Mutability of Bifunctional Thigh Muscle Activity in Pedaling due to Contralateral Leg Force Generation

Locomotion requires uninterrupted transitions between limb extension and flexion. The role of contralateral sensorimotor signals in executing smooth transitions is little understood even though their participation is crucial to bipedal walking. However, elucidating neural interlimb coordinating mechanisms in human walking is difficult because changes to contralateral sensorimotor activity also affect the ipsilateral mechanics. Pedaling, conversely, is ideal for studying bilateral coordination because ipsilateral mechanics can be independently controlled. In pedaling, the anterior and posterior bifunctional thigh muscles develop needed anterior and posterior crank forces, respectively, to dominate the flexion-to-extension and extension-to-flexion transitions. We hypothesized that contralateral sensorimotor activity substantially contributes to the appropriate activation of these bifunctional muscles during the limb transitions. Bilateral pedal forces and surface electromyograms (EMGs) from four thigh muscles were collected from 15 subjects who pedaled with their right leg against a right-crank servomotor, which emulated the mechanical load experienced in conventional two-legged coupled-crank pedaling. In one pedaling session, the contralateral (left) leg pseudo-pedaled (i.e., EMG activity and pedal forces were pedaling-like, but pedal force was not allowed to affect crank rotation). In other sessions, the mechanically decoupled contralateral leg was first relaxed and then produced rhythmic isometric force trajectories during either leg flexion or one of the two limb transitions of the pedaling leg. With contralateral force production in the extension-to-flexion transition (predominantly by the hamstrings), rectus femoris activity and work output increased in the pedaling leg during its flexion-to-extension transition, which occurs simultaneously with contralateral extension-to-flexion in conventional pedaling. Similarly, with contralateral force production in the other transition (i.e., flexion-to-extension; predominantly by rectus femoris), hamstrings activity and work output increased in the pedaling leg during its extension-to-flexion transition. Therefore rhythmic isometric force generation in the contralateral leg supported the ongoing bifunctional muscle activity and resulting work output in the pedaling leg. The results suggest that neural interlimb coordinating mechanisms fine-tune bifunctional muscle activity in rhythmic lower-limb tasks to ensure limb flexion/extension transitions are executed successfully.

An intracellular study of muscle primary afferents during fictive locomotion in the cat

1. Presynaptic activity of identified primary afferents from flexor, extensor, and bifunctional hindlimb muscles was studied with intra-axonal recordings during fictive locomotion. Fictive locomotion appeared spontaneously in decorticate cats (n = 9), with stimulation of the mesencephalic locomotor region (n = 4), and in spinal cats injected with clonidine or nialamide and L-DOPA (n = 4). Representative flexor and extensor muscle nerves, recorded to monitor the locomotor pattern and dorsal rootlets of the sixth and seventh lumbar segments, were recorded simultaneously to monitor dorsal root potentials (DRPs). 2. From responses to muscle stretches and, in some instances, twitch contractions of the parent muscle, 75% of the single units examined were putatively identified as spindle afferents (40/53). On the basis of conduction velocity and stimulation threshold, 73% of these were further classified as group I fibers (29/40), the rest as group II fibers. 3. All units (n = 53 with resting potential more negative than -45 mV) showed fluctuations of their membrane potential (up to 1.5 mV) at the rhythm of the fictive locomotion. Subsequent averaging of these fluctuations over several cycles revealed that 89% of all units displayed a predominant wave of depolarization during the flexor phase, followed by a trough of repolarization. In 79% of the units, there was also a second, usually smaller, depolarization during the extensor phase. The relative size of each wave of depolarization could vary with different episodes of fictive locomotion in the same unit and among various afferents from the same muscle in the same experiment. 4. The firing frequency of some afferents from the ankle flexor tibialis anterior (5/16) and the bifunctional muscle posterior biceps-semitendinosus (4/15) was phasically modulated along the fictive step cycle. The maximum frequency always occurred during the flexor phase, i.e., during the largest depolarization of the unit. Because of the absence of phasic sensory input in the curarized animal, we assume that the phasic discharges were generated within the spinal cord and antidromically propagated. Phasic firing was never encountered in afferents from extensor muscles such as triceps surae (0/15) and vastus lateralis (0/4). 5. The results demonstrate that the pattern of rhythmic depolarization accompanying fictive locomotion is similar for the majority of flexor, extensor, and bifunctional group I (and possibly group II) muscle spindle primary afferents. They further indicate that there is a specific phasic modulation of antidromic firing for some flexor and bifunctional muscle spindle afferents.(ABSTRACT TRUNCATED AT 400 WORDS)