Passive knee joint range of motion is unrelated to the mechanical properties of the patellar tendon

The physiological factors that govern passive joint range of motion (ROM) are poorly understood. The present study investigated the relation between passive knee joint ROM and the mechanical properties of the patellar tendon. Knee joint ROM was assessed in 43 individuals, and the subjects with the greatest ROM (flexible group, n = 10) and lowest ROM (inflexible group, n = 10) were selected for further analysis. In these groups an overall "lower extremity joint ROM score" was determined with 11 clinical tests. The elongation of the patellar tendon was assessed during graded maximal isometric knee extensor contractions using ultrasonography, and the mechanical properties of the patellar tendon were determined from corresponding load and tendon deformation data. The two groups were similar with respect to weight, height, tendon cross-sectional area and length, and were, furthermore, equally physically active. The knee joint ROM and lower extremity joint ROM score was significantly different between the groups (flexible: 136 +/- 7 degrees vs inflexible: 76 +/- 16 degrees, P < 0.001 and flexible: - 4.7 +/- 1.3 vs inflexible: 3.1 +/- 4.1, P < 0.001). There was no difference between groups in maximal knee extensor force or the corresponding tendon deformation. The tendon stiffness (flexible: 3269 1591 vs inflexible: 3185 +/- 1457 N/mm), stress (flexible: 22.4 +/- 6.5 vs inflexible: 34.0 +/- 17.6 N/mm(2)), strain (flexible; 6.5 +/- 1.6 vs inflexible: 7.2 +/- 1.9%) and Young's modulus (flexible: 0.81 +/- 0.35 vs inflexible: 1.22 +/- 0.52 GPa) were not different between the two groups of subjects. These data suggest that differences in knee joint ROM cannot be explained by the mechanical properties of the patellar tendon.