Influence of loading rate on patellar tendon mechanical properties in vivo.

Background: Rate-dependent properties of tendons have consistently been observed in vitro but in vivo studies comparing the effects of loading duration on this feature remain conflicting. The main purpose of the present study was to evaluate whether tendon loading rate per se would affect in vivo tendon mechanical properties.

Methods: Twenty-two physically active male subjects were recruited. Patellar tendon deformation was recorded with ultrasonography under voluntary isometric contractions at rates of 50, 80 and 110Nm/s, controlled via visual feedback.

Findings: Subjects were able to accurately generate all three loading rates (Accuracy=2% to 15%), with a greater steadiness at 50 (CV=12.4%) and 110Nm/s (CV=13.1%) than at 80Nm/s (CV=22.9%). Loading rate did not appreciably affect strain or stress. However, stiffness (ɳp(2)=0.555) and Youngs's Modulus (ɳp(2)=0.670) were significantly higher at 80Nm/s (21.4% and 21.6%, respectively) and at 110Nm/s (32.5% and 32.0%, respectively) than at 50Nm/s. Similarly, stiffness and Young's modulus were 9.9% and 8.8% higher, respectively, at 110Nm/s than at 80Nm/s.

Interpretation: These results indicate that in vivo measurements of patellar tendon mechanics are influenced by loading rate. Moreover, they bear important methodological implications for in vivo assessment of mechanical properties of this tendon and possibly other human tendons.