Locomotor limb synergism through short latency afferent links

Reports of facilitation of lower limb synergists through afferent stimulation in resting humans lead to study of short latency reflex responses during activity of the legs. Four synergistic muscles of the right leg were examined following electrical stimulation of low threshold afferents of the posterior tibial (PTN) and femoral (FN) nerves. Four males cycled on an ergometer with pedals modified to measure the force applied. Movement frequency was 0.8 Hz with 15 N ergometer resistance for two 6 min trials, one involving PTN and the other FN. Collection of EMGs of soleus, lateral gastrocnemius, vastus medialis and rectus femoris lasted for 200 msec after stimulation, which was triggered when the pedal was 56 degrees beyond top-dead-center. Control EMGs were similarly collected during intervening, non-stimulated, pedal cycles within the same work bout. Collection of force on pedals occurred over the same 200 msec period as well as the following two complete pedal revolutions. From PTN stimulation short latency increase occurred not only in homonymous EMGs but also in heteronymous as distant as rectus femoris. FN stimulation similarly resulted in short latency changes in quadriceps and also in triceps surae. Pedal force changes accompanied the EMGs. M response confounding in many cases resulted in only tentative identification of the reflex force evoked. Little change with stimulation was seen in the force in subsequent cycles. The responses represent a net of pathways which could affect within-limb coordination of muscles during human locomotion and extend previous investigations of Ia projections in spinal cats and resting humans.