Time-of-day effect on patella tendon stiffness alters vastus lateralis fascicle length but not the quadriceps force–angle relationship

Aim To examine the time-of-day (TOD) effect on torque–force/angle, fibre length (FL), tendon stiffness (K), stress, and strain using the quadriceps muscle–tendon complex as a model. Methods Twelve healthy young men (aged 27±2.0 years) were studied at AM (7h45) and PM (5h45). Maximal isometric contractions were carried out on an isokinetic dynamometer, with real-time recordings of vastus lateralis (VL) FL and patella tendon K using B-mode ultrasonography. Percutaneous electrical twitch doublets superimposed on maximal torque were used to test for muscle activation capacity (AC). Results At PM, torque and force increased by 16±3.0% (P<0.01) over 30–90° knee angles. Where the load was standardised (at 250 N) in order to discriminate between torque generation capacity and tendon K changes, PM relative to AM, there were 8% and 13% (P<0.01) reductions in relaxed and contracted FL, respectively. Average K decreased by 21% (P<0.001) and the maximal stress and strain were increased at PM by 11% and 16%, respectively (P<0.01). No TOD effect on AC was seen. Conclusion The quadriceps torque or force–angle relationships shift upwards at PM vs. AM, with no shift in the position of the optimal knee angle. This torque or force increase appears not to be centrally modulated. Although K decreases with TOD thereby potentially shortening the working length of the sarcomeres, these changes overall do not affect the ability of the muscle to produce greater torque in the evening.