Objectives: Our study was undertaken to investigate reciprocal inhibition in humans both from ankle flexors to extensors and from ankle extensors to flexors.
Methods: Changes in the firing probability of single motor units in response to electrical stimulation of muscle nerves (the peristimulus time histogram technique) were used to derive the reciprocal projections of muscle spindle Ia afferents to the motoneurones of ankle muscles. Discharges of units in ankle flexors (the tibialis anterior muscle [TA]) and extensors (soleus [SOL] and medial gastrocnemius [MG] muscles) were investigated respectively after stimulation of the posterior tibial (PTN) and common peroneal (CPN) nerves (predominantly on the deep branch). In eight normal subjects aged 24 to 40 years, one motor unit per each muscle was studied.
Results: CPN stimulation produced reciprocal Ia inhibition in the SOL of 5 of 7 of them and in the MG of 3 of 5, whereas PTN stimulation produced reciprocal Ia inhibition in the TA of only 2 of 6 subjects.
Conclusions: These findings suggest that at low level contraction reciprocal Ia inhibition from ankle flexors to extensors may be stronger than that from ankle extensors to flexors.