscle EMG responses. Most arm muscle responses appeared with a short latency (65-80 ms) and were larger in Delt and Tric than in Bic. They were strongest when deceleration impulses were released during mid stance, associated with a right compensatory TA response. A similar response pattern in arm muscles was obtained following tibial nerve stimulation. The arm muscle responses were small or absent when stimuli were applied during standing or sitting. The arm muscle responses correlated more closely with the compensatory TA than with the compensatory GM responses. The amplitude of the responses in most arm muscles correlated closely with the background EMG activity of the respective arm muscle. The observations suggest the existence of a task-dependent, flexible neuronal coupling between lower and upper limb muscles. The stronger impact of leg flexors in this interlimb coordination indicates that the neuronal control of leg flexor and extensor muscles is differentially interconnected during locomotion. The results are compatible with the assumption that the proximal arm muscle responses are associated with the swinging of the arms during gait, as a residual function of quadrupedal locomotion.

The primary sensorimotor cortex of the adult brain is capable of significant reorganization of topographic maps after deafferentation and de-efferentation. Here we show that patients with spinal cord injury exhibit extensive changes in the activation of cortical and subcortical brain areas during hand movements, irrespective of normal (paraplegic) or impaired (tetraplegic patients) hand function. Positron emission tomography ([O-15]-H2O-PET) revealed not only an expansion of the cortical 'hand area' towards the cortical 'leg area', but also an enhanced bilateral activation of the thalamus and cerebellum, The areas of the brain which were activated were qualitatively the same in both paraplegic and tetraplegic patients, but differed quantitatively as a function of the level of their spinal cord injury. We postulate that the changes in brain activation following spinal cord injury may reflect an adaptation of hand movement to a new body reference scheme secondary to a reduced and altered spino-thalamic and spine-cerebellar input.