The present study investigated an effective method of eliciting medium and long latency cutaneous reflexes in normal human subjects. The effect of changes in stimulus conditions (number of pulse train, duration of electrical pulse and inter-stimulus interval) on cutaneous reflexes in the first dorsal interosseous muscle (FDI) following non-noxious electrical stimulation to the hand digits (digit 1 ; D1, digit 2 ; D2 and digit 5 ; D5) were investigated in seven healthy volunteers. Cutaneous reflexes were elicited while the subjects performed isolated isometric contraction of FDI (D2 abduction). Under all experimental conditions, the level of muscle contraction was set at 10% of the maximal EMG amplitude, which was determined during maximal voluntary contraction. Intensity of the electrical stimulation was set at 2.0 times the perceptual threshold under all experimental conditions.Although the amplitude of E2 (excitatory response, peak latency ∼60∼90 ms) was independent of the number of pulses (1, 2, 3, and 5 pulses, pulse frequency at 333 Hz), that of I1 (inhibitory response, ∼45∼60 ms), I2 (inhibitory response, ∼90∼120 ms) and E3 (excitatory response, ∼120∼180 ms) was significantly increased depending on the number of pulses (p<0.001). Amplitudes of E2 and I2 were significantly affected by the digit stimulated (p<0.01). For all four components of the cutaneous reflexes, there were no significant differences in magnitude even by alternating both the inter-stimulus interval (fixed at 1, 2 and 3 Hz and random between at 0.7 and 2 Hz) and the duration (0.1, 0.5 and 1 ms) of the electrical stimulation.These findings suggest that the susceptibility of responsible interneurons impinging on each reflex pathway to temporal summation of the test impulse differs depending on the digit stimulated. It is also likely that almost the same population of the cutaneous afferent fibers were activated by test stimulation with different durations as far as the same stimulus intensity was utilized. As a practical application, double or more pulses up to 3 Hz without causing pain is recommended to effectively evoke medium and long latency cutaneous reflexes in FDI, which would reduce possible effects arising from fatigue.

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.