Effect of arm or leg cycling on motor-evoked potentials and H-reflexes in static limbs

Modulation of the excitability of the corticospinal tract and spinal reflex in static upper and lower limbs was investigated during arm or leg cycling. The excitability of the corticospinal tract was examined with motor-evoked potentials (MEPs) following transcranial magnetic stimulation (TMS). H-reflexes were evoked by electrical stimulation of peripheral nerves in the upper and lower limbs. MEPs and H-reflexes were recorded from the soleus while the subject performed arm cycling and the soleus was at rest. In addition, MEPs and H-reflexes were recorded from the flexor carpi radialis (FCR) during leg cycling while the FCR was at rest. MEPs and H-reflexes were also evoked without arm or leg cycling as a control. TMS or electrical stimulation was delivered at 4 different pedal positions. The subjects performed arm or leg cycling at 30 and 60 rpm. The amplitudes of MEP in the soleus significantly increased during arm cycling compared to the control. In contrast, H-reflexes in the soleus significantly decreased during arm cycling compared to control values. The same results were obtained in FCR during leg cycling. MEPs and H-reflexes were not modulated in a phase-dependent manner during either arm or leg cycling. The degree of modulations in MEP and H-reflex amplitudes depended on the cadence of arm and leg cycling. These findings suggest that a differential regulation of spinal and supraspinal excitability in the static limb was induced by arm and leg cycling. The corticospinal tract and the reflex arc independently would be responsible for coordination between the upper and lower limbs.