Meeting Abstract
Pennate skeletal muscles can operate with a range of gear ratios, i.e. the ratio of muscle shortening velocity to fiber shortening velocity, where a high gear favors speed and a low gear favors force. A previous study has shown that age-related increases in muscle stiffness result in a loss of variable gearing and compromised force production. Here we test the hypothesis that age-related changes in connective tissue result in restricted shape change during a contraction, leading to the loss of variable gearing and reduced force capacity. The dynamic 3D shape of the muscle belly and aponeurosis were tracked with a set of eight external markers during a series of isotonic contractions in young (n= 5, 6-8 months) and old rat (n=5, 30-32 months) lateral gastrocnemius muscles. We confirmed that the aged muscle lost variable gearing and produced less stress (aged= 15.6 ± 2.7 Ncm-2, young= 24.0 ± 3.5 Ncm-2, P<0.01). The fiber rotation, or degree of pennation angle change per mm fiber shortening, decreased with force in the young muscle, but in the aged muscle, rotation stayed elevated across all force levels (b= 2.73 ± 1.27 deg mm-1). In young muscles, strain measured in aponeurosis width and muscle belly width decreased as force increased (P=0.0025 and 0.0001, respectively). In the aged muscle there was no change in width of the aponeurosis or muscle belly across force (P=0.99 and 0.72). Increased connective tissue within old muscles likely restricts the change in width during shortening, resulting in greater fiber rotation across all force levels. These results show that directional restriction of muscle shape leads to a fixed gear, and lower forces in aged muscle.
