Imaging and Simulation of Inter-muscular Differences in Triceps Surae Contributions to Forward Propulsion During Walking

Human muscle activity and lower limb biomechanics of overground walking at varying levels of simulated reduced gravity and gait speeds

“…If gravity level influenced the length and speed of muscle contraction, then electromyography values may not change even though the resultant muscle force may be different. Recent combinations of ultrasound imaging with musculoskeletal modeling show that the medial gastrocnemius, lateral gastrocnemius, and soleus do not all behave similarly in response to altered mechanical demand at the ankle [58]. Specific reasons for this differential would need to be studied with ultrasound or computer simulations to fully understand how the muscles are changing within the force-length-velocity relationship estimate muscle fascicle length and velocity during walking at different gravity levels [59,60].…”

Age-related changes to triceps surae muscle-subtendon interaction dynamics during walking

“…Consistent with our overarching hypothesis that more uniform AT tissue displacements disrupts TS contractile dynamics and thus ankle joint mechanical output, we did not observe any significant positive correlations between TS muscle length change heterogeneity and push-off intensity in older adults. Recently, we combined electromyography, ultrasound imaging, and musculoskeletal modeling in the same subjects and revealed that the biarticular gastrocnemius muscles play a more significant role than the uniarticular SOL in governing changes in forward propulsion in young adults 8 . It follows that any disruption in muscle contractile dynamics would deleteriously impact their relative contribution to forward propulsion, thereby affecting push-off intensity.…”

“…More specifically, net ankle joint moment and mechanical power (i.e., push-off intensity) is largely governed by the forces generated by the lateral and medial gastrocnemius (GAS) and soleus (SOL) muscles that make up the triceps surae (TS) 3 , 4 . Despite collectively transferring their force through a common distal tendon, the TS muscles undergo different magnitudes of fascicle length change during constant-velocity walking and, biomechanically, contribute differently to forward propulsion (primarily from GAS) and vertical support (primarily from SOL) 5 – 8 . These muscle-level differences may be facilitated by the architectural complexity of the Achilles tendon (AT), which itself is comprised of three distinct bundles of tendon fascicles, known as “subtendons”, that originate from GAS and SOL muscles 9 – 13 .…”

“…In young adults, the majority of empirical studies suggest the gastrocnemius muscles are primarily responsible for governing changes in forward propulsion while the SOL is primarily responsible for governing changes in vertical support (e.g. 5 – 8 , though see 28 , 29 for alternative theories). However, experimental manipulations of walking speed affects both walking subtasks, thus confounding our interpretations in the specific context of forward propulsion 5 , 30 .…”

“…Fortunately, the application of horizontal aiding and impeding forces at fixed speeds has provided a more direct means to manipulate the mechanical demand for forward propulsion 6 , 31 . Using horizontal forces in young adults, we recently revealed that compared to walking normally, increased mechanical demand for forward propulsion elicits larger peak GAS fascicle shortening than that of the SOL 8 . Therefore, it is likely that any disruption of GAS fascicle length via a reduction in the capacity for sliding between adjacent subtendons could deleteriously impact contributions to forward propulsion.…”