Single vs double leg countermovement jump tests not half an apple ! –

INTRODUCTION Historically, following anterior cruciate ligament reconstruction (ACLR), limb symmetry indexes calculated using postinjury contralateral performance in a series of single leg hop tests have been used to guide return to sport (RTS) decision making1. However, two major limitations of this approach have been highlighted: 1. Declines in contralateral healthy limb performance undermine the value of a limb symmetry index as a benchmark for RTS2. 2. Normalisation of output variables such as distance hopped does not equate to recovery of underlying functional deficits identified by biomechanical assessment of movement strategy1,3. In most sports and clinical environments, financial and/or time costs limit systematic use of 3D motion capture and tri-axial force plates for biomechanical assessments. However, assessments use of dual force platform single axis technology, allowing the assessment of vertical ground reaction forces (vGRF), and asymmetries thereof, during double and single leg jump-land activities is now commonplace in these settings. This has led to an increase in the availability of healthy individual limb kinetic data, reducing the dependence on contralateral limb as a benchmark during rehabilitation. In addition, while these measurements do not permit the quantification of jointspecific contributions that 3D kinematics provides; however, associations between vRGF and knee kinetic asymmetries following ACLR4,5 mean that these data are considered clinically relevant in the context of rehabilitation to quantify the magnitude of inter-limb asymmetries5-8 and the effect of specific interventions9. Furthermore, specific bilate-ral (combined limb output) variables in the countermovement jump (CMJ) also appear to be provide additional insight on injury induced alterations in movement “strategy”9,10. It is well documented that dual force platform jump-land tests reveal kinetic asymmetries months to years after RTS following ACLR, with landing phase asymmetries in the double limb (DL) drop jump (DJ) a consistent finding, particularly in female athletes11. More recent reports show similar associations between heightened asymmetries in the take-off (eccentric and concentric) and landing phases of the DLCMJand prior ACLR and other lower-limb injuries10,12,13. The increased use of force platforms in performance settings and published research14 has however highlighted that an athlete’s interlimb asymmetries derived from single leg (SL) and the double leg (DL) CMJ tests may not align either in their magnitude or direction. This observation has in turn led many practitioners to ask: which of these provides a better or more accurate measure of asymmetry? We highlight two opposing viewpoints from the literature which frame this question, and suggest that a simple answer is likely not apparent: