A Low-Cost Apparatus for Measuring Muscle Force-Velocity Relationships

The relationship between muscle force, contractile velocity, and power represents a nearly century-old paradigm in physiology and a cornerstone of our understanding of musculoskeletal function. However, this paradigm is rarely studied directly in physiology and biomechanics classrooms because of the prohibitive cost of the required equipment. This study sought to design and validate a low-cost apparatus using a 3D-printed pulley system incorporating a ratchet-and-pawl gear that can be easily constructed and configured to collect force-velocity-power data from human finger muscles via a smartphone accelerometer using the free Phyphox application. A custom Python script converts raw accelerometer measurements into velocity and power outputs, which are then compared on a subject-by-subject basis with measurements obtained using a gold-standard dual-mode lever system. Pairwise comparisons of five summary variables (peak force, Fmax; peak speed, Vmax; peak power, Pmax; optimal speed, Vopt; and force-velocity curvature, a/Fmax) revealed no statistically significant differences between force-velocity-power data collected using the two apparatuses (paired t-tests, p > 0.05). Therefore, without a significant reduction in data quality, the inexpensive apparatus offers an accessible, safe, and cost-effective means of teaching and studying biomechanical and physiological principles. This apparatus has the potential to democratize muscle physiology and biomechanics education, particularly in underfunded and resource-limited educational settings worldwide.