Key result
Varying the pattern of stimulation and movement in rabbit muscle fibres demonstrated that locomotion demanding high work, high impulse, or economical work output each require distinct stimulation patterns.
Why the study?
Energetic costs of muscle mechanical functions during locomotion are important components of the energy budget, prompting investigation into how stimulation and movement patterns affect mechanics and energetics.
Different patterns of stimulation and movement are required to optimize work, impulse, or energetic efficiency in skeletal muscle during locomotion.
Does not yet guide human stimulation strategies; leaves open optimal timing for work versus impulse in locomotion.
Active muscle performs various mechanical functions during locomotion: work output during shortening, work absorption when resisting (but not preventing) lengthening, and impulse (force-time integral) whenever there is active force. The energetic costs of these functions are important components in the energy budget during locomotion. We investigated how the pattern of stimulation and movement affected the mechanics and energetics of muscle fibre bundles isolated from wild rabbits (Oryctolagus cuniculus, Linnaeus). The fibres were from muscles consisting of mainly fast-twitch, type-2 fibres. Fibre length was either held constant (isometric) or a sinusoidal pattern of movement was imposed at a frequency similar to the stride frequency of running wild rabbits. Duty cycle (=stimulation duration x movement frequency) and phase (timing of stimulation relative to movement) were varied. Work and impulse were measured as well as energy produced as heat. The sum of net work (work output - work input) and heat was taken as a measure of energetic cost. Maximum work output was produced with a long duty cycle and stimulation starting slightly before shortening and was produced quite efficiently. However, efficiency was even higher with other stimulation patterns that produced less work. The highest impulse (considerably higher than isometric impulse) was produced when stimulation started while the muscle fibres were being lengthened. High impulse was produced very economically due to the low cost of producing force during lengthening. Thus, locomotion demanding high work, high impulse or economical work output or impulse, each require a distinct and different pattern of stimulation and movement.
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Curtin et al. (2019) studied this question. Pattern of stimulation and movement (duty cycle and phase) vs. Isometric (constant length) or different stimulation patterns was evaluated on Work, impulse, and energetic cost (heat + net work). Varying the pattern of stimulation and movement in rabbit muscle fibres demonstrated that locomotion demanding high work, high impulse, or economical work output each require distinct stimulation patterns.
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