Despite the explosion of research on ACL injury during the last 30-years, the exact mechanism of injury is still not fully understood. However, a combination of internal (e.g. anatomic, hormonal, neuromuscular) and external factors (e.g. environment, footwear, ground, opposing players) may adequately explain the mechanism [8, 11].
The greatest strain on the ACL occurs during 3-dimensional knee loading, which includes a knee extension moment, proximal anterior tibial shear force, knee valgus/varus moment, as well as a rotational contribution . After ACL reconstruction, altered movement mechanics in jump-landing tasks are often apparent  and have shown to increase the risk of re-injury by 5-15 fold .
Focussing on particular movements that have been associated to ACL injury during landing tasks , such as valgus rotation at the knee joint, movements screens such as the LESS have been developed to identify athletes who may be at risk of sustaining an ACL injury .
What is the Landing Error Scoring System (LESS)?
The Landing Error Scoring System (LESS) is a relatively easy-to-use assessment tool to analyse the biomechanics of the lower extremities in a landing and jumping task. As a reliable clinical screening tool, it is described to offer the greatest value for the identification of individuals at risk of attaining non-contact ACL-injury . However, recent research has questioned its ability to identify those at-risk of ACL injury due to a lack of validity .
Based on a 19-point continuous scale (see FREE downloadable scoring sheet), the LESS assesses the positioning of the trunk and lower extremities at various stages through the Drop-Vertical Jump (DVJ) movement. Global fluidity and range of motion in the landing phase are analysed from frontal and sagittal plane video data. Movement patterns deviating from the ‘biomechanical optimum’ can possibly predispose an athlete to lower-extremity injuries . Those are scored as “error-points”, and add up to a LESS score. Poor landing technique is indicated by higher LESS scores (more “error-points”) and vice versa.