Effect of foot position during plantarflexion on the neural drive to thegastrocnemiiin runners with Achilles tendinopathy

Abstract

Runners with Achilles tendinopathy (AT) have lower neural drive to gastrocnemius lateralis (GL) and it has been shown that modified foot position during plantarflexion activity affects motor unit discharge properties of GL. This paper tested the motor unit (MU) responses of GL and gastrocnemius medialis (GM) to feet-neutral and feet-in. We recruited runners with (n=18) and without (n=19) AT for experiment 1(EXP1) and nine healthy individuals for experiment 2(EXP2). High-Density electromyography was used to analyse MU discharge rates (MUDR) for GL and GM, during an isometric plantarflexion (EXP1) with feet-neutral and feet-in; and during a 30%-peak torque (EXP2) using a dynamometer. In EXP1 MUDR were lower during feet-in in GL (p<0.001) and in GM (p<0.001), without group difference (p=0.84). Furthermore, cumulative spike train analysis showed no difference in GL or GM neural drive between group or condition. EXP2 showed no condition differences in GL (p=0.91); and lower MUDR in GM (p=0.01). Finally, the AT group have lower triceps surae endurance observed during single leg heel raise. Our data indicate that feet-in reduces gastrocnemii MUDR, compared to feet-neutral, irrespective of tendinopathy. Current understanding of modulation strategies to GL driven by feet position is conflicting; thus, plantarflexion using feet in may not be a viable treatment strategy for AT.