Patellar tendon in vivo regional strain with varying knee angle

Descriptive data on the aspects of site specific in vivo tendon strain with varying knee joint angle are non-existent. The present study determines and compares surface and deep layer strain of the patellar tendon during isometric contractions across a range of knee joint angles.

Male participants (age 22.0±3.4) performed ramped isometric knee extensions at knee joint angles of 90°, 70°, 50° and 30° of flexion. Strain patterns of the anterior and posterior regions of the patellar tendon were determined using real-time B-mode ultrasonography at each knee joint angle. Regional strain measures were compared using an automated pixel tracking method.

Strain was seen to be greatest for both the anterior and posterior regions with the knee at 90° (7.76±0.89% and 5.06±0.76%). Anterior strain was seen to be significantly greater (p<0.05) than posterior strain for all knee angles apart from 30°, 90°=(7.76vs. 5.06%), 70°=(4.77vs. 3.75%), and 50°=(3.74vs. 2.90%). The relative strain (ratio of anterior to posterior), was greatest with the knee joint angle at 90°, and decreased as the knee joint angle reduced.

The results from this study indicate that not only are there greater absolute tendon strains with the knee in greater flexion, but that the knee joint angle affects the regional strain differentially, resulting in greater shear between the tendon layers with force application when the knee is in greater degrees of flexion. These results have important implications for rehabilitation and training.