Investigating single-leg landing strategies and movement control across changes in task demands
Variability is an intrinsic characteristic of human movement, with hypothesized connections to neuromotor functioning and mechanisms of injury. The purpose of this study was to evaluate changes in movement variability among kinematic, kinetic, and electromyographic (EMG) variables following mechanical task demand manipulations during single-leg drop landings. Biomechanical outcome variables included 3 kinematic (sagittal, hip, knee, and ankle angles), 4 kinetic (sagittal hip, knee, ankle moments and vertical ground reaction force; GRFz), and 5 EMG variables (gluteus maximus, vastus medialis, biceps femoris, medial gastrocnemius, and tibialis anterior muscles). Mechanical task demands were altered using load and landing height manipulations, computed as percentages of participant anthropometrics (bodyweight: BW, BW+12.5%, BW+25%, and height: H12.5% and H25%, respectively). Fewer emergent strategies were identified under greater mechanical task demands, defined using the load accommodation strategies model, alongside decreased movement variability, assessed using principal component analysis (PCA). Joint-specific biomechanical adjustments were identified, highlighting mechanisms for the observed load accommodation strategies and changes in movement variability. An increasingly upright landing posture was observed under greater mechanical task demands, decreasing effective landing height and reducing landing impulse. Alterations in movement variability were interpreted in the context of the available functional degrees of freedom at each lower extremity joint, aligning with physiological predictions and theories from motor control. The holistic approach taken in this investigation provided a more complete understanding of mechanisms contributing to changes in movement variability and factors that may underlie landing injuries.