Contractility of Dystrophic Mouse Muscle

Excised gastrocnemii from 2–4-month-old normal and genetically dystrophic mice (Jackson Memorial Laboratory strains) were studied at 15°C which permits prolonged uniform maintenance of standard responses. Results are averages from muscles of generally 10–14 diseased or normal animals. Normal muscles (87 mg) produce maximal twitch and tetanus tensions of respectively 23.9 and 53.1 gm; dystrophic muscles (35 mg) correspondingly yield 3.3 and 14.6 gm. Thus, compared to normal, dystrophic muscles are atrophied, weaker (directly or in tension output/gm muscle), and show a smaller twitch/tetanus tension ratio. The contraction period time is the same for both muscle types (about 59 msec.), but the dystrophic relaxation period is 3 times longer (676 as against 224 msec.). The dystrophic active state duration (to end of plateau) is shorter than the normal (9.8 as against 14.4 msec.). At rest, both muscle types possess nonlinear elasticity, but the diseased are stiffer.