LeMoine JK, Lee JD, Trappe TA. Impact of sex and chronic resistance training on human patellar tendon dry mass, collagen content, and collagen cross-linking. Am J Physiol Regul Integr Comp Physiol 296: R119 -R124, 2009. First published October 22, 2008 doi:10.1152/ajpregu.90607.2008.-Collagen content and cross-linking are believed to be major determinants of tendon structural integrity and function. Sex and chronic resistance training have been shown to alter tendon function and may also alter the key structural features of tendon. Patellar tendon biopsies were taken from untrained men [n ϭ 8, 1 repetition maximum (RM) ϭ 53 Ϯ 3 kg], untrained women (n ϭ 8, 1 RM ϭ 29 Ϯ 2 kg), and resistance-trained (10 Ϯ 1 yr of training) men (n ϭ 8, 1 RM ϭ 71 Ϯ 6 kg). Biopsies were analyzed for dry mass, collagen content, and collagen cross-linking (hydroxylysylpyridinoline). We hypothesized that these elements of tendon structure would be lower in women than men, whereas chronic resistance training would increase these parameters in men. Tendon dry mass was significantly lower in women than men (343 Ϯ 5 vs. 376 Ϯ 8 g dry mass/mg tendon wet wt, P Ͻ 0.01) and was not influenced by chronic resistance training (P Ͼ 0.05). The lower tendon dry mass in women tended to reduce (P ϭ 0.08) collagen content per tendon wet weight. Collagen content of the tendon dry mass was not influenced by sex or resistance training (P Ͼ 0.05). Similarly, cross-linking of collagen was unaltered (P Ͼ 0.05) by sex or training. Although sex alters the water content of patellar tendon tissue, any changes in tendon function with sex or chronic resistance training in men do not appear to be explained by alterations in collagen content or cross-linking of collagen within the dry mass component of the tendon. human tendon; water content; hydroxyproline; hydroxylysylpyridinoline TENDONS ARE THE FIBROELASTIC structures that connect muscle to bone and convey muscular force (5,22,23). The tensile strength of tendon tissue is estimated to be 50 -100 N/mm 2 (5), a key feature as tendons are subjected daily to high forces. The composition of tendon tissue influences its strength and integrity, playing a key role in force transmission through the muscle-tendon complex (23). Animal and human cadaver studies indicate that tendon tissue is ϳ55-70% water (5, 23), with the remaining dry mass containing the extracellular matrix and tendon cells (5,22). Collagen is the main protein component of tendon tissue, comprising ϳ65-75% of the dry mass (5,22,23). Collagen molecules are joined into fibrils and fibers by lysine-based cross-links [hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP)] (7,22,23,30). Tendon dry mass, collagen content, and cross-links have been shown to impact tendon strength and mechanical properties (22,23,30,52).Tendon tissue is dynamic, highly metabolic, and quite responsive to exercise (23,24,29,34). Tendons should therefore adapt in a positive manner to long-term loading, becoming more damage resistant and ensuring optimal muscular force transmissi...