Changes in countermovement jump performance and the force-time waveform after anterior cruciate ligament reconstruction

Abstract

Anterior cruciate ligament (ACL) ruptures pose a substantial injury burden on athletes. Surgery (ACLR) is commonly recommended after ACL rupture. Only a small fraction of athletes return to pre-injury performance levels after ACLR with a high risk of non-contact re-injury. Significant functional impairments and altered movement patterns occur following ACLR which may further increase the risk of future re-injury. Objective return to play protocols are not well established after ACLR with traditional hop tests showing insufficient sensitivity for detecting compensatory movement patterns. The countermovement jump (CMJ) is a common neuromuscular test after ACLR, and bilateral force production is frequently measured on a dual force platform. Athletes with ACLR demonstrate significantly poorer jump performance post ACLR in the CMJ test, specifically within the braking and propulsive phases. However, limitations of these studies include the isolated use of discrete or CMJ phase-specific metrics to measure performance after ACLR instead of assessing the force-time data across the entire jump movement. Between-limb asymmetry indices are also used to quantify differences between involved and uninvolved limbs, but this may lead to a false positive indication of readiness given that ACLR impacts the strength of the uninvolved limb. Pre-injury data, CMJ force-time curve waveform analysis, and a longitudinal study design can help to address these gaps and develop new knowledge surrounding return to play protocols. This study therefore aimed to assess differences in CMJ force production before and after ACLR utilising traditional performance and asymmetry measures and statistical parametric mapping (SPM) analysis throughout an athlete’s rehabilitation up to two years post-surgery. Twenty (age = 21.6 ± 3.8 years) competitive athletes from alpine skiing, freestyle skiing, football, ski jump, and wrestling performed the CMJ as part of routine testing and monitoring before and after ACLR over a five year study period. Dual force plates measured the ground reaction force (GRF), and these force-time data were analysed using the Shiny vertical jump analysis app (https://github.com/mattsams89/shiny-vertical-jump) in RStudio. The difference in time between surgery and testing was calculated and stratified into five time intervals: pre-injury (T0), 24 ± 3 weeks (T1), 36 ± 3 weeks (T2), 48 ± 3 weeks (T3), and between 72 – 100 weeks (T4) after surgery. Traditional measures of jump performance and asymmetry were assessed using paired sample t-tests. SPM analysed differences in the continuous force-time data between the involved and uninvolved limbs at each of the post-surgical time periods and compared the involved limb to itself post-surgically to pre-surgical baseline testing values. Results showed that, compared to pre-injury baseline values, traditional discrete jump performance outcomes (jump height, contact time, RSImod) were decreased after ACLR, with lower peak force production for the involved limb and greater peak force asymmetry at six months post ACLR. Traditional discrete CMJ phase metrics showed lower impulse production of the involved limb after surgery up until one-year post-ACLR, although this was not statistically significant. There was a significant increase in braking impulse asymmetry at two years post ACLR compared to pre-injury values, favouring the involved limb. Propulsive impulse production was lower for the involved limb versus pre-injury baseline ~ two years after surgery, versus the uninvolved limb up to nine months post-surgery and showed greater asymmetry six months after surgery. SPM analysis confirmed that for the continuous force-time data, the involved limb had a lower unweighting and propulsive impulse production compared to pre-injury baseline values at six months post ACLR and showed a lower propulsive impulse production of the involved limb versus the uninvolved limb at six months and at two years after surgery compared to pre-injury baseline values. It is hoped that the results from this study contribute to the knowledge surrounding return to play protocols after ACLR and help inform practitioners with new evidence to promote a safer return to sport past traditionally determined return to play timelines.