title: Doublet potentiation in the triceps surae is limited by series compliance and dynamic fascicle behavior.
abstract: Activation of skeletal muscle twice in quick succession results in nonlinear force summation (i.e., doublet potentiation). The force contributed by a second activation is typically of augmented amplitude, longer in duration, and generated at a greater rate. The purpose of this study was to examine force summation in a muscle attached to a compliant tendon, where considerable internal shortening occurs during a fixed-end contraction. The triceps surae of 21 (Experiment 1) and 9 (Experiment 2) young adults were maximally activated with doublet stimulation of different interstimulus intervals (ISIs) (5-100 ms) at several muscle lengths. Ultrasound images acquired from lateral gastrocnemius and soleus muscles allowed quantification of dynamic fascicle behavior. Force summation was muscle length dependent. Force augmentation was limited to a short muscle length. Lateral gastrocnemius and soleus fascicles underwent large amounts of active shortening and achieved high velocities in response to doublet stimulation, dynamics unfavorable for force production. Summation amplitude and the sensitivity of summation to ISI were dramatically depressed in the triceps surae after comparison to muscles with less fixed-end compliance. We propose that the internal shortening permitted by high series compliance limited force augmentation by offsetting and/or interfering with activation and cross-bridge processes driving augmentation. High series compliance may also reduce the sensitivity of the summated response to ISI, an assertion supported by predictions from a Hill-type muscle model. These muscles may exhibit greater force augmentation during more accustomed stretch-shorten tasks (i.e., hopping), where the compliance of the Achilles tendon actually enables near-isometric fascicle behavior.
authors.affiliation: School of Human Movement and Nutrition Sciences, The University of Queensland, Brisbane, Australia;
authors.forename: Dean L
authors.identifier: http://orcid.org/0000-0003-4036-7117
authors.initials: DL
authors.lastname: Mayfield
authors.affiliation: School of Human Movement and Nutrition Sciences, The University of Queensland, Brisbane, Australia;
authors.forename: Glen A
authors.initials: GA
authors.lastname: Lichtwark
authors.affiliation: Department of Biology of Physical Activity, University of Jyväskylä, Jyväskylä, Finland.
authors.forename: Neil J
authors.initials: NJ
authors.lastname: Cronin
authors.affiliation: Department of Biology of Physical Activity, University of Jyväskylä, Jyväskylä, Finland.
authors.forename: Janne
authors.initials: J
authors.lastname: Avela
authors.affiliation: School of Human Movement and Nutrition Sciences, The University of Queensland, Brisbane, Australia; a.cresswell@uq.edu.au.
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Cresswell
mesh_terms: D000125:Achilles Tendon / Q000502:physiology; D001696:Biomechanical Phenomena; D003187:Compliance / Q000502:physiology; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D007719:Knee Joint / Q000502:physiology; D007866:Leg / Q000000981:diagnostic imaging / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000502:physiology*; D010525:Peripheral Nerves / Q000502:physiology; D013710:Tendons / Q000502:physiology; D019415:Torque; D014463:Ultrasonography; D055815:Young Adult
keywords: force summation; muscle force; muscle mechanics; muscle-tendon interaction