Absence of lateral gastrocnemius activity and differential motor unit behavior in soleus and medial gastrocnemius during standing balance

Abstract

In a standing position, the vertical projection of the center of mass passes in front of the ankle, which requires active plantar-flexor torque from the triceps surae to maintain balance. We recorded motor unit (MU) activity in the medial (MG) and lateral (LG) gastrocnemius muscle and the soleus (SOL) in standing balance and voluntary isometric contractions to understand the effect of functional requirements and descending drive from different neural sources on motoneuron behavior. Single MU activity was recorded in seven subjects with wire electrodes in the triceps surae. Two 3-min standing balance trials and several ramp-and-hold contractions were performed. Lateral gastrocnemius MU activity was rarely observed in standing. The lowest thresholds for LG MUs in ramp contractions were 20-35 times higher than SOL and MG MUs (P < 0.001). Compared with MUs from the SOL, MG MUs were intermittently active (P < 0.001), had higher recruitment thresholds (P = 0.022), and greater firing rate variability (P < 0.001); this difference in firing rate variability was present in standing balance and isometric contractions. In SOL and MG MUs, both recruitment of new MUs (R(2) = 0.59-0.79, P < 0.01) and MU firing rates (R(2) = 0.05-0.40, P < 0.05) were associated with anterior-posterior and medio-lateral torque in standing. Our results suggest that the two heads of the gastrocnemius may operate in different ankle ranges with the larger MG being of primary importance when standing, likely due to its fascicle orientation. These differences in MU discharge behavior were independent of the type of descending neural drive, which points to a muscle-specific optimization of triceps surae motoneurons.

Doublet potentiation in the triceps surae is limited by series compliance and dynamic fascicle behavior

“…Given that the amplitude of anteriorposterior sway appears to be modulated by the accuracy of such impulses (52), high series compliance may serve to benefit control by minimizing fluctuations in torque in response to rate coding and motor unit recruitment. Although soleus has been evidenced to generate an ongoing background plantar flexor torque during standing (15,58), a compliant tendon may still act to limit inaccuracies in torque brought about by variability in motor unit firing rate (33).…”

Motor units in the human medial gastrocnemius muscle are not spatially localized or functionally grouped

“…Recently, we investigated the discharge behaviour of triceps surae MUs in standing balance with intramuscular single‐unit recordings (Héroux et al . ) and noted several instances where the same MU (time‐locked spike trains) was recorded from intramuscular electrodes located ∼5–6 cm from one another along the length of the muscle. Although anecdotal, this observation contradicted previous reports of small MU territories and regional activation.…”

Agenesis of the Medial Gastrocnemius and Plantar Muscle: Case Presentation

“…It has been reported that the flexor and extensor action exerted by the sural triceps is essential in maintaining the center of gravity during standing (Héroux et al, 2014). Besides the biomechanical action of MGM and PM, they help protect nerves and vessels coursing through the popliteal region; therefore, risk of neurovascular lesion increases with the agenesis of these muscles.…”