The few published studies of cutaneous reflex modulation during cyclic movement have focused mainly on EMG responses (Zehr et al. 2001). The purpose of the present investigation was to extend muscle activation studies and to involve functional measures of lower limb kinematic responses during steady-state bicycling to brief electrical stimulation of the sural nerve, which innervates the dorsolateral foot surfaces.

Seven healthy male subjects (26.7 ± 7.2 years, 82 ± 13.7 kg, 182 ± 4.8 cm) gave informed consent and participated in the study approved by the local ethics committee. Subjects performed cycle ergometry (90-100 W; 60 r.p.m.). EMG was recorded from the ankle flexor tibialis anterior (TA) and ankle extensor soleus (SOL) using surface bipolar electrodes ipsilateral to the stimulation. Electrogoniometers positioned over the Achilles tendon recorded ankle angular position (dorsi/plantar flexion and eversion/inversion). Sural nerve was stimulated at an intensity equal to 75 % of the predetermined pain threshold via electrodes placed inferior to the lateral malleolus. The stimulation trains (5 X 1.0 ms pulses at 200 Hz) were delivered pseudorandomly at eight equidistant points in the crank cycle. Two stimuli were separated by at least two non-stimulated cycles. Net reflex responses were obtained by averaging 30 crank cycles for each of nine conditions (8 with and 1 without stimulation, control) followed by average control subtraction from the corresponding stimulation data.

The main observations of this study were: (1) cutaneous reflexes were dependent upon the point of delivery within a crank cycle and were predominantly inhibitory for both TA and SOL (Fig. 1) in contrast to mainly facilitatory responses observed during static contractions and walking (e.g. Zehr et al. 1998); (2) ankle kinematic responses were suppressed by sural stimulation as a result of TA and SOL response inhibition; (3) in both muscles the reflex peak occurred at latencies at or above 100 ms, suggesting that transcortical reflex pathways may contribute to reflex burst generation in bicycling as during human walking (Nielsen & Sinkjaer, 2002).

We conclude that cutaneous reflexes are an important part of the sensory feedback used by the central drive to modulate muscle activation during bicycling as a protective mechanism against sudden ankle movement disturbance.