In vivo muscle function is negatively affected by the presence of the Ho p.F4976L Ryr1 mutation. (A) Voluntary running distance and speed of WT (n = 13) and Ho (n = 14) mice (starting age = 10–13 wk) measured for a period of 22 days. Data points are expressed as mean ± SEM. Left panel shows the spontaneous locomotor cumulative distance (km) recorded during the dark phase period (5 pm–5 am). Right panel shows the average running speed (km/hour) recorded during the night phase (5 pm–5 am). Statistical analysis was performed using ANOVA, followed by the Bonferroni post hoc test. *P < 0.05. (B) Analysis of forelimb (anterior two paws) grip strength in WT (n = 15) and Ho (n = 15) littermates. Mice were tested once per week for a period of 12 wk starting from 6 wk of age. Five measurements for each mouse were averaged. Each point represents the mean ± SEM. ANOVA, followed by the Bonferroni post hoc test. *P < 0.05. (C) Comparison between the distance run by WT and Ho mice during the exhaustion treadmill test. The distance run by Ho is 36% shorter than the one run by WT littermates. Each point represents the measurement coming from a single mouse. Bars represent mean ± SD. Homozygous mice run significantly less than WT. Statistical analysis was performed using ANOVA, followed by the Bonferroni post-hoc test. ***P < 0.001.