Weight Loss Reduces Biomechanically Modeled Knee Joint And Muscle Forces

Abstract

Obesity is characterized by excessive body mass relative to height and is a predictor of knee osteoarthritis (OA). Despite the assumption that reduced body mass reduces knee joint loads and knee OA risk, these loads have not been identified in obese people or in formerly obese people after weight loss. PURPOSE: To identify the effects of weight loss on knee joint and muscle forces in obese adults. METHODS: Ground reaction forces and 3D kinematics were measured during walking prior to (baseline, BMI = 43.2) and 6 (BMI = 31.6) and 12 months (BMI = 28.6) after bariatric surgery in 10 adults. Musculoskeletal modeling was applied to these data to predict knee joint and muscle forces which were analyzed with 1-way ANOVA and post-hoc tests comparing baseline vs 6 months and 6 vs 12 months. All subjects gave written informed consent. RESULTS: Body mass decreased 27% from baseline to 6 months and 9% more at 12 months (126 vs 92 vs 84 kg, both p<0.05). Self-selected walking speed was statistically unchanged at 6 months (1.30 vs 1.35 m/s, N.S.) but increased 7% from 6 to 12 months (1.45 m/s, p<0.05). Maximum knee compressive force decreased 18% (2,943 vs 2,417, p<0.05) at 6 months then was statistically unchanged at 12 months (2,557 N). Maximum hamstrings (661 vs 550 N, p<0.05) and gastrocnemius (1,055 vs 805 N, p<0.05) forces were reduced at 6 months compared to baseline. Quadriceps force was statistically unchanged. CONCLUSIONS: Reduced body weight was the primary mechanism in changing knee joint compressive force. Indeed, knee force and body weight were correlated at r=0.82. Knee force most likely did not change from 6 to 12 months because increased walking speed probably counteracted the effect of reduced weight. Speed was correlated to knee force at r=0.49. Quadriceps force went unchanged despite weight loss most likely because step length (r=0.72 with quadriceps force) and knee flexion (r=0.44 with quadriceps force) both increased through the trial (data not shown) increasing quadriceps demand. Our data suggest 1) reduced weight led to reduced hamstrings and gastrocnemius forces which then led to reduced knee force and 2) reduced body weight may serve as a protective mechanism against knee OA incidence by reducing knee joint forces.