Using induced accelerations to understand knee stability during gait of individuals with muscle weakness.

The purpose of this case series was to quantify the effectiveness of different compensatory strategies used by individuals with muscle weakness to produce knee extension during the stance phase gait. Subjects were three males with less than anti-gravity strength in the quadriceps femoris and a variable pattern of weakness elsewhere in the lower extremity. They walked independently at a self-selected speed without assistive devices. Gait analysis of the pelvis and bilateral lower extremity segments was performed with a six-camera, two force platform motion capture system. Joint angles and net internal moments were computed from the motion data. Induced acceleration analysis was performed to quantify the relative ability of each lower extremity joint moment and gravity to produce knee angular acceleration. Results showed that a variety of adaptive strategies both within and across limbs can control knee position during gait. One subject generated knee extension almost exclusively via the hip extensor moment. Another relied less on the hip extensor moment (39%) and more on the ankle plantar flexor moment (61%) to create knee extension. The third subject used the ipsilateral hip extensor moment (24%) and the contralateral ankle plantar flexor moment (67%) to assist knee extension. The strategy selected by each subject likely was influenced by both their total pattern of impairments and the effectiveness of the available compensations. This case series demonstrates how an induced acceleration analysis can augment a traditional gait analysis to expand and enhance our understanding of compensatory movement control strategies.