Abstract
Here, we used an obstacle treadmill experiment to investigate the neuromuscular control of locomotion in uneven terrain. We measured in vivo function of two distal muscles of the guinea fowl, lateral gastrocnemius (LG) and digital flexor-IV (DF), during level running, and two uneven terrains, with 5 and 7 cm obstacles. Uneven terrain required one step onto an obstacle every four to five strides. We compared both perturbed and unperturbed strides in uneven terrain to level terrain. When the bird stepped onto an obstacle, the leg became crouched, both muscles acted at longer lengths and produced greater work, and body height increased. Muscle activation increased on obstacle strides in the LG, but not the DF, suggesting a greater reflex contribution to LG. In unperturbed strides in uneven terrain, swing pre-activation of DF increased by 5 per cent compared with level terrain, suggesting feed-forward tuning of leg impedance. Across conditions, the neuromechanical factors in work output differed between the two muscles, probably due to differences in muscle-tendon architecture. LG work depended primarily on fascicle length, whereas DF work depended on both length and velocity during loading. These distal muscles appear to play a critical role in stability by rapidly sensing and responding to altered leg-ground interaction.
Highlights
The muscles of animal legs must function to allow versatile and stable movement through a wide range of terrain conditions
Changes in kinematics and leg posture during perturbed strides in uneven terrain When guinea fowl negotiated an obstacle, the leg contacted the obstacle during late swing, initiating an early transition to stance compared to level terrain (Figs. 1 and 2)
Muscle force-length-activation dynamics during perturbed strides in uneven terrain Significant changes occurred in muscle force-length dynamics during obstacle strides in relation to altered leg loading and posture
Summary
The muscles of animal legs must function to allow versatile and stable movement through a wide range of terrain conditions. Animal movement requires a complex integration of central, peripheral and physical control mechanisms (Pearson et al, 1998, Koditschek et al, 2004, Nishikawa et al, 2007). Due to the complexity of the neuromuscular system, multiple possible combinations of muscle activity and sensory feedback can achieve any given target task (Bernstein, 1967, Misiaszek and Pearson, 2002, Ting et al, 2009). Different solutions for achieving a task may result in different characteristics such as stability, required total muscle activity and fatigue rate (e.g., Bunderson et al, 2008, Ting et al, 2009). Animals likely select among motor control strategies in a context-dependent manner based on numerous criteria (Chiel et al, 2009)
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More From: Philosophical Transactions of the Royal Society B: Biological Sciences
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