Do Training Induced Changes in Quadriceps Tendon Stiffness Result in Reflective Changes in Jump Kinematics?

Abstract

Both high-load strength training and high-velocity power training are effective means of increasing vertical jump height. However, only strength training is able to elicit increases in tendon stiffness. Because a stiffer tendon will require more force to adequately lengthen it and will recoil at a greater velocity when lengthened changes in tendon stiffness may alter joint kinematics differently than power training. PURPOSE: To determine if training induced changes in tendon stiffness will alter jump kinematics. HYPOTHESIS: Strength training will increase tendon stiffness that will elicit subsequent changes in jump kinematics that differ from those experienced by power training. METHODS: 36 physically active but non-strength trained males were randomly assigned to one of four groups: a strength training group that performed high-resistance parallel-depth squats (SQ-P, n = 8), one of two power training groups that performed high-speed jump squats from either a self selected depth (JS, n = 10) or parallel depth (JS-P, n =9) or a control group (C). Each intervention was 8-weeks in duration before and after which subjects had their quadriceps tendon stiffness measured during a ramp isometric contraction with tandem ultrasonography and their knee kinematics measured using high-speed cinematography during a maximal vertical jump. Comparisons were made between groups and time using a 4x2 mixed model MANOVA with Bonferoni post-hoc (p < 0.05). RESULTS: Quadriceps tendon stiffness increased in SQ-P (p = 0.001) but not JS, JS-P or C. Vertical jump knee kinematics showed a main effect of time on peak knee flexion (p = 0.001) and eccentric knee velocity (p = 0.005), but not in peak concentric knee velocity or acceleration or movement duration. Post-hoc analysis revealed JS and JS-P increased eccentric velocity and knee flexion while SQ-P and C had no significant changes to jump kinematics. CONCLUSION: While SQ-P resulted in an increase in quadriceps tendon stiffness it did not result in any alterations in vertical jump knee kinematics. In contrast, power training, which did not influence tendon properties, resulted a faster and shallower countermovement. These results do not support our primary hypothesis and suggest that factors outside of quadriceps tendon stiffness will have a greater effect on joint kinematics.