Silva, K. E., Riebe, D. A., & Earp, J. E. (2016). Do training induced changes in quadriceps tendon stiffness result in reflective changes in jump kinematics? Medicine & Science in Sports & Exercise, 48(5), Supplement abstract number 694.

"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."

This study determined if training induced changes in tendon stiffness altered jump kinematics. The underlying hypothesis of this investigation was: Strength training will increase tendon stiffness that will elicit subsequent changes in jump kinematics that differ from those experienced by power-training. Physically active but non-strength trained males (N = 36) were randomly assigned to one of four groups: a strength-training group that performed high-resistance parallel-depth squats (N = 8), one of two power-training groups that performed high-speed jump-squats from either a self-selected depth (N = 10) or parallel depth (N = 9), or a control group (N = 9). Each intervention lasted eight weeks before and after which Ss 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.

Quadriceps tendon stiffness increased in the high-resistance parallel depth-squat group but not in the other three groups. Vertical-jump knee kinematics showed a main effect of time on peak knee-flexion and eccentric knee-velocity, but not in peak concentric knee-velocity or acceleration or movement duration. Post-hoc analysis revealed self-selected depth jump-squats and parallel jump-squats increased eccentric velocity and knee flexion while the high-resistance parallel depth-squats group and the control group had no significant changes to jump kinematics.

Implication. While high-resistance parallel squatting resulted in an increase in quadriceps tendon stiffness it did not result in any alterations in vertical jump knee-kinematics. Contrastingly, power-training, which did not influence tendon properties, resulted in a faster and shallower countermovement. These results do not support the primary hypothesis and suggest that factors outside of quadriceps-tendon stiffness will have a greater effect on joint kinematics.

The increased muscle and tendon stiffness that results from high-resistance training did not translate into performance or movement-factor improvements. Performance of the knee was only influenced positively by training activities that concentrated on movement (power-training). High-resistance strength training is not an avenue for improving movement characteristics.

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