gms | German Medical Science

Objective: It is well recognized that muscle strength is influenced by body mass. In the assessment of muscle strength, either clinically or in research studies, normalization for body mass is thus often recommended. However, there is a lack of consensus on the best method for normalizing muscle strength measures. We therefore sought to compare various methods recommended in the literature to normalize knee extension strength (both force and torque) in a community-based cohort of young men and women.

Material/Methods: In 24 men and 25 women, representing a random sample of community adults aged 20 to 29 years, we assessed both isometric knee extension force (in Newtons [N]) and torque (in N-meters [N-m]) using a custom-built chair dynamometer. Total lean muscle mass was measured from whole body DXA scans. Skeletal muscle mass was estimated using the appendicular lean muscle mass from the arms and legs regions from these scans multiplied by 1.33, while leg lean mass was determined from the leg region of scans. Using ratio standards, we normalized knee extension force for each subject by different measures of body mass (in N): total weight, lean mass, skeletal mass and lean leg mass. Knee extension torque was normalized using each of these same measures of mass as well as by height (in m). For knee extension force and torque respectively, the coefficient of variability (CV) of their normalized variables was determined, and the one with the lowest CV was considered the optimal normalization method for each measure of strength. Men and women were analyzed separately.

Results: In men as well as women, normalization of knee extension force using skeletal mass yielded the smallest CV, followed by normalization using leg lean mass. The highest CV was normalization by body weight. For knee extension torque, again in both men and women, the smallest CV was normalization by skeletal mass and height, followed by leg lean mass and height. The highest CV was with normalization by weight and height.

Conclusion: In a community-based sample of young adults, we found that using skeletal mass measurements derived from DXA scans was the optimal method for normalizing knee extension strength, either force or torque. However, normalizing by body weight, which is by far the most common approach used in the literature, appeared to be the least favorable method.