Isometric maximal handgrip strength (GSmax) has been used as an expedient test of overall muscle strength and index of fat-free mass (FFM), We tested this relationship in 55 fit young men undergoing high rates of FFM loss in an 8.5-wk military training course involving multiple stressors including nutritionally uncomplicated energy deficit. GSmax, was measured by a hand dynamometer interfaced with a computer providing visual feedback; another strength test, measuring dynamic strength of larger muscle groups (Cleansim), was also performed. GSmax did not change (530 ± 57 vs 529 ± 63 N) in the face of a 15.6% loss of body weight (12.1 ± 3.4 (SD) kg), including 6.9% loss of FFM (4.6 ± 2.6 kg), but Cleansim decremented significantly (77.4 ± 9.6 to 58.7 ± 8.9 kg) and changes were significantly correlated with ΔFFM for GSmax (r = 0.31) and Cleansim (r = 0.49). We conclude that GSmax is not a good representation of changes in total FFM in healthy young men even though it appears to be useful in more severely catabolic patients with extreme losses of FFM and in pubertal boys making large gains in FFM. Other aspects of physical performance are clearly affected by high rates of weighl loss, as demonstrated by decrements in the Cleansim and its stronger relationship to ΔFFM.