title: 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.
authors: Centre for Sensorimotor Performance, School of Human Movement and Nutrition Sciences, The University of Queensland, Brisbane, 4072 QLD, Australia d.mayfield@uq.edu.au.
authors: Dean L
authors: 0000-0003-4036-7117
authors: DL
authors: Mayfield
authors: Centre for Sensorimotor Performance, School of Human Movement and Nutrition Sciences, The University of Queensland, Brisbane, 4072 QLD, Australia.
authors: Andrew G
authors: AG
authors: Cresswell
authors: Centre for Sensorimotor Performance, School of Human Movement and Nutrition Sciences, The University of Queensland, Brisbane, 4072 QLD, Australia.
authors: Glen A
authors: GA
authors: Lichtwark
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology; D003187:Compliance; D004548:Elasticity; D004558:Electric Stimulation; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D012399:Rotation; D013997:Time Factors; D019415:Torque
keywords: Active shortening; History-dependent properties; Rate of force development; Tendon stiffness; Triceps surae; Twitch