Mechanical and neural factors underlying muscle performance during submaximal voluntary shortening contractions

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Many everyday movements require that muscles change length while producing force to accelerate or decelerate the body, yet our ability to predict in vivo muscle force under these conditions is poor. This might be because the conventional muscle models used to predict muscle force typically neglect the muscle’s history of length change, which affects the muscle’s force-producing capacity and its underlying neural control. Specifically, following active muscle shortening, force output is depressed compared with what is expected based on generally-accepted theories of muscle contraction. This phenomenon is termed residual force depression (rFD) and our current understanding of how rFD affects neuromuscular performance is incomplete. This is because we typically assess a muscle’s maximum force-producing capacity under constant muscle-tendon unit (MTU) length conditions and we neglect that MTU compliance permits muscle shortening under these conditions, which induces rFD. Consequently, we underestimate the true in vivo isometric force capacity of human locomotor muscles and we fail to account for the changes in neuromuscular function that might occur due to rFD. This current lack of understanding contributes to inaccurate muscle force predictions during everyday movements, which severely limits our ability to effectively restore, improve and supplement human movement across the lifespan. This project therefore aims to uncover how rFD interacts with (I) the muscle’s force-producing capacity, (II) the upstream excitability of the motor cortex and spinal cord, and (III) the muscle’s underlying neural drive. These aims will be addressed across four studies that will systematically manipulate human tibialis anterior (a major dorsiflexor muscle important for walking and postural control) in vivo muscle lengths, length changes and muscle activity levels while the excitability of the corticospinal system and the muscle’s motor unit behaviour are probed with a unique combination of cutting-edge experimental techniques from biomechanics and neurophysiology (e.g. ultrasound imaging, non-invasive brain and spinal cord stimulation, high-density surface electromyography). Preliminary results indicate that muscle shortening during submaximal voluntary fixed-end contractions induces rFD due to MTU compliance, which might subsequently increase corticospinal excitability and change motor unit behaviour compared with constant muscle length contraction conditions. This effect of rFD on neuromuscular function has previously been neglected and is useful information for developing more effective strategies to enhance muscle performance, reduce injury risk or prevent age-and-disability-related functional decline. This research is also important for advancing current neuromusculoskeletal models, which are used to interpret muscle function during everyday movements and assess the consequences of clinical conditions, as well as inform surgical planning and mechatronic design.