In vivo changes in the human patellar tendon moment arm length with different modes and intensities of muscle contraction

The purpose of this study was to examine the effect of different muscle contraction modes and intensities on patellar tendon moment arm length (dPT)(dPT). Five men performed isokinetic concentric, eccentric and passive knee extensions at an angular velocity of 60 deg/s and six men performed gradually increasing to maximum effort isometric muscle contractions at 90∘90∘ and 20∘20∘ of knee flexion. During the tests, lateral X-ray fluoroscopy imaging was used to scan the knee joint. The dPTdPT differences between the passive state and the isokinetic concentric and extension were quantified at 15∘15∘ intervals of knee joint flexion angle. Furthermore, the changes of the dPTdPT as a function of the isometric muscle contraction intensities were determined during the isometric knee extension at 90∘90∘ and 20∘20∘ of knee joint flexion. Muscle contraction-induced changes in knee joint flexion angle during the isometric muscle contraction were also taken into account for the dPTdPT measurements. During the two isometric knee extensions, dPTdPT increased from rest to maximum voluntary muscle contraction (MVC) by 14–15%. However, when changes in knee joint flexion angle induced by the muscle contraction were taken into account, dPTdPT during MVC increased by 6–26% compared with rest. Moreover, dPTdPT increased during concentric and eccentric knee extension by 3–15%, depending on knee flexion angle, compared with passive knee extension. These findings have important implications for estimating musculoskeletal loads using modelling under static and dynamic conditions.