4.6 Article

Mechanical and neural stretch responses of the human soleus muscle at different walking speeds

Journal

JOURNAL OF PHYSIOLOGY-LONDON
Volume 587, Issue 13, Pages 3375-3382

Publisher

WILEY-BLACKWELL PUBLISHING, INC
DOI: 10.1113/jphysiol.2008.162610

Keywords

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Funding

  1. The Obel Family Foundation
  2. The Spar Nord Foundation
  3. KAKENHI [20800061]
  4. Grants-in-Aid for Scientific Research [20800061] Funding Source: KAKEN

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During human walking, a sudden trip may elicit a Ia afferent fibre mediated short latency stretch reflex. The aim of this study was to investigate soleus (SOL) muscle mechanical behaviour in response to dorsiflexion perturbations, and to relate this behaviour to short latency stretch reflex responses. Twelve healthy subjects walked on a treadmill with the left leg attached to an actuator capable of rapidly dorsiflexing the ankle joint. Ultrasound was used to measure fascicle lengths in SOL during walking, and surface electromyography (EMG) was used to record muscle activation. Dorsiflexion perturbations of 6 deg were applied during mid-stance at walking speeds of 3, 4 and 5 km h(-1). At each walking speed, perturbations were delivered at three different velocities (slow: similar to 170 deg s(-1), mid: similar to 230 deg s(-1), fast: similar to 280 deg s(-1)). At 5 km h(-1), fascicle stretch amplitude was 34-40% smaller and fascicle stretch velocity 22-28% slower than at 3 km h(-1) in response to a constant amplitude perturbation, whilst stretch reflex amplitudes were unchanged. Changes in fascicle stretch parameters can be attributed to an increase in muscle stiffness at faster walking speeds. As stretch velocity is a potent stimulus to muscle spindles, a decrease in the velocity of fascicle stretch at faster walking speeds would be expected to decrease spindle afferent feedback and thus stretch reflex amplitudes, which did not occur. It is therefore postulated that other mechanisms, such as altered fusimotor drive, reduced pre-synaptic inhibition and/or increased descending excitatory input, acted to maintain motoneurone output as walking speed increased, preventing a decrease in short latency reflex amplitudes.

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