Chimpanzee super strength and human skeletal muscle evolution.

Area of Science:

• Comparative muscle physiology
• Evolutionary biology
• Primate biomechanics

Background:

• Common chimpanzees (Pan troglodytes) are anecdotally reported to possess
• super strength
• compared to humans.
• Previous research suggested differences in isometric force, shortening velocity, or myosin heavy chain (MHC) isoform content.
• However, a critical review indicates chimpanzee mass-specific muscle performance is only modestly higher (approx. 1.5x) than humans.

Purpose of the Study:

• To investigate the underlying muscular basis for the observed differences in muscle performance between chimpanzees and humans.
• To test hypotheses regarding isometric force, shortening velocity, and MHC isoform content.
• To model whole-muscle performance using species-specific parameters.

Main Methods:

• Analysis of single-fiber contractile properties in chimpanzee and human skeletal muscle.
• Quantification of myosin heavy chain (MHC) isoform composition.
• Computer simulations of species-specific whole-muscle models.

Main Results:

• Chimpanzee and human single muscle fibers exhibit similar contractile properties.
• Chimpanzee muscle has a significantly higher proportion of fast-twitch MHC II isoforms (approx. 67% MHC IIa+IId) compared to humans.
• Simulations predict 1.35 times higher maximum dynamic force and power output in chimpanzee muscle models.

Conclusions:

• Superior mass-specific muscle performance in chimpanzees is primarily due to a higher fraction of fast-twitch MHC II isoforms and differences in fiber length, not isometric force or shortening velocity.
• The hominin lineage may have experienced a decline in maximum dynamic force and power output over the last 7-8 million years.
• This decline could be linked to evolutionary selection for repetitive, low-cost contractile behaviors.