Muscle force cannot be directly inferred from muscle activation: illustrated by the proposed imbalance of force between the vastus medialis and vastus lateralis in people with patellofemoral pain.

François Hug, Paul W Hodges, Kylie Tucker

Synopsis Muscle force cannot be directly inferred from neural drive assessed using electromyography (EMG). Although the limitations associated with inferring force from EMG are well known, this has received little attention in the clinical literature. This commentary discusses these limitations within the context of the issue of imbalance of force production between the vastus medialis (VM) and vastus lateralis (VL) muscles, which has been speculated to contribute to the development and/or persistence of patellofemoral pain. The balance of neural drive between Vastii muscles is most frequently measured with 2 approaches: 1) the onset of VM EMG relative to that of VL and 2) the ratio of the EMG signal amplitude of the VM and VL. Here we demonstrate that this classical approach cannot resolve the issue of whether an imbalance of force exists between VM and VL. Considerations such as altered electromechanical delay (time between the onsets of muscle activation and patella motion) in people with patellofemoral pain may lead to a reconsideration of the classical interpretation of the onset of VM EMG signal relative to that of VL. Also, beyond the amplitude of the neural drive, muscle force depends on several biomechanical factors (eg, specific tension and physiological cross sectional area). Therefore, VL/VM activation ratio does not provide information about VL/VM force ratio, which is ultimately the most important information from a clinical perspective. Although the literature includes defence for both the existence and absence of this force imbalance in people with patellofemoral pain, a reconsideration of the methods used to assess this imbalance is needed. J Orthop Sports Phys Ther, Epub 26 Mar 2015. doi:10.2519/jospt.2015.5905.