Including jump height when normalizing single hop impact kinetics can change the directionality of findings

Highlights

• Different methods to normalize kinetic outcomes can influence their interpretations.

• Hop height and raw impact forces are reduced in the surgical limb of anterior cruciate ligament reconstruction patients.

• Impact forces are greater in the surgical limb when normalizing by potential energy.

Abstract

Background

Assessing landing kinetics during hop testing could improve return to sport decisions following anterior cruciate ligament reconstruction (ACL) reconstruction. However, different methods for normalizing kinetic outcomes could influence the interpretation of landing kinetics and therefore the clinical recommendations.

Methods

Twenty-one females who had returned to sport following primary unilateral ACL reconstructed completed two single hops for maximum distance on each limb. Hop distance, hop height, peak impact force, and impulse were computed for each hop, and peak impact force and impulse magnitudes were assessed when 1) non-normalized 2) normalized by bodyweight, and 3) normalized by peak potential energy during the hop.

Findings

Along with hop distance and height, peak impact force and impulse were found to be lower on the surgical limb relative to the non-surgical limb for both non-normalized data and when normalized to bodyweight only (p < 0.001, d > 0.95). However, peak impact force and impulse were found to be higher on the surgical limb relative to the non-surgical limb when normalizing outcomes to peak potential energy (p < 0.001, d > 1.03).

Interpretation

Different normalization methods result in different interpretations of single hop kinetics. ACL reconstruction patients have shorter hop distances, lower hop heights, lower force magnitudes, and worse energy absorption when hopping on their surgical limb, relative to their non-surgical limb. We believe that normalizing landing kinetics using bodyweight and using peak potential energy provide different information, and as such, we suggest that future research use both methods based on the research question.

Introduction

The annual incidence of anterior cruciate ligament (ACL) injuries continues to rise, especially in sport (Agel et al., 2016). Athletes returning to sport after ACL reconstruction (ACLR) surgery currently have a high-risk of staining a second ACL injury (Webster and Feller, 2016). Single leg hop testing is one clinic-based method widely used to determine readiness for return to sport (Ithurburn et al., 2015; Orishimo et al., 2010). While patients with more symmetric hop distances are more likely to successfully return to sport and have better patient reported outcomes (Ardern et al., 2011), there is limited evidence that better hop distance symmetry is associated with a decrease in second ACL injury risk (Kyritsis et al., 2016). As poor and asymmetric landing mechanics are associated with the risk for second ACL injuries (Paterno et al., 2010), evaluating landing mechanics in addition to hop distance could improve the diagnostic ability of hop testing. Impact force is an important biomechanical measure when assessing ACL injury risk, as cadaveric modeling has shown that greater peak impact force is positively correlated with ACL strain (Bakker et al., 2016), and human subjects testing has demonstrated that peak impact force is predictive of primary ACL injuries in young female athletes (Leppänen et al., 2017). Additionally, impact force can be reliably measured using load sensing insoles or portable force plates (Peebles et al., 2018; Walsh et al., 2006), which are ideal for use in clinical settings.

As there is a strong association between bodyweight and kinetic outcomes (e.g. peak force), normalizing kinetic outcomes by dividing each outcome by the participant's bodyweight is commonly performed in biomechanics research to reduce the influence of bodyweight on study outcomes (Messier et al., 2018; Paterno et al., 2010; Peebles et al., 2019, Peebles et al., 2018; Stickley et al., 2018). However, this normalization procedure has been debated in recent literature, (Stickley et al., 2018), with some authors moving away from normalizing kinetic outcomes by bodyweight, and instead including bodyweight as a covariate in their statistical analysis (Messier et al., 2018). Examining the non-normalized magnitude of impact forces during hop testing could provide additional insight into injury risk, as a recent modeling study found that non-normalized peak vertical ground reaction force had the strongest association with peak ACL strain when assessing sagittal-plane hip and knee kinematic and kinetic outcomes during a simulated unilateral landing tasks (Bakker et al., 2016). In light of this discussion, an analysis of normalization technique on single hop impact kinetics is warranted.

Conflicting results have been reported when assessing impact force during unilateral hop testing in ACLR patients (Myer et al., 2012; Peebles et al., 2019). Peebles et al., found that ACLR patients have lower peak impact force and impulse when completing a single hop for maximum distance on their surgical limb, relative to their non-surgical limb (Peebles et al., 2019). In contrast, Myer et al., found that ACLR patients have higher peak impact forces when completing the single hop for maximum height on their surgical limb, relative to their non-surgical limb (Myer et al., 2012). While these two studies had contradicting conclusions, the methods used to normalize impact forces were different and could explain the outcome differences. Peebles et al., normalized impact force to bodyweight alone (Peebles et al., 2019), however, to account for between-limb differences in hop height, Myer et al., normalized impact force by peak hop potential energy, taken as the product of bodyweight and hop height (Myer et al., 2012). It is currently unclear if the differing conclusions from these previous studies is the result of the different methods used to normalize landing kinetics.

It is unclear what impact normalization choices have on kinetic outcomes and their interpretation during return to sport hop testing. The purpose of this study was to compare impact kinetic symmetry between the surgical and non-surgical limbs of ACLR patients when using 1) non-normalized measures, 2) normalizing impact force with bodyweight only and 3) normalizing with bodyweight and jump height (i.e. peak potential energy). Based on the potential disparities in hop height between the surgical and non-surgical limbs of ACLR patients, we hypothesized that including hop height when normalizing impact force will lead to differences in study results and the interpretation of these results.