Relationship between fat-to-fat-free mass ratio and decrements in leg strength after downhill running

The purpose of this study was to determine whether greater body fat mass (F M) relative to lean mass would result in more severe muscle damage and grea ter decrements in leg strength after downhill running. The relationship bet ween the FM-to-fat-free mass ratio (FM/FFM) and the strength decline result ing from downhill running (-11% grade) was investigated in 24 male runners [age 23.4 +/- 0.7 (SE) yr]. The runners were divided into two groups on the basis of FM/ FFM: low fat (FM/FFM = 0.100 +/- 0.008, body mass = 68.4 +/- 1.3 kg) and normal fat (FM/FFM = 0.233 +/- 0.020, body mass = 76.5 +/- 3.3 kg, P < 0.05). Leg strength was reduced less in the low-fat (-0.7 +/- 1.3%) than in the normal-fat individuals (-10.3 +/- 1.5%) 48 h after, compared w ith before, downhill running (P < 0.01). Multiple linear regression analysi s revealed that the decline in strength could be predicted best by FM/FFM ( r(2) = 0.44, P < 0.05) and FM-to-thigh lean tissue cross-sectional area rat io (r(2) = 0.53, P < 0.05), with no additional variables enhancing the pred iction equation. There were no differences in muscle glycogen, creatine pho sphate, ATP, or total creatine 48 h after, compared with before, downhill r unning; however, the change in muscle glycogen after downhill running was a ssociated with a higher FM/FFM (r = -0.56, P < 0.05). These data suggest th at FM/FFM is a major determinant of losses in muscle strength after downhil l running.