Consumption of protein supplements can induce muscle hypertrophy when combined with exercise, but the isolated effects of differing protein diets on muscle mechanical performance remain poorly understood. We sought to determine the effects of protein origin (animal- vs plant-based) on muscle mechanical properties (mass-specific force, and peak contraction speed). Mouse cohorts were fed diets with protein derived from casein (N = 7), whey (N= 5), and pea (N=4). Dietary amino acid content was carefully balanced to ensure equivalent protein synthesis. After a minimum of a 6-week dietary intervention, we performed muscle ergometry to measure force-length-velocity relationships from two muscles: soleus (SOL), a plantarflexor with a mixed fiber type composition in near parallel-fibered configuration, and tibialis anterior (TA), a fast-twitch, relatively pennate-fibered dorsiflexor. Casein outperformed both whey and pea in terms of muscle-mass specific force, while the inverse was true for peak contraction speed, in accordance with the force-velocity trade-off. TA from casein-fed mice produced mass-specific forces of 23.2 ± 7.9 compared to 18.4 ± 4.2 and 15.9 ± 0.4 N/g for whey and pea respectively. Peak contraction speed of TA (in optimal lengths, Lo/sec) were 6.4 ± 1.7, 5.9 ± 1.6, and 6.5 ± 0.7 Lo/sec for casein, whey, and pea respectively. These results may inform advocacy for dietary protein sourcing, especially casein, a slow-absorbing protein source.