Lower limb stiffness testing in athletic performance: a critical review

Abstract

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.

Limits in reliability of leg-spring and joint stiffness measures during single-leg hopping within a sled-based system

“…Similar to the findings of Hobara and colleagues [21] k leg and k joint increased with increases in HopFreq in the present study. Values of 13.9–28.9 kN.m -1 have been reported for k leg during natural hopping conditions on different surfaces and at different frequencies [22,23,30,45,46]. In addition, values of 6.9–12.0 Nm.deg -1 have been reported for k ankle during natural hopping [45].…”

“…drop / rebound jumps, hopping) while attached to a chair / platform (sled) that slides up and down the inclined rails. SBS regulate levels of mechanical load experienced during SSC tasks by (i) altering rail angle [27]; (ii) employing a winch system to control initial eccentric loading [19,29]; and (iii) reducing movement degrees-of-freedom [10,30]. For lower body SBS, performance of SSC tasks while attached to the chair limits upper body movement and allows researchers to control impact velocities and eccentric loading [8,19].…”

Measuring biomechanical loads in team sports – from lab to field

“…Net moments about the knee joint structure for example, can be used as an indicator of loading magnitude and injury risk of the anterior cruciate ligament [23,24]. Likewise, measures of joint or leg stiffness, which is the resistance of a structure to withstand the forces acting on it, have been demonstrated to be sensitive to training status [25], running speed [26] and exercise-induced fatigue [27,28] (see [29] for an extensive discussion of the use of stiffness measures in sports). Quantifying structure-specific loading parameters can thus be informative for evaluating the risk of injury or biomechanical adaptations to training.…”

Leg Stiffness and Vertical Stiffness of Habitual Forefoot and Rearfoot Strikers during Running

“…Previous studies confirm that a stiff leg may facilitate the efficient storage and reutilization of elastic energy, thereby enhancing the athletic performance in stretch–shortening cycles during running [ 18 ]. The Kleg and the Kvert play an important role during running efficiency, but limited evidence is available regarding the effect of the foot strike pattern on stiffness during running [ 19 , 20 ].…”