Effect of localized muscle fatigue on vertical ground reaction forces and ankle joint motion during running.

The purpose of this study was to examine the changes in the vertical ground reaction force (VGRF) and ankle joint motion during the first 50% of the stance phase of running following fatiguing exercise of either the dorsiflexors or the invertors of the foot. VGRFs, sagittal and rearfoot kinematic data were collected from 11 female recreational runners running at 2.9 m/second on a treadmill prior to and following localized muscle fatigue of either the invertors or dorsiflexors of the right foot. Loading rate of the impact peak force significantly increased following fatiguing exercise of the dorsiflexors, while the peak magnitudes of the impact and push-off forces remained unchanged. There were significant decreases in dorsiflexion at heel contact, but no significant difference in any rearfoot motion parameters tested following dorsiflexor fatigue. Following fatiguing exercise of the invertors, impact peak magnitude, push-off peak magnitude and the rate of decline of the impact peak force significantly decreased; there was no change in the loading rate of the impact peak force. Invertor fatigue also resulted in a less inverted foot position at heel contact, but there were no significant differences in any other kinematic parameters tested. The results demonstrate that localized muscle fatigue of either the invertors or dorsiflexors can have a significant effect on the loading rates, peak magnitudes and ankle joint motion seen during running. These changes, due to localized muscle fatigue, may play a role in many common lower extremity running injuries.

A multisegment computer simulation of normal human gait.

The goal of this project was to develop a computer simulation of normal human walking that would use as driving moments resultant joint moments from a gait analysis. The system description, initial conditions and driving moments were taken from an inverse dynamics analysis of a normal walking trial. A nine-segment three-dimensional (3-D) model, including a two-part foot, was used. Torsional, linear springs and dampers were used at the hip joints to keep the trunk vertical and at the knee and ankle joints to prevent nonphysiological motion. Dampers at other joints were required to ensure a smooth and realistic motion. The simulated human successfully completed one step (550 ms), including both single and double support phases. The model proved to be sensitive to changes in the spring stiffness values of the trunk controllers. Similar sensitivity was found with the springs used to prevent hyperextension of the knee at heel contact and of the metatarsal-phalangeal joint at push-off. In general, there was much less sensitivity to the damping coefficients. This simulation improves on previous efforts because it incorporates some features necessary in simulations designed to answer clinical science questions. Other control algorithms are required, however, to ensure that the model can be realistically adapted to different subjects.

A two-part, viscoelastic foot model for use in gait simulations.

A three-dimensional, two-part model of the foot, for use in a simulation of human gait, is presented. Previous simulations of gait have not included the foot segment (e.g. Siegler et al., 1982, J. Biomechanics 15, 415-425) or have fastened it to the ground (e.g. Onyshko and Winter, 1980, J. Biomechanics 13, 361-368). A foot model based on viscoelastic elements (e.g. Meglan, 1991, Ph.D. thesis, Ohio State Univ.), allows more freedom of movement and thus models the physical system more closely. The current model was developed by running simulations of the foot in isolation from just before heel contact to just after toe-off. The driving inputs to the simulation were the resultant ankle joint forces and moments taken from a gait analysis. Nine linear, vertically oriented spring/damper systems, positioned along the midline of the foot were used to model the combined viscoelastic behaviour of the foot, shoe and floor. Associated with each vertical spring/damper system were two orthogonally placed, linear, horizontal dampers used to provide the shear components of the ground reaction force. Torques at the metatarsal-phalangeal joint were supplied by a linear, torsional spring and damper. Control about the vertical axis and the long axis of the foot was achieved by the use of linear, torsional dampers. The predicted kinetic and kinematic values are very similar to those taken from the gait analysis. The model represents an improvement over previous work because the transition from swing to stance was smooth and continuous without the foot being constrained to any specific trajectory.