Thomas Bjørnsen

Introduction: Force–velocity profiling has been proposed in the literature as a method to identify the overall mechanical characteristics of lower extremities. A force–velocity profile is obtained by plotting for jumps at different loads the effective work as a function of the average push-off velocity, fitting a straight line to the results, and extrapolating this line to find the theoretical maximum isometric force and unloaded shortening velocity. Here we investigated whether the force–velocity profile and its characteristics can be related to the intrinsic force–velocity relationship. Methods: We used simulation models of various complexity, ranging from a simple mass actuated by a linearly damped force to a planar musculoskeletal model comprising four segments and six muscle–tendon complexes. The intrinsic force–velocity relationship of each model was obtained by maximizing the effective work during isokinetic extension at different velocities. Results: Several observations were made. First, at the same average velocity, less effective work can be done during jumping than during isokinetic lower extremity extension at this velocity. Second, the intrinsic relationship is curved; fitting a straight line and extrapolating it seem arbitrary. Third, the maximal isometric force and the maximal velocity corresponding to the profile are not independent. Fourth, they both vary with inertial properties of the system. Conclusions: For these reasons, we concluded that the force–velocity profile is specific for the task and is just what it is: the relationship between effective work and an arbitrary estimate of average velocity; it does not represent the intrinsic force–velocity relationship of the lower extremities.