MUSCLE STRENGTH AND SOFT-TISSUE COMPOSITION AS MEASURED BY DUAL-ENERGY X-RAY ABSORPTIOMETRY IN WOMEN AGED 18-87 YEARS

Dual energy x-ray absorptiometry (DEXA) offers the possibility of asse ssing regional soft tissue composition, i.e. lean mass (LM) and fat ma ss : LM may be considered a measure of muscle mass. We examined age-re lated differences in LM, percentage fat (%fat) and muscle strength in 100 healthy non-athletic women aged 18-87 years. Relationships between muscle strength and leg LM in 20 elite female weight lifters and in 1 8 inactive women with previous hip fractures were also studied. The LM and %fat of the whole body, trunk, arms and legs were derived from a whole body DEXA scan. Isokinetic knee extensor strength (KES) and flex or strength (KFS) at 30 . s(-1) were assessed using an isokinetic dyna mometer. The women aged 71-87 years had 35% lower KES and KFS than the women aged 18-40 years (P < 0.0001). Differences in LM were less pron ounced. The LM of the legs, for instance, was 15% lower in the old tha n in the young women (P < 0.0001). In a multiple regression analysis w ith age, body mass, height and leg LM or KES as independent variables and KES or leg LM as the dependent variable, age was the most importan t predictor of KES (r(partial) = -0.74, P < 0.0001). The same applied to KFS. Body mass, not age, was the most important predictor of leg LM (r(partial) = 0.65, P < 0.0001) and of LM at all other measurement si tes. The LM measured at different regions decreased equally with incre asing age. The KES:leg LM ratio was negatively correlated with age (r = -0.70, P < 0.0001). The weight lifters had significantly higher KES: leg LM ratios than age-matched controls (+12%, P < 0.0001) and vice ve rsa for the women with previous hip fractures (-36%, P < 0.0001). In c onclusion, from our study it would seem that in healthy nonathletic wo men, age is a more important determinant of muscle strength than is LM as measured by DEXA. Muscle strengthening exercises and inactivity se em to have a considerably stronger influence on muscle strength than o n LM.