The influence of external load on quadriceps muscle and tendon dynamics during jumping

Purpose: The influence of external loading on muscle and tendon dynamics during maximal vertical jumping was explored. Methods: Ten resistance-trained men performed parallel-depth, countermovement vertical jumps with and without additional load (0%, 30%, 60%, and 90% of maximum squat lift strength), while joint kinetics and kinematics, quadriceps tendon length (LT) and patellar tendon FT and RFDT were estimated using integrated ultrasound, motion analysis and force platform data and muscle tendon modelling. Results: Estimated FT and RFDT, but not peak LT, increased with external loading. Temporal comparisons between 0% and 90% loads revealed that FT was greater with 90% loading throughout the majority of the movement (11%–81% and 87%–95% movement duration). However, RFDT was greater with 90% load only during the early movement initiation phase (8%–15% movement duration) but was greater in the 0% load condition later in the eccentric phase (27%–38% movement duration). LT was longer during the early movement (12%–23% movement duration) but shorter in the late eccentric and early concentric phases (48%–55% movement duration) with 90% load. Discussion: External loading positively influenced peak FT and RFDT but tendon strain appeared unaffected, suggesting no additive effect of external loading on patellar tendon lengthening during human jumping. Temporal analysis revealed that external loading resulted in a large initial RFDT that may have caused dynamic stiffening of the tendon and attenuated tendon strain throughout the movement. These results suggest that external loading influences tendon lengthening in both a load- and movement-dependent manner.