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

Runners with Achilles tendinopathy have reduced neural drive to the gastrocnemius lateralis. Positioning feet-inwards (feet-in) can influence gastrocnemii activity in healthy individuals. Therefore, this study investigated if pointing (feet-in) during isometric plantarflexion would increase gastrocnemius lateralis electromyography root mean square amplitude (RMS) and motor unit discharge rates (MUDR), compared to feet-neutral (feet-neutral), in Achilles tendinopathy. High-density electromyograms were recorded from gastrocnemius lateralis and medialis, during 20-s feet-in and feet-neutral contraction, in runners with (n = 18) and without (n = 19) Achilles tendinopathy. During feet-in, gastrocnemius lateralis RMS was higher in both groups and gastrocnemius medialis RMS was lower in the Achilles tendinopathy, compared to feet-neutral. MUDR were lower during feet-in in gastrocnemius lateralis (p < 0.001) and in gastrocnemius medialis in the Achilles tendinopathy group. The Achilles tendinopathy group had lower triceps surae endurance during single leg heel raise. In summary, feet-in increases gastrocnemius lateralis RMS in both groups, conversely reducing MUDR in the Achilles tendinopathy group, compared to feet-neutral. Additionally, feet-in reduces gastrocnemius medialis RMS and MUDR only in the Achilles tendinopathy group, compared to feet-neutral. This would shift the gastrocnemius lateralis/medialis ratio excitation, favouring gastrocnemius lateralis. Nonetheless, while this strategy holds promise, it remains uncertain whether performing plantarflexion exercise with feet pointed inwards would provide additional benefits for the treatment of runners with Achilles tendinopathy. Our findings suggest that the increased gastrocnemius lateralis RMS during feet-in may not be as consequence of increased MUDR and, but it might be a result of recruitment of more motor units.