Greater Acute Changes in Passive Mechanical and Morphological Properties Following Constant Force Than Constant Length Muscle Stretching

Stretching is applied to lengthen shortened muscles in pathological conditions such as joint contractures. We investigated (i) the acute effects of different types of stretching, i.e. constant length (CL) and constant force (CF) stretching, on changes in passive mechanical and morphological properties of medial gastrocnemius muscle (MG) and (ii) the relationship between the changes in mechanical and morphological properties and impulse or maximal strain of stretching. Forty-eight hindlimbs from 13 male and 12 female Wistar rats (13 weeks old, respectively 424.6 ± 35.5 and 261.8 ± 15.6 grams) were divided into six groups (n=8 each). The MG was initially stretched to a length at which the force was 75%, 95%, or 115% of the force corresponding to maximal dorsiflexion and held at either CF or CL for 30 min. Before and after the stretching protocol, the MG peak force and peak stiffness were assessed by lengthening the passive muscle to the length corresponding to maximal ankle dorsiflexion. Also, the muscle belly length and tendon length were measured. CF stretching affected peak force, peak stiffness, muscle belly length and tendon length more than CL stretching (p<0.01). Impulse was associated only with the decrease in peak force, while maximal strain was associated with the decrease in peak force, peak stiffness, and the increase in muscle belly length. We conclude that CF stretching results in greater acute changes in mechanical and morphological properties than CL stretching, which appears to be dependent predominantly on the differences in imposed maximal strain.