Effects of series elastic compliance on muscle force summation and the rate of force rise

Abstract

Compliant tendons permit mechanically unfavourable fascicle dynamics during fixed-end contractions. The purpose of this study was to reduce the effective compliance of tendon and investigate how small reductions in active shortening affect twitch kinetics and contractile performance in response to a second stimulus. The series elastic element (SEE) of the human triceps surae (N=15) was effectively stiffened by applying a 55 ms rotation to the ankle, through a range of 5 deg, at the onset of twitch and doublet [interstimulus interval (ISI) of 80 ms] stimulation. Ultrasonography was employed to quantify lateral gastrocnemius and soleus fascicle lengths. Rotation increased twitch torque (40-75%), rate of torque development (RTD; 124-154%) and torque-time integral (TTI; 70-110%) relative to constant-length contractions at the initial and final joint positions, yet caused only modest reductions in shortening amplitude and velocity. The torque contribution of the second pulse increased when stimulation was preceded by rotation, a finding unable to be explained on the basis of fascicle length or SEE stiffness during contraction post-rotation. A further increase in torque contribution was not demonstrated, nor was an increase in doublet TTI, when the second pulse was delivered during rotation and shortly after the initial pulse (ISI of 10 ms). The depressant effect of active shortening on subsequent torque generation suggests that compliant tendons, by affording large length changes, may limit torque summation. Our findings indicate that changes in tendon compliance shown to occur in response to resistance training or unloading are likely sufficient to considerably alter contractile performance, particularly maximal RTD.

Additional in-series compliance reduces muscle force summation and alters the time course of force relaxation during fixed-end contractions

“…We argue that the higher velocity of active shortening afforded by the compliant tendon was sufficient to cause a considerable reduction in the force-generating capacity of the contractile apparatus during force development, such that the relative force contribution of a second stimulating pulse was reduced with respect to stiffer conditions. Twitch force has been shown to increase significantly in response to a modest reduction in the velocity of shortening induced by an increase in SEE effective stiffness (Mayfield et al, 2016). Sandercock (2000) also concluded that a higher velocity of active shortening during force development could likely explain the reduction in force summation demonstrated when individual parts of whole muscle were activated simultaneously rather than individually, arguing that a common SEE permitted greater internal shortening during synchronous activity.…”

“…A similar response was observed when active shortening against the stretch of the SEE was largely prevented by a feedback control signal that adjusted the length of an isolated bullfrog plantaris MTU (Sawicki and Roberts, 2009). We recently demonstrated for the human triceps surae, which has high fixed-end compliance, that the mechanical properties of the twitch could be dramatically increased by only a modest reduction in fascicle shortening amplitude and velocity induced by a rapid stretch of relatively low amplitude (Mayfield et al, 2016). Stretch of the MTU early in the twitch was shown to also influence the force generated by a second stimulating pulse delivered at the post-stretch MTU length, suggesting that the depressant effect of active shortening on contractile force may influence force summation under conditions where internal shortening is permitted.…”

“…Further to this, the degree to which the effective stiffness is modified by a given amplitude of stretch is dependent on the active state of the muscle during the stretch. There is less relative resistance to active shortening when activation is achieved with two stimuli, particularly at a high frequency of stimulation, owing to higher rates of shortening and force rise (Mayfield et al, 2016). A more suitable model for studying the direct influence of series compliance on force summation would permit the compliance of the SEE to be manipulated to align with different physiological compliances for comparative purposes, but also be maintained for the duration of the contraction.…”

The effect of muscle length on transcranial magnetic stimulation-induced relaxation rate in the plantar flexors

“…; Mayfield et al. ), of which only Sale and colleagues measured components of plantar flexor relaxation. Using electrical stimulation of the tibial nerve at rest, Sale and colleagues found half‐relaxation time to be slower at 20° DF (139.0 ± 26.5 ms) compared to 30° PF (89.5 ± 21.4 ms).…”

“…; Mayfield et al. ). Hence, no study has paired TMS with ultrasound to assess intrinsic contractile properties while the muscle is actively contracting.…”