Growth changes in the elastic properties of human tendon structures

The purpose of this study was to investigate the growth changes in the elastic properties of human tendon structures. 9 younger boys (age 10.8 +/- 0.9 years, YBG), 9 elder boys (14.8 +/- 0.3 years, EBG), and 14 young adult men (24.7 +/- 1.6 years, ADG) volunteered to take part in the present study. Using a B-mode ultrasonic apparatus, the elongation of tendon and aponeurosis of vastus lateralis muscle (VL) was noninvasively measured in vivo, while subjects performed the extension of knee joint isometrically with force production levels from zero (relax) to maximal voluntary contraction (MVC) within 5 seconds. A curvilinear relationship was found between elongation of tendon structures (dL) and muscle force (Fm). This relationship consisted of two components, a steep initial change in length followed by a linear-region. The relationship between dL and Fm was fitted to a linear regression, and then the dL and dFm within 50 to 100% MVC was defined as compliance of tendon structures. The MVC force was the greatest in ADG and the lowest in YBG among the three groups. Significant age-related differences were found in compliance; 4.1 +/- 0.9 x 10(-2) mm/N for YBG, 2.9 +/- 1.1 x 10(-2) mm/N for EBG and 1.8 +/- 0.3 x 10(-2) mm/N for ADG. The dL/thigh length (TL) was significantly greater in YBG than in the other two groups above 0.35 MPa of Fm per muscle cross-sectional area (muscle stress). However, there was no significant difference between EBG and ADG in the relationship between dL/TL and muscle stress. The ratio of fascicle length to TL in YBG was significantly lower than those in the other two groups. These results suggest that the tendon structures in younger boys are more compliant than those in older boys and young men. The observed properties of tendon structures in the younger boys may play a role in protecting younger boys from athletic injuries associated with immature muscle-tendon complex.