Short communicationMuscle-specific indices to characterise the functional behaviour of human lower-limb muscles during locomotion
Introduction
Muscles generate force and do work to produce body movement. Muscles often contract to generate positive mechanical work and power. However, muscles may also contract isometrically or absorb energy, performing a range of functions across movement tasks and species (Dickinson et al., 2000). These functions can be characterised into four distinct behaviours: (1) a motor that generates positive work; (2) a spring that stores and recovers elastic strain energy; (3) a strut that generates significant force with minimal length change; and (4) a damper that lengthens to absorb energy. These functions depend on a range of factors, from interactions between the external environment and the body to the intrinsic properties of the muscle (Biewener, 2016). One clear example is the change in human ankle plantarflexor work that occurs with a shift in whole body mechanical demands during sprinting (Lai et al., 2016), or in turkey leg extensors during incline running (Roberts et al., 1997, Roberts and Scales, 2004). Other studies have shown that muscle function varies with a proximo-distal gradient of lower-limb muscle organisation (Biewener, 2016), where more distal muscles have been shown to exhibit strut-like, quasi-isometric muscle fibre behaviour favouring force development and spring-like storage of elastic strain energy in humans (e.g. Lai et al., 2015), wallabies (Biewener et al., 1998), and turkeys Roberts et al., 1997). In contrast, more proximal muscles generally favour work modulation (Biewener, 1998, Biewener and Daley, 2007).
Despite our understanding of how muscle function can vary with mechanical demand and anatomical location, there is yet to be a quantitative approach capable of comparing the function of different muscles and how function varies across locomotor demands. Addressing these limitations can assist in tuning the design and control strategies of physiological-inspired robotics and assistive devices that can mimic the diversity of human movement (Grimmer and Seyfarth, 2014). A promising index-based approach was introduced by Qiao and Jindrich (2016) that characterised joint function during locomotion. Our study adapts this approach to define muscle-specific parameters and, based on experimental data and computational simulations, applies the approach to characterise the functional behaviours of the human lower-limb muscles during locomotion. Using simulations of walking and running, we evaluated the approach by differentiating existing understanding of muscle function, including (1) greater spring-like function during running compared with walking, (2) greater motor-like function in the proximal limb muscles, and (3) greater strut-like function in distal muscle fibres compared with the MTU.
Section snippets
Experimental protocol
Experimental data were taken from ten participants (9 males, 1 female; 27 ± 5.6 y.o.; 1.81 ± 0.07 m; 80.2 ± 11.7 kg) who were part of a larger study (Lai et al., 2015). Each participant gave their informed consent and the relevant ethics committees approved the study (University of Queensland ethics #: 2012001215). Marker trajectories and ground reaction forces were extracted during walking and running at steady-state speeds of 1.4 m s−1 and 4 m s−1, respectively.
3D trajectories of 36
Results
The selected muscles generated the majority of total negative (68%) and positive (69%) MTU power and work done by the lower-limb during walking and running (Fig. 1, Fig. 2). Specifically, GMAX, VL, MG, and SO generated substantial MTU and muscle fibre power during the stance phase of walking and running; whereas, bi-articular RF and BF generated substantial negative and positive power during the swing phase of walking and running. The functional indices of the selected muscles varied with
Discussion
A muscle’s function is commonly characterised by its mechanical force and work output. We show that the functional index approach introduced here is capable of quantitatively characterising and differentiating functional variations of several human lower-limb muscles across gait-related whole-body mechanical demands, their anatomical location within the limb, and between the MTU and its muscle fibres. For example, our muscle-specific index approach demonstrated the shift to more spring-like
Conflict of interest statement
There are no conflicts of interest.
Acknowledgements
We thank Glen Lichtwark for assistance during data collection. We gratefully acknowledge funding from NIH Grant 2R01AR055648.
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