Heterogeneity of passive elastic properties within the quadriceps femoris muscle-tendon unit

The purpose of this study was to compare regional elastic properties between anterior and posterior regions of the patellar tendon, and individual quadriceps muscles, over a range of knee flexion angles.

An isokinetic dynamometer passively positioned the non-dominant knee of 19 young, healthy participants, at 25°, 40°, 55°, 70° and 85° flexion. Shear wave velocity (SWV, an index of tissue elasticity) was measured using ultrasound shear wave elastography in a relaxed (passive) state, confirmed by electromyography.

SWV of the patellar tendon and quadriceps muscles increased with knee flexion (longer muscle-tendon unit; P < 0.001). Within the proximal third of the patellar tendon, SWV was lower in the posterior than anterior region at 70° (P = 0.002) and 85° (P < 0.001), but not at 25°, 40° or 55° (region-by-angle interaction, P = 0.007). No differences were found between anterior and posterior regions within the middle third of the patellar tendon (P = 0.332). For the quadriceps muscles, a significant muscle-by-angle (P < 0.001) interaction was also observed. SWV of VL was greater than VM at 55° (P = 0.005), 70° (P = 0.001) and 85° (P < 0.001), but not at 25° or 40°. SWV of RF was lower than VL at all angles (all P < 0.002) and lower than VM at 55°, 70° and 85° (all P < 0.002).

Passive knee flexion at and beyond 70° was associated with non-uniform elastic properties within the proximal patellar tendon and between individual quadriceps muscles. To what extent this heterogeneity of passive elastic properties contributes to injury remains unknown.