HUMAN FLEXOR REFLEX MODULATION DURING CYCLING

1. Human flexor reflex (HFR) responses were elicited during ergometer cycling in neurologically intact humans with the objective of understa nding the influence of lower limb muscle activity on phase-dependent r eflex modulation during movement. The experimental setup permitted con trol over background muscle activity and stimulus intensity without si gnificantly interfering with the cycling motion. 2. All experiments in volved cycling on an ergometer at a set rate and workload. A 333-Hz, 1 5-ms pulse train of electrical stimulation was randomly delivered to t he skin over the tibial nerve at the ankle at selected lower limb posi tions. In the first group of experiments, subjects were stimulated at six cycling phases while pedaling with normal, phasic ankle activity ( free-form cycling). The second and third group of experiments involved stimulation under static limb positioning conditions and during activ e pedaling while subjects were asked to maintain a consistent backgrou nd level of isolated tibialis anterior (TA) or soleus (SOL) electromyo graphic (EMG) activity. 3. Control criteria were established to assure similar isolated muscle EMG levels and sensory stimulation intensitie s throughout the experiments. With the aid of the application of a low er extremity brace and visual EMG feedback, SOL and TA activity were c onfined by the subject to a narrow range during the task of cycling. S timulus consistency was achieved through maintenance of flexor halluci s brevis M-waves to within an envelope encompassing the mean value +/- 5% of the maximum M-wave amplitude in all experimental conditions. 4. When the subject's limb was statically positioned, the HFR responses i n the SOL muscle showed no significant changes in pattern when compare d at various limb positions. During cycling with consistent SOL activi ty, a response waveform pattern of early-latency-long-duration depress ion was followed by a later-latency facilitation response in all posit ions except the initial power phase. The initial power phase was chara cterized by an additional early-latency facilitation in all but one su bject. 5. In the TA muscle response, no change in onset latency (57.5 +/- 0.8 ms, mean +/- SD), waveform pattern, or response amplitude (7.9 +/- 1.1% maximal voluntary contraction, mean +/- SD) was observed dur ing static limb positioning. Significant increases in response amplitu de (P < 0.05) coupled with significant increases (9.2 ms, P < 0.05) in onset latency were seen during the transition from the recovery phase to the power phase during cycling. In addition, there was no correlat ion between the prestimulation baseline level and the onset latency du ring controlled TA cycling activity conditions. 6. The results from th is investigation demonstrate that modulation of the waveform pattern ( in SOL), onset latency (in TA), and response amplitude (in TA) of the ipsilateral HFR during cycling can occur independent of concurrent mus cle activity. On the basis of the assumption that mean EMG levels in a muscle represent the motoneuron excitability level of that muscle's m otoneuron pool, it is proposed that these data support the hypothesis that interneuronal mechanisms involved with rhythmic modulation, eithe r pre- or postsynaptically, of reflex pathways are likely responsible for the observed phase-dependent modulation. Functionally, these mecha nisms may have evolved to assure forward progression during locomotive movements.