Effect of ankle joint position on triceps surae motor unit firing rates

Abstract

The triceps surae is an interesting model to study the effect of muscle length changes on motor unit properties for two reasons. First, the gastrocnemii components show lower motor unit discharge rates (MUDRs) when shortened by knee joint flexion as compared to an extended knee joint, whereas all other muscles studied display either higher or similar MUDRs in a shortened compared to a lengthened position. Second, the soleus receives strong afferent feedback due to its predominantly type I muscle fiber composition indicating that muscle length changes may have a great effect on its motor unit properties. The limited previous studies have not explored how changes in ankle joint position affect the triceps surae muscles. Thus, the purpose of this study was to compare MUDRs of the bi‐articular medial gastrocnemius (MG) and mono‐articular soleus at two ankle joint positions, 30° dorsiflexion and 20° plantar flexion, during the common task of plantar flexion. Four young, healthy men (~24 years old) were seated upright in a leg dynamometer with both the hip and knee joint angles at 90°. Subjects performed two ~3s isometric plantar flexion maximal voluntary contractions (MVC) at each ankle joint position. Tungsten electrodes were then used to record MUDRs from the MG and soleus during separate steady state 3–10s isometric plantar flexion contractions at 25, 50, 75, and 100% MVC at one ankle joint position. After a series of contractions, the ankle joint angle was changed and the subjects performed another series of plantar flexion contractions at each torque level. Initial ankle joint position and the order of contractions performed were randomized prior to each testing session. Subjects visited the lab multiple times to repeat the intervention in order to sample from many motor units. The mean MVC torque was 53% lower in the shortened (20° plantar flexion; 105Nm) compared to the lengthened (30° dorsiflexion; 223Nm) position. A total of 487 motor units from the MG (n=228) and soleus (n=259) were identified. Mean MG MUDRs at 25, 50, 75 and 100% MVC were lower by 8, 0, 17 and 14%, respectively in the dorsiflexed compared to the plantar flexed position. Similarly, mean soleus MUDRs were lower by 8, 12, 25 and 25% at 25, 50, 75 and 100% MVC, respectively in the dorsiflexed compared to the plantar flexed position. These preliminary findings indicate that shortening the MG and soleus by placing the ankle joint in a plantar flexed position leads to higher MUDRs as compared to a dorsiflexed position. This may serve as a compensatory mechanism for the decreased torque producing capability of the triceps surae in a shortened (plantar flexed) position.Support or Funding InformationFunded by NSERC.