Fig. 1.

In vivo direct muscle stimulation of CB6F1 mouse gastrocnemius to induce fatigue. Five adult (11 month) male CB6F1 mice were anesthetized and subject to a 15 min fatiguing protocol before a 1 h recovery and collection of the left and right gastrocnemius (A). 100 Hz submaximal stimulations for 300 ms every fourth second over fifteen minutes induced fatigue to approximately 30% initial force (B). The prolonged force output of type 2 fibers is observed for up to 2 min with force-time integration analysis (C). The maximum rate of contraction (D) and relaxation (E) during the 15 min of fatigue suggest a shift from fast-twitch to slow-twitch fibers.