Foot position during plantarflexion changes gastrocnemii motor unit discharge rate in runners with Achilles tendinopathy but not in the way you might think

Runners with Achilles tendinopathy (AT) have reduced neural drive to the gastrocnemius lateralis (GL). This study investigated if the strategy of pointing feet-inward (feet-in) during isometric plantarflexion would increase gastrocnemius lateralis electromyography root mean square amplitude (RMS) and motor unit discharge rates, compared to feet-in neutral position (feet-neutral), in runners with Achilles Tendinopathy (AT). High-density electromyograms were recorded from gastrocnemius lateralis and medialis, during 20-s feet-in and feet-neutral isometric heel raise, in runners with (n=18) and without (n=19) AT. During feet-in, GL RMS was higher during feet-in in both groups and GM RMS was lower only during feet-in in the AT. Conversely, motor unit discharge rates were lower during feet-in in GL (p<0.001) and in GM in the AT group. The AT group had lower triceps surae endurance during single leg heel raise. In summary, feet-in increases GL RMS in both groups, conversely reducing motor unit discharge rates in the AT group, compared to feet-neutral. Additionally, feet-in reduces GM RMS and motor unit discharge rates only in the AT 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 GL RMS during feet-in is effective in increasing GL excitation but not as consequence of increased MUDR and, but it might be a result of recruitment of more motor units.