Twitch force potentiation of fast-twitch skeletal muscle is produced by repetitive stimulation that can be achieved from either (1) the staircase effect (continual low frequency stimulation) or (2) post-tetanic potentiation (a 1–2 s high-frequency tetanic stimulation). Previous studies examining twitch force potentiation have been conducted in vitro and shown that it is related to phosphorylation of myosin regulatory light chain (pRLC). We previously found, in vitro, reduced potentiation of twitch force and decreased pRLC in ovariectomized (Ovx, estrogen-deficient) compared with sham-operated (estrogen-replete) mice. Thus, we questioned whether this phenomenon occurred in vivo and whether age and sex would affect the potentiation of twitch force. Using an in vivo post-tetanic potentiation method (one twitch contraction followed by a tetanic contraction—100 Hz for 1,000 ms with 0.01 ms pulses, and two post-tetanic twitch contractions), we investigated twitch torque potentiation in C57BL/6 young and old, male and female mice. There were significant main effects of sex (P < 0.001) and age (P < 0.001) on body mass and significant main effects of sex (P < 0.001) on tibialis anterior and extensor digitorum longus muscle masses, with males and aged being relatively greater. Analysis of twitch torque using a three-way ANOVA across time, age, and sex showed a significant main effect of time (pre < post; P < 0.001), time × age (P = 0.038), and time × sex (P = 0.028), indicating potentiation occurred in young and old, males and females. Analysis of twitch torque potentiation (percent increase) using a two-way ANOVA revealed a significant main effect of age (young = 45.16 ± 2.04 versus old = 27.88 ± 9.96; P < 0.001) with no effect of sex (P = 0.215). In summary, enhanced generation of twitch force of skeletal muscle using a post-tetanic potentiation method does occur in vivo and is affected by age but not sex, as there is greater twitch torque potentiation in young than old mice.

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