Relaxation is not accelerated by prior contraction in plantaris muscle of rana pipiens assessed in situ

A transient period of accelerated relaxation occurs when mammalian muscle is repeatedly stimulated. We aimed to determine if a similar effect occurs during repetitive stimulation of frog plantaris muscle. The existence of this effect would provide insight into the mechanisms controlling the rate of muscle relaxation. Repeated contractions were evoked at 200 Hz for 25 ms at 1-second intervals for 11 s. We found that relaxation became slower with each successive contraction, and that all contractile parameters had fully recovered after an 8-minute rest period. We conclude that frog and mammalian muscle exhibit differences in history-dependent properties of relaxation, but future study is needed to determine a causative explanation.