How much impact does one pound have on your knee?
We tend to think that 1 extra pound of body weight adds one pound of pressure to the knee, but that would be inaccurate. Think about your walking motion for a moment. When you walk you transfer all of your weight from one leg to the other leg. There is a period of time when all of your weight is only on one leg and the other leg is swinging in the air. The impact you are having on your knee is not equal to one pound, but instead is equal to 4 lbs (specific to those with osteoarthritis in a recent study). That means that the extra one pound of fat (or even muscle) has an impact of 4,800 pounds for every mile walked. If you are ten pounds overweight, that is equivalent to 48,000 pounds of extra compression on your knee joint. If you were curious why your knee was hurting so much, this might give you a better idea as to why that is occurring. Just another reason to lose that unwanted body fat!
To determine the relationship between change in body mass and knee‐joint moments and forces during walking in overweight and obese older adults with knee osteoarthritis (OA) following an 18‐month clinical trial of diet and exercise.
Data were obtained from 142 sedentary, overweight, and obese older adults with self‐reported disability and radiographic evidence of knee OA who underwent 3‐dimensional gait analysis. Gait kinetic outcome variables included peak knee‐joint forces and peak internal knee‐joint moments. Mixed regression models were created to predict followup kinetic values, using followup body mass as the primary explanatory variable. Baseline body mass was used as a covariate, and thus followup body mass was a surrogate measure for change in body mass (i.e., weight loss).
There was a significant direct association between followup body mass and peak followup values of compressive force (P = 0.001), resultant force (P = 0.002), abduction moment (P = 0.03), and medial rotation moment (P = 0.02). A weight reduction of 9.8N (1 kg) was associated with reductions of 40.6N and 38.7N in compressive and resultant forces, respectively. Thus, each weight‐loss unit was associated with an ∼4‐unit reduction in knee‐joint forces. In addition, a reduction in body weight of 9.8N (1 kg) was associated with a 1.4% reduction (0.496 Nm) in knee abduction moment.
Our results indicate that each pound of weight lost will result in a 4‐fold reduction in the load exerted on the knee per step during daily activities. Accumulated over thousands of steps per day, a reduction of this magnitude would appear to be clinically meaningful.