2Current musculoskeletal modeling approaches cannot account for variability in muscle activation 3 patterns seen across individuals, who may differ in motor experience, motor training, or 4 neurological health. While musculoskeletal simulations typically select muscle activation patterns 5 that minimize muscular effort, and generate unstable limb dynamics, a few studies have shown 6 that maximum-effort solutions can improve limb stability. Although humans and animals likely 7 adopt solutions between these two extremes, we lack principled methods to explore how effort 8 and stability shape how muscle activation patterns differ across individuals. Here we 9 characterized trade-offs between muscular effort and limb stability in selecting muscle activation 10 patterns for an isometric force generation task in a musculoskeletal model of the cat hindlimb. We 11 define effort as the sum of squared activation across all muscles, and limb stability by the 12 maximum real part of the eigenvalues of the linearized musculoskeletal system dynamics, with 13 more negative values being more stable. Surprisingly, stability increased rapidly with only small 14 increases in effort from the minimum-effort solution, suggesting that very small amounts of muscle 15 coactivation are beneficial for postural stability. Further, effort beyond 40% of the maximum 16 possible effort did not confer further increases in stability. We also found multiple muscle 17 activation patterns with equivalent effort and stability, which could underlie variability observed 18 across individuals with similar motor ability. Trade-off between muscle effort and limb stability 19 could underlie diversity in muscle activation patterns observed across individuals, disease, 20 learning, and rehabilitation.
31new muscle activation pattern, potentially providing a mechanism to explain individual-specific 32 muscle coordination patterns in health and disease. Finally, we provide a straightforward method 33 for improving the physiological relevance of muscle activation pattern and musculoskeletal 34 stability in simulations.
157Muscles are listed in alphabetic order.