Functional diversification within and between muscle synergists during locomotion

Area of Science:

• Biomechanics
• Animal locomotion
• Muscle physiology

Background:

• Locomotion relies on coordinated limb muscle function, which is dynamic during strides and varies with speed and terrain.
• Understanding how work is distributed within and between synergistic muscles in vivo is crucial but not well-established.

Purpose of the Study:

• To investigate the distribution of mechanical work within and between synergistic hindlimb muscles during locomotion in helmeted guinea fowl.
• To determine how this work distribution changes with variations in gait and speed.

Main Methods:

• In vivo analysis of muscle function during locomotion in helmeted guinea fowl (Numida meleagris).
• Quantification of positive work done by the lateral gastrocnemius (LG) and medial gastrocnemius (MG) muscles at different speeds (0.5 m s-1 and 2.0 m s-1).
• Comparison of work distribution between proximal (pMG) and distal (dMG) regions of the MG muscle.

Main Results:

• The LG muscle performed significantly more work than the MG muscle, with this difference increasing at higher speeds.
• The proximal MG (pMG) performed more work than the distal MG (dMG).
• Differences in work output were attributed to varying knee moments, motor unit recruitment, muscle stiffness, and fiber type composition (slow-twitch in dMG).

Conclusions:

• Synergistic muscles exhibit functional diversity in work distribution, which is modulated by gait and speed.
• The pMG's greater compliance enhances fascicle stretch and force production compared to the dMG.
• This study highlights the complex, adaptable nature of muscle function during locomotion.