Mechanical control: parallel pathways for modulating movement.

Animals across diverse taxa navigate complex and unpredictable environments by modulating movement to maintain stability and efficiency. Active neural feedback has traditionally been viewed as the sole and primary mechanism for control. In this Review, we highlight the importance of non-neural, mechanical control mechanisms that allow for rapid modulation of locomotor systems. As the speed of movement increases or organisms face perturbations that rapidly change loading conditions, neural responsiveness may become too slow to adapt effectively. In such cases, pre-tuned modulation through intrinsic muscle properties, elastic structures and tissue compliance can provide a faster, more reliable response, sometimes outperforming active control in maintaining stability and energetic efficiency. We explore how these forms of mechanical control complement neural feedback and enhance control across size scales, particularly in systems where rapid adjustments are critical. We synthesize recent findings to provide a framework for understanding the trade-offs between passive and active control and highlight the potential mechanisms that function in parallel, across levels of biological organization to modulate locomotor output.