Mechanical power output during stretch–shortening cycles of rat medial gastrocnemius muscle: Influence of various muscle length trajectories

The average mechanical power output (AMPO) over a stretch–shortening cycle produced by a muscle depends on muscle length and stimulation over time. While the effects of cycle frequency and muscle length excursion on AMPO are well-known, several questions remain about the effects of muscle length and stimulation over time on the maximally attainable AMPO. For example, which precise muscle length and stimulation over time yield maximal AMPO? In situ experiments are inherently limited to a finite set of muscle length and stimulation over time. To overcome this limitation, we combined in situ experiments on rat m. gastrocnemius medialis with Hill-type muscle modelling. We first performed dedicated trials to estimate the muscle-tendon-complex (MTC) properties of each rat. Subsequently, we performed various stretch–shortening cycles with substantial differences in cycle frequency, shortening-to-lengthening time ratio and MTC length excursion. Model-predicted AMPO correlated nearly perfectly with experimentally measured AMPO (r² > 0.98). This justified further exploration using the Hill-type MTC model. Using the Hill-type MTC model, we predicted that AMPO peaks at a cycle frequency of 3.5 Hz, with a shortening-to-lengthening time ratio of 6:1 and an MTC length excursion of 8 mm. Notably, cycle frequency and MTC length excursion showed a strong interaction: increasing one necessitated a decrease in the other to maximise AMPO. By contrast, the optimal shortening-to-lengthening time ratio remained remarkably constant across all tested combinations of cycle frequency and MTC length excursion. This shows that muscles should spend substantially more time shortening than lengthening to maximise AMPO.Author summary: The average mechanical power output (AMPO) of cyclic contractions, in which a muscle shortens and lengthens, is a key determinant of performance in tasks like sprint cycling and rowing. Apart from stimulation, muscle AMPO depends on three key factors: cycle frequency (the number of shortening–lengthening cycles per second), length excursion (the change in muscle length during each cycle), and the relative duration of shortening and lengthening. To study the dependency of AMPO on these key factors, we combined experiments on rat calf muscle with a mathematical muscle model incorporating key physiological properties. The model accurately reproduced the experimental results, which allowed us to explore a much wider range of the three factors than would be possible using experiments alone. Cycle frequency and length excursion interacted strongly, such that increasing one required decreasing the other to maximise AMPO. By contrast, the optimal relative duration of shortening and lengthening was consistent across all tested cycle frequencies and length excursions: AMPO was maximised when muscles spent about 85% of each cycle shortening. These findings improve our understanding of muscle function and may help explain animal behaviour as well as inform sports equipment design to enhance performance.