Why muscle is an efficient shock absorber.

Area of Science:

• Muscle physiology
• Biophysics
• Skeletal muscle mechanics

Background:

• Skeletal muscles convert ATP hydrolysis energy into mechanical work for movement.
• During locomotion's landing phase, muscles undergo stretch, absorbing impact energy and protecting joints.
• Muscle stretch exhibits higher force and stiffness than isometric contraction with lower ATP consumption.

Purpose of the Study:

• To elucidate the structural and interactive changes in actomyosin molecules during muscle stretch.
• To understand the molecular mechanisms behind the mechanical and thermodynamic properties of muscle during stretch-induced energy absorption.

Main Methods:

• Utilized synchrotron radiation to monitor low-angle x-ray diffraction patterns of rabbit skeletal muscle fibers.
• Compared diffraction patterns during ramp stretch versus isometric contraction at physiological temperatures.
• Resolved intensities of off-meridional layer lines and M3 myosin reflection fine structure.

Main Results:

• The fraction of actin-bound myosin heads increased during stretch compared to isometric contraction.
• Intensities of actin layer lines were lower during stretch than during isometric contraction.
• Mechanical and structural data indicate a higher proportion of non-stereo-specifically bound myosin heads during stretch.

Conclusions:

• During stretch, a significant fraction of actin-bound myosin heads are disordered azimuthally but stiff axially.
• This non-stereo-specific binding explains the reduced ATP consumption and low metabolic cost during muscle stretch.
• The findings clarify how muscles effectively absorb landing impact energy, protecting skeletal structures.