Cross bridges as the major source of compliance in contracting skeletal muscle

Abstract

IT has been established that the elastic properties of contracting skeletal muscle are dominated by structures within the sarcomeres. We have demonstrated¹ a decrease in the instantaneous stiffness of a contracting toad sartorius at lengths greater than body length (l₀) coinciding directly with the decline in maximum isometric tetanic tension. This finding suggested that the cross bridges between actin and myosin filaments may be the major site of compliance in an active muscle, a conclusion in accordance with that reached by Huxley and Simmons² on the basis of a study of tension transients following rapid releases in single fibres of frog semitendinosus. Also in agreement with this conclusion is the work of Cleworth and Edmond³ who used a light diffraction technique to demonstrate that any change in the length of the sarcomere during isometric contraction was less than 50 Å, or less than 0.1 %. These studies suggest that the cross links could be the major site of elasticity within the sarcomere, but it is possible that significant compliance resides in the unbonded thin filaments, although X-ray diffraction studies of contracting muscle⁴ indicate that the I-filament length does not increase by more than 0.02%. The experiments reported here were carried out to extend previous observations of muscle stiffness into the range of lengths below the resting length.