Background: In Duchenne muscular dystrophy (DMD), the absence of dystrophin makes muscle prone to damage caused by eccentric contractions. Varying levels of eccentric contractions in level walking may lead to selective degeneration in lower limb muscles in DMD. Declined walking encountered in our lives is thought to cause higher levels of eccentric contractions than level walking, leading to more detrimental effect to patients with DMD. However, it is unclear to what extent the levels of eccentric contractions increase in declined walking for lower limb muscles.
Objectives: The purpose of this study was to examine eccentric contraction in lower limb muscles during declined walking using dynamic musculoskeletal walking simulations.
Approaches: The level of muscle eccentric contraction was estimated based on a published musculoskeletal simulation study of walking. Specifically, muscle negative work, the product of muscle force and lengthening in eccentric contraction, was calculated in simulations of declined (-9°) and level (0°) walking and used as a measure of the level of eccentric contraction. To account for architectural differences across muscles, negative work was normalized by muscle optimal fiber length and maximum isometric force. Differences in negative work between declined and level walking were assessed using ANOVA with p=0.05.
Results: Seventeen of 35 lower limb muscles had significantly more negative work in declined walking than in level walking. Only two had less negative work in declined walking. Particularly, negative work increased the most in knee extensors, especially rectus femoris.
Conclusions: Our results suggest that declined walking with greater eccentric contractions may be more detrimental to patients with DMD than level walking, and thus may need to be avoided. Our results also highlight the knee extensors as the most adversely affected muscles in the declined walking, which may contribute to the selective muscle degeneration in DMD.