Frequency-dependent activation. A. An increase in frequency led to a decrease in force development in both muscles from NBD treated and vehicle treated dko mice. B. When normalized to their individual initial forces at 4 Hz, NBD treated muscles do not exhibit the negative force-frequency behavior displayed by the vehicle treated group at the lower frequency range at 37°C. All muscles were kept at their optimal length during this protocol. ANOVA (repeated measures) indicated that both the factors treatment and frequency, as well as the interaction between these two factors was significantly different. * indicates a difference of P < 0.05 between the two groups.