Vertical stiffness asymmetries during drop jumping are related to ankle stiffness asymmetries.

Asymmetry in vertical stiffness has been associated with increased injury incidence and impaired performance. The determinants of vertical stiffness asymmetry have not been previously investigated. Eighteen healthy men performed three unilateral drop jumps during which vertical stiffness and joint stiffness of the ankle and knee were calculated. Reactive strength index was also determined during the jumps using the ratio of flight time to ground contact time. "Moderate" differences in vertical stiffness (t17  = 5.49; P < 0.001), "small" differences in center of mass displacement (t17  = -2.19; P = 0.043), and "trivial" differences in ankle stiffness (t17  = 2.68; P = 0.016) were observed between stiff and compliant limbs. A model including ankle stiffness and reactive strength index symmetry angles explained 79% of the variance in vertical stiffness asymmetry (R2  = 0.79; P < 0.001). None of the symmetry angles were correlated to jump height or reactive strength index. Results suggest that asymmetries in ankle stiffness may play an important role in modulating vertical stiffness asymmetry in recreationally trained men.

Do stiffness and asymmetries predict change of direction performance?

Change of direction speed (CODS) underpins performance in a wide range of sports but little is known about how stiffness and asymmetries affect CODS. Eighteen healthy males performed unilateral drop jumps to determine vertical, ankle, knee and hip stiffness, and a CODS test to evaluate left and right leg cutting performance during which ground reaction force data were sampled. A step-wise regression analysis was performed to ascertain the determinants of CODS time. A two-variable regression model explained 63% (R2 = 0.63; P = 0.001) of CODS performance. The model included the mean vertical stiffness and jump height asymmetry determined during the drop jump. Faster athletes (n = 9) exhibited greater vertical stiffness (F = 12.40; P = 0.001) and less asymmetry in drop jump height (F = 6.02; P = 0.026) than slower athletes (n  = 9); effect sizes were both "large" in magnitude. Results suggest that overall vertical stiffness and drop jump height asymmetry are the strongest predictors of CODS in a healthy, non-athletic population.

Lower limb stiffness testing in athletic performance: a critical review.

Stiffness describes the resistance of a body to deformation. In regard to athletic performance, a stiffer leg-spring would be expected to augment performance by increasing utilisation of elastic energy. Two-dimensional spring-mass and torsional spring models can be applied to model whole-body (vertical and/or leg stiffness) and joint stiffness. Various tasks have been used to characterise stiffness, including hopping, gait, jumping, sledge ergometry and change of direction tasks. Appropriate levels of reliability have been reported in most tasks, although they vary between investigations. Vertical stiffness has demonstrated the strongest reliability across tasks and may be more sensitive to changes in high-velocity running performance than leg stiffness. Joint stiffness demonstrates the weakest reliability, with ankle stiffness more reliable than knee stiffness. Determination of stiffness has typically necessitated force plate analyses; however, validated field-based equations permit determination of whole-body stiffness without force plates. Vertical, leg and joint stiffness measures have all demonstrated relationships with performance measures. Greater stiffness is typically demonstrated with increasing intensity (i.e., running velocity or hopping frequency). Greater stiffness is observed in athletes regularly subjecting the limb to high ground reaction forces (i.e., sprinters). Careful consideration should be given to the most appropriate assessment of stiffness on a team/individual basis.