Children with cerebral palsy often walk with diminished knee extension during the terminal-swing phase, resulting in a troublesome "crouched" posture at initial contact and a shortened stride. Treatment of this gait abnormality is challenging because the factors that extend the knee during normal walking are not well understood, and because the potential of individual muscles to limit terminal-swing knee extension is unknown. This study analyzed a series of three-dimensional, muscle-driven dynamic simulations to quantify the angular accelerations of the knee induced by muscles and other factors during swing. Simulations were generated that reproduced the measured gait dynamics and muscle excitation patterns of six typically developing children walking at self-selected speeds. The knee was accelerated toward extension in the simulations by velocity-related forces (i.e., Coriolis and centrifugal forces) and by a number of muscles, notably the vasti in mid-swing (passive), the hip extensors in terminal swing, and the stance-limb hip abductors, which accelerated the pelvis upward. Knee extension was slowed in terminal swing by the stance-limb hip flexors, which accelerated the pelvis backward. The hamstrings decelerated the forward motion of the swing-limb shank, but did not contribute substantially to angular motions of the knee. Based on these data, we hypothesize that the diminished knee extension in terminal swing exhibited by children with cerebral palsy may, in part, be caused by weak hip extensors or by impaired hip muscles on the stance limb that result in abnormal accelerations of the pelvis.
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The authors are grateful to Joyce Trost and Roy Wervey for assistance with data collection, to Saryn Goldberg, May Liu, and Matt Walker for input related to data processing, to Ilse Jonkers, George Chen, and Felix Zajac for helpful discussions related to simulation and interpretation of muscle function, and to Jim Gage, Tom Novacheck, and Sylvia Õunpuu for many stimulating discussions about the causes of crouch gait. This work was supported by the National Institutes of Health through the NIH Roadmap for Medical Research U54 GM072970, NIH RO1 HD33929, and NIH R01 HD046814.
- Cerebral palsy
- Dynamic simulation
- Muscle function
- Swing phase