Strength and power assessment in rehabilitation: profiling the athlete’s return to sport journey following ACL reconstruction

Residual deficits in athletic performance are common despite rehabilitation guidelines following anterior cruciate ligament (ACL) reconstruction including criterion-based progressions to protect healing structures, ensure safe restoration of fundamental physical capacities, and guide appropriate return to sports (RTS) activities. The aim of the proposed research was to examine strength and power in the rehabilitation pathway of soccer players following ACL reconstruction. This enabled us to explore interrelationships between physical capacities, movement strategies, and subsequent injury risk. These data were also compared with pre-injury values and matched controls to more fully examine the overall level of physical preparedness following ACL reconstruction, and to examine the validity of alternative approaches to determine RTS status such as composite physical capacity profiling. To address our aims, a systematic review was completed to examine the physical ability of athletic populations in the later stages of rehabilitation in comparison to healthy controls. The results showed deficits in knee peak extension and flexion in adult males at more than 6 months post ACL reconstruction, which were influenced by graft type, and can be mitigated by targeted rehabilitation programs. Insufficient evidence was available to examine rate of force development and reactive strength. The relationships between fundamental physical capacities and biomechanical variables during dynamic movement tasks were then explored through a narrative review using a structured search criteria. Quadriceps strength and rate of torque development explained a moderate portion of the variance in aberrant kinetic and kinematic strategies commonly detected in ACL reconstructed cohorts in the later stages of rehabilitation, prior to RTS. Increasing our understanding of these inter-connected aspects is required to improve rehabilitation outcomes and to reduce the risk of secondary injury following RTS. The findings from our comprehensive review of the available literature led us to explore the recovery pattern of strength and power qualities during rehabilitation and at the time of RTS relative to pre-injury baseline data and those of healthy matched controls. We also examined the effect of these physical qualities has on performance and task execution during tasks which may be associated with subsequent re-injury risk. In addition to the systematic review, three experimental studies were designed each with specific aims.

The aim of study 1 was to examine changes in strength and power characteristics at the time of RTS relative to pre-injury baseline data and healthy matched controls. The main finding was that strength and power in professional soccer players at RTS following ACL reconstruction were often reduced compared to preinjury values and controls. Compared to pre-injury, ACL normalised quadriceps peak torque of the involved limb, SLCMJ height and Reactive Strength Index modified (RSImod) were reduced following ACL reconstruction, even after the completion of rehabilitation. No significant reductions in bilateral CMJ height, RSImod and relative peak power were indicated at RTS in the ACL group when compared to pre-injury values, but deficits were present relative to controls. The uninvolved limb significantly improved quadriceps and hamstring strength from pre-injury to RTS. No significant differences from baseline were shown in SLCMJ height, power and reactive strength of the uninvolved limb following ACL reconstruction.

In study 2 we investigated if greater physical capacity results in different SLDJ mechanics in adult male soccer players following ACL reconstruction. Moderate to large significant differences between the ACL reconstructed and uninjured limb in SLDJ performance (d = 0.92 – 1.05), kinetic (d = 0.62 – 0.71) and kinematic variables (d = 0.56) were evident. Stronger athletes jumped higher (p = 0.002; d = 0.85), produced greater concentric (p = 0.001; d = 0.85) and eccentric power (p = 0.002; d = 0.84). Similar findings were present for RSI, but the effects were larger (d = 1.52 – 3.84). Weaker players, and those who had lower RSI, displayed landing mechanics indicative of a “stiff” knee movement strategy and this may be indicative of heightened injury risk.

The final experiment chapter (study 3) examined the utility of the Total Score of Athleticism (TSA), a composite score including strength, power, and reactive strength assessments to aid RTS decision making, moving beyond the current practice of limb symmetry thresholds and their inherent limitations. A large difference was evident between ACL reconstructed and uninjured players in TSA score (d = 0.84). For every additional increase of one unit in the TSA, the odds of belonging to the ACL reconstructed group decreased by 74% (95%CI 0.19, 0.56). A case series also showed there was a higher frequency of low TSA scores in players who sustained a second injury following RTS. These preliminary findings indicate the TSA may be a useful RTS readiness tool and can be used to set benchmarks, and rehabilitation goals for restoration of physical performance.

This thesis provides an original and significant contribution to the existing research. The cumulative findings suggest that: i) strength, power and reactive strength are reduced in elite male professional soccer players at the later stages of rehabilitation and at the time of RTS; ii) these have detrimental effects on kinetic and kinematic variables in dynamic tasks, with weaker players who also have a lower RSI displaying aberrant strategies commonly associated with increased re-injury risk; iii) when assessing recovery of physical capacities, clinicians and coaches should consider both absolute scores on each limb and not just symmetry values. In situations where baseline pre-injury data are not available, comparisons to uninjured matched controls should be made to ensure minimum standards are met; and iv) a TSA can be used to aid RTS decision making due to its ability to differentiate between injured and un-injured athletes, and preliminary evidence suggesting TSA scores are likely to be lower in players who sustain a re-injury after RTS. Further research could prospectively monitor rebounding tasks (e.g., SLDJ) performance and biomechanics, with the implementation of wearable technology (e.g. IMU system) and include these data along with the existing tests which comprised the TSA in the current thesis. Also, a broader range of physical capacities (e.g. aerobic fitness, speed, change of direction, etc.) could be added to our TSA to ensure the all relevant physical performance characteristics are assessed. Using this broader test battery and subsequent TSA composite profile, prospective analysis of secondary injuries following return to sport is warranted to more clearly elucidate its ability to identify associations with re-injury risk.