Relaxation after extreme contraction, studied with isotonic technique

¹ Department of Physiology, Harvard Medical School, Boston, Massachusetts; and National Institute of Neurological Diseases and Blindness, National Institutes of Health, Bethesda, Maryland

Posttetanic relaxation was studied under nearly isotonic conditions, using an electromagnetic muscle lever and frog gastrocnemius. Greatly prolonged relaxation occurred when, during contraction, the muscle was thrown into slack, reaching lengths less than the minimum length in situ but greater than 40% of initial length. Contraction under such conditions has been called extreme contraction. The relaxing muscle afterward resisted stretching with tensions estimated to average 42% of twitch tension, and a shift in a length-tension characteristic seemed to have taken place. However, adequate stretching or loading overcame this resistance, and appeared to result in complete reversibility. The similarity of relaxation curves after normal and extreme contractions was demonstrated objectively by fitting the curves with the same mathematical relationship between length and time. Possible mechanisms accounting for extreme contraction are discussed.