Proceedings of The Physiological Society

University College London December 2005 (2006) Proc Physiol Soc 1, PC15

Poster Communications

Reciprocal inhibition between knee extensors and knee flexors in humans

Hamm, K; Alexander, C M; Harrison, P J;

1. Department of Physiology, UCL , London, United Kingdom. 2. Department of Physiotherapy, Hammersmith Hospitals NHS Trust, London, United Kingdom.


The reciprocal inhibition evoked from the knee extensors to the knee flexors is particularly potent in the cat and has therefore been the subject of detailed analysis (Hultborn, 1972). More recently reciprocal inhibition has been described between various muscles in humans. Surprisingly, however, the reciprocal inhibition between knee extensors and flexors has not been studied in humans. We have therefore investigated the reciprocal inhibition between quadriceps and hamstrings in 8 healthy subjects. EMG was recorded from vastii medialis and lateralis, and the medial and lateral hamstrings. Either the femoral or sciatic nerves were stimulated using 1 ms pulses at strengths up to 2x motor threshold (MT). The femoral nerve was stimulated at the femoral triangle. The sciatic nerve was stimulated at mid-thigh level between the bellies of biceps femoris and semitendinosus. Data were collected using two approaches. In the first approach, the EMG was rectified and averaged following 100 stimuli to either the femoral or the sciatic nerve. In the second approach, the H reflexes of hamstrings and quadriceps were conditioned with stimuli to the antagonist's nerve at various test-condition intervals. Short latency inhibition, at strengths below 2T, was observed in the medial and lateral hamstrings upon stimulating the femoral nerve and in the vastii medialis and lateralis upon stimulating the sciatic nerve. The occurrence of short latency inhibition was observed more often in both the medial (21/31 trials) and lateral (15/22 trials) hamstrings than in vastus medialis (4/27 trials) and vastus lateralis (8/27 trials). In addition, the inhibition evoked in the hamstrings by femoral nerve stimulation was significantly larger than the short latency inhibition evoked in quadriceps by sciatic nerve stimulation (p<0.05, t test). Using H reflex testing in three subjects, increasing the stimulus strength to the femoral nerve produced a progressive reduction in the hamstring H reflex amplitude, reaching a maximum at 1.5MT and with a condition-test interval of 4 ms. In contrast, varying the stimulus strength to the sciatic nerve produced no change to the quadriceps H reflex, even at 2MT. The inhibition evoked by quadriceps in hamstring muscles observed here seems likely to be the equivalent of the Ia reciprocal inhibition observed in the cat. In contrast, caution needs to be observed in interpreting the inhibition evoked by stimulating the sciatic nerve, since such stimuli will activate afferents from many other muscles as well as the hamstring afferents. However, despite this, only small inhibitions were observed by comparison to the inhibition evoked from quadriceps to hamstring. These results show that reciprocal inhibition at this joint is not actually reciprocal i.e. not equal in both directions. The implications of this in understanding the function of reciprocal inhibition needs to be assessed.

Where applicable, experiments conform with Society ethical requirements