Leg, vertical, and joint stiffness levels in rear-foot and fore-foot strike landings
Gainer, Allison Nichol
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Bouncing gait, specifically hopping, running, and jumping, involves a complex combination of legs, joints, muscles, and nerves coordinated to perform simple biomechanical tasks. The findings associated with spring-mass modeling of bouncing gait suggest that hopping and running humans maintain center of mass (CoM) motions by adjusting vertical leg stiffness. Overall, lower extremity stiffness increases with the demands of the activity such as increased hopping frequency, hopping or jumping height, and running speed, which are all associated with increased stiffness. The increase in leg, vertical, and joint stiffness occurs because as more physical demands are imparted on the body, greater resistance to movement is needed to produce controlled movements. Studies comparing fore-foot strike (FFS) and rear-foot strike (RFS) patterns in running and hopping have shown converse results regarding the contribution of knee and ankle joint stiffness levels in preserving total leg stiffness. It is known that fore-foot strike runners generate smaller collision forces than rear-foot strike runners. However, an understanding of how joint stiffness levels differ when in a fore-foot strike pattern compared to a rear-foot strike patterns is unknown. Moreover, it is unclear how leg, vertical, and joint stiffness are affected when humans run at increasing speeds with both a fore-foot and rear-foot strike pattern. Investigations that assess the relationship between strike patterns and changes in velocity are needed in order to clarify joint contributions to changes in performance tasks. We completed a study on vertical hopping, fore-foot strike running, and rear-foot strike running to determine how ankle and knee joint stiffness values vary across different performance tasks. Throughout the study, leg stiffness remained constant (P>0.05) and vertical stiffness increased as the step frequency increased (P<0.05). During the fore-foot strike running trials, there were greater increases in ankle joint stiffness in comparison to knee joint stiffness. This suggests that the knee joint was stiffer than the ankle joint throughout the fore-foot strike running performance. In contrast, there was a greater increase in knee joint stiffness than ankle joint stiffness throughout the rear-foot strike performance, which implies that the ankle joint was stiffer than the knee joint. The changes in joint stiffness levels across the two strike patterns could be attributed to the small decrease in knee excursion and increase in ankle excursion in the fore-foot strike pattern compared to the rear-foot strike pattern. Understanding how these joint-level responses to differentiating in tasks influence the stability of leg stiffness may aid robotic, lower limb prosthetic, and even running shoe design.