Force During Stretches of Rat Skeletal Muscles after Hypertonia at Short and Long Lengths

Abstract

Following injection of tetanus toxin into rat gastrocnemius muscle to produce hypertonia, plantar flexor muscles were allowed to shorten (S, n = 5) without restraint or held lengthened (L, n = 3) by splinting. Saline injected rats served as control (n = 5). One week after injection, peak forces during 3 stretches with passive muscles and acute isometric force deficits produced by 15 stretches of electrically stimulated muscles were examined under pentobarbital anesthesia. Isometric force and mass of plantar flexors were similar in S rats but 16% lower in L rats compared to control. Peak passive forces were highest in S rats but not different between L rats and control. At the end of the stretch protocol, isometric force deficits were 26% larger in S rats compared to L rats and 17% smaller in L rats compared to control. Acute isometric force deficits produced by stretches of active skeletal muscles were dependent on the muscle length maintained during hypertonia. Our animal model could be used to test rehabilitation interventions during hypertonia of skeletal muscles.