Abstract
Objectives:
Athletes returning to sport following anterior cruciate ligament reconstruction (ACLR) face many challenges, including low rate of return to pre-injury level of sport, and high risk of reinjury. Milestone-based rehabilitation and return to sports testing protocols have been adopted to assess an athlete’s functional readiness to handle the rigors of their sport and to help make clinical decisions about returning to play. Strength testing, particularly of the quadriceps is always a standard test. Vertical countermovement jump (CMJ) tests on force plates have more recently become an integral part of return to sport testing due to the fact that they assess coordinated lower extremity stretch-shortening cycle function. The analysis of ground reaction force (GRF) profiles during the unweighting, braking and propulsive phases of takeoff provides valuable information pertaining to the storage and release of elastic energy and indicates biomechanical efficiency. Comparing involved versus noninvolved CMJ metrics from bilateral and unilateral CMJs in patients following ACLR provides a measure of lower extremity functional asymmetries. The purpose of this study was to assess the biomechanical efficiency of CMJs in athletes prior to return to sports following ACLR. It was hypothesized that CMJ metrics from the unweighting, braking and propulsive phases of the jump would be impaired on the involved side. Additionally, it was hypothesized that these CMJ functional asymmetries would only be partially explained by quadriceps strength asymmetries and that asymmetries in unweighting and braking phases of the jumps would be mostly independent of quadriceps weakness.
Methods:
Twenty-four patients who had undergone ACLR (18 male, 6 female; height: 176.2 ± 10.4 cm, mass: 76.5 kg ± 12.6 kg, age: 26 ± 7 years, time since surgery: 11.6 ± 7.7 months; range: 5 to 43 months) underwent isokinetic knee extension strength testing and performed maximal-effort bilateral and unilateral CMJs as part of a comprehensive return to sports testing protocol. Unilateral CMJs were performed with the propulsive foot on a single force plate while bilateral CMJs were performed with each foot on a separate force plate (BTS Bioengineering, Quincy, MA). Isokinetic knee extension strength was tested concentrically at 60° per second on the Biodex System 4 Dynamometer (Biodex Medical Systems, Inc., Shirley, NY). There were 15 CMJ metrics (Table 1): 2 in unweighting phase, 7 in braking phase, 4 in propulsive phase and 2 performance metrics. Separate paired t-tests were used to compare each CMJ force metrics during each type of jump, as well as the peak isokinetic knee extension torque, between the involved and noninvolved legs. Associations between asymmetries in CMJ metrics and quadriceps strength were assessed using Pearson correlation analysis. For bilateral jumps 7 of the 15 metrics could be compared between limbs (i.e. metrics such as jump height or power that use center of mass position or velocity cannot be compared between limbs during bilateral jumps).
Results:
During the bilateral CMJs (Table 1, Figure 1), asymmetries were evident for all 4 braking phase metrics and both propulsive phase metrics (Braking phase: force at low position 12% lower on involved side, p = 0.001; rate of force development 18% lower, p=0.031; eccentric force 17% lower, p=0.001; eccentric stiffness 18% lower, p = 0.001; Propulsive phase: mean propulsive force 12% lower, p < 0.001 and peak propulsive force 11% lower, p < 0.001). During unilateral CMJs (Table 1) asymmetries were evident for 1 of 2 unweighting phase metrics, 4 of 7 braking phase metrics, 3 of 4 propulsive phase metrics and both performance metrics (Unweighting phase: 12% less unweighting on the involved side p = 0.006; Braking phase: countermovement depth 10%, less, p = 0.001; peak braking power 16% lower, p < 0.001; force at low position 5% lower, p=0.004; eccentric force 15% lower p < 0.001; Propulsive phase: peak propulsive power 12% lower, p < 0.001; mean propulsive force 4% lower, p = 0.001; peak propulsive force 6% lower, p = 0.001; Performance metrics: jump height 19% less; reactive strength index 14% lower, p = 0.001). The involved quadriceps were 24 ± 19% weaker than the noninvolved side (p < 0.001). Strength asymmetry was not correlated with any of the 7 CMJ asymmetry metrics during the bilateral jumps. Strength asymmetry was correlated with only 2 of the 15 CMJ asymmetry metrics during the unilateral jumps (Braking phase: countermovement depth r = 0.465, p = 0.029. Propulsive phase: peak propulsive power r = 0.507, p = 0.016).
Conclusions:
Quadriceps weakness is a well-recognized problem after ACLR. However, biomechanical inefficiencies in coordinated lower extremity stretch-shortening cycle function have not been studied extensively. With the wider availability of force plates in clinics and training facilities, CMJ testing provides a necessary compliment to strength testing in assessing functional recovery after ACLR. CMJ metrics during bilateral and unilateral jumps clearly identify impairments in the storage (unweighting and braking phases) and release (propulsive phase) of elastic energy. Importantly these impairments were mostly independent of quadriceps weakness, indicating that full restoration of strength does not guarantee restoration of stretch-shortening cycle function across joints. Force plate CMJ analyses can provide important practical and clinical assessments to better inform readiness to return to play after ACLR.
