Variability is inherent to human motion and is the product of the interaction of the neural, muscular and skeletal systems with the physical surround. Three sources of variability (random error, systemic error, and motor program) were postulated and modeled using experimental and stochastic mathematical techniques. The outputs from the models (systems of time normalized torque curves) served as inputs into a differential equation computer simulation landing (DECSL) model producing kinematic and ground reaction force (GRF) data.

An initial investigation (4 subjects, 4 conditions) utilizing the developed simulation tools was conducted. Two simulation conditions (random error and systemic error) and two experimental conditions (no load, added load, 2 subjects 1000 g., 2 subjects 2000 g.) of 25 trials were generated. Four male volunteers performed landings (60 cm. from hanging position) onto an AMTI force platform (2 footed landing, 1 foot on platform, 1000 Hz. ) while simultaneous sagittal kinematics were recorded (Motion Analysis, 200 Hz). Joint torques and powers were calculated from the kinematic (5th metatarsal, calcaneus, malleolus, knee, hip and rib) and the ground reaction force data (Fx, Fy, Fz, Mx, and My) . The simulation conditions assigned observed variance from the no load experimental condition to either random error (no mechanical propagation of stochastically induced error) or to systemic error (mechanical propagation of induced error).

Within subject comparisons of no load versus load, no load versus random error (RB), and no load versus systemic error (SE) were conducted. Two types of analysis were performed using Model Statistics;​ 1) point by point curve comparisons (torque and power curves), and 2) discrete point analysis (Fl, T1, F2, T2).

Initial results support a motor program origin for normal movement variability. Preliminary experimentation with DECSL parameters suggest that tolerance to neural noise is sensitive to velocity dependent damping. The results of this investigation indicate that more information on the energetics of landing is needed and that simulation techniques can provide useful insights into movement problems that can not be addressed by experimental methods alo