Reduced Muscle Strength Can Alter the Impact of Gait Modifications on Knee Cartilage Mechanics

Abstract

Muscle strength can substantially influence knee joint loading, stability, and cartilage biomechanics, all of which are important factors in the onset and progression of knee osteoarthritis (KOA). Noninvasive rehabilitation methods, such as gait modifications, are suggested to effectively alter knee joint loading, potentially helping to prevent or slow KOA progression. However, no studies to date have assessed if reduced lower limb muscle strength can influence the effects of rehabilitation exercises such as gait modifications. This study aimed to reveal how reducing the strength of three lower limb muscle groups (knee extensors, hip abductors, and ankle extensors) impacts the effects of gait modifications (Toe-out, Toe-in, and Wide gait) on knee cartilage stresses. We analyzed motion and ground reaction force data from seven healthy male participants using a musculoskeletal-finite element workflow, where we systematically reduced the isometric strength of the selected muscle groups in the musculoskeletal models. Our findings indicate that reductions in hip abductor and ankle extensor strength had the most pronounced impact on the effects of gait modifications on cartilage mechanics (maximum decrease of 6 percentage points (PP) in the changes of maximum principal stress). Overall, Toe-out gait was least affected by reduced muscle strength compared to other gait styles, although responses varied greatly between participants (0-14 percentage point differences). This study emphasizes the importance of considering participant-specific muscle strengths when designing personalized rehabilitation strategies such as gait modifications and provides theoretical insights into optimizing rehabilitation exercises for managing KOA based on a given motion and joint kinetics.

Figure 1

General workflow of the study. Motion and ground reaction force data (A) from seven participants were used in the musculoskeletal modeling analysis (B; AnyBody Modeling System). For each participant, we created four models with different gait styles and four models with different muscle strength modifications. Twelve outputs were analyzed from each participant: knee flexion angle, tibiofemoral forces, tibiofemoral axial rotation and abduction moments, and patellofemoral forces and moments. These outputs were then utilized as inputs in the finite element analysis (C; ABAQUS) which was used to estimate the maximum principal stresses on the medial and lateral tibial cartilage surface during one stance phase of the gait.

Figure 2

The average effect of reduced muscle strength on maximum principal stresses in the medial (a, c, e, g) and lateral (b, d, f, h) tibial cartilage across all seven participants walking with normal and modified gait. Plots show the mean change in stress (±95% confidence intervals represented by the shaded areas) between the reduced strength and reference (unmodified strength) models. The “*” symbol represents statistical significance (p < 0.05). Overall, in the medial tibial cartilage, reduced hip abductor strength was associated with increased stresses while reduced ankle extensor strength was associated with decreased stresses in all gait styles.

Figure 3

Sensitivity of the effects of gait modifications (on cartilage mechanical stress responses) to the reduced lower limb muscle strengths. Average difference in the absolute values of maximum principal stress percentage changes from all seven participants (relative to the reference (unmodified muscle strength) model represented by the value zero) between different gait modifications and normal gait. The “*” symbol represents statistical significance (p < 0.05) at a certain time point. The stresses during the Toe‐out gait were significantly less sensitive to the reduced ankle extensor strength (i.e., negative PP_difference, smaller stress difference in Toe‐out gait vs. normal gait when ankle extensor strength was reduced) in medial (a) tibial cartilage, whereas the stresses during the Toe‐in and Wide gait were significantly less sensitive to the reduced hip abductor strength in the medial tibial cartilage (c, e). No statistically significant differences were observed in the lateral tibial cartilage (b, d, f).

Figure 4

Estimated axial joint contact forces, abduction moments, and maximum principal stresses for Participant 1 during each gait style. Reducing the ankle extensor muscle strength led to lower abduction moments and maximum principal stress mainly during normal gait (b, c) and the Toe‐in gait (h, i).

Figure 5

Estimated axial joint contact forces, abduction moments, and maximum principal stresses for Participant 6 during each gait style. Reducing the knee extensor muscle strength decreased the joint contact forces (a, b, g, j), increased the tibiofemoral adduction moments (b, e, h, k) and decreased the maximum principal stresses in the lateral tibial cartilage in all gait styles.

Figure 6

Differences in the maximum principal stress changes (relative to the Reference model represented by the zero line) for Participants 1 and 6 due to reduced muscle strength during each gait modification compared to normal gait. For Participant 1, the stresses during Toe‐in gait (e, f) were more sensitive to reduced ankle extensor strength and less sensitive during the other gait modifications. For Participant 6, the stresses appeared to be more sensitive to reduced knee extensor strength across all gait modifications.

Figure 7

Activations of rectus femoris muscle in Toe‐out and Toe‐in gait with all the muscle strength modifications for Participants 1 and 6. For Participant 1, the activation increased with reduced hip abductor muscle strength in both gait styles, whereas with Participant 6 the activation only increased in Toe‐out gait. For Participant 6, the reduced strength of the knee extensors caused a notable increase in the muscle activation during Toe‐in gait, which was not present with Participant 1.