Muscle Stiffness, Not Force, Limits Human Force Production

Humans have a remarkable ability to manage physical interaction despite a tremendously complex musculoskeletal system, comparatively slow muscles and slow neural communication. Most physical interaction tasks require force generation, and that can compromise stability. To ensure stability, muscles are required to produce a certain mechanical impedance (especially stiffness and damping). In this work we investigate the human limits of force generation and test whether and how those limits are constrained by our ability to stabilize the task. Moreover, we also study how strongly limb configuration - known to play a key role in transmitting muscle tension to hand force - matters in the generation of force. We devised an experiment in which healthy individuals performed a maximum voluntary pushing task using a custom-designed gimbal apparatus which allowed us to conditionally change the ability to transmit torque at the wrist, thus enabling or disabling stabilization via wrist pronation-supination, ulnar-radial deviation, and flexion-extension. The experiment was repeated in two different arm configurations (preferred and undesired) to assess the effect of limb configuration. The results showed: (i) a statistically significant and substantial decrease in maximum output force with less wrist ability to generate stabilizing torque, and (ii) a significant but minimal difference in maximum pushing force between the two arm configurations. Interestingly, subjects unexpectedly improved their maximum pushing performance between pre- and post-experiment, suggesting a negligible effect of fatigue and a possible effect of learning. Our findings confirm our hypothesis that force production appears to be limited by stiffness production. Muscles do not primarily generate force, but generate mechanical impedance.