Lower limb strength and biomechanics after anterior cruciate ligament reconstruction

Rupture of the anterior cruciate ligament (ACL) is one of the most significant injuries to the knee joint, with the frequency of injury increasing over the last 10 years. Of these injuries, the increase in incidence among young female athletes (<18 years) has been especially significant. Direct and indirect management of ACL injuries range from $100 million in countries like Australia, to as much as $2 billion in the United States. The increasing rates of ALC injury and significant associated costs places significant pressure on the healthcare system.

The high economic cost of ACL injuries is typically associated with ACL reconstruction (ACLR) and the subsequent rehabilitation period. Restoration of lower limb muscle strength, function, and coordination, as well as a gradual return to activities like running, jumping, landing, and agility tasks are all components of a structured rehabilitation program and criteria for return to sports (RTS). Following the completion of rehabilitation, up to 80% of people are able to RTS of some level. Despite the high rate of RTS, a significant number of ACLR individuals will report poor subjective knee function (e.g., knee pain during activity), be subjected to a high risk of reinjury and be prone to early onset of knee osteoarthritis. There is evidence that these poor outcomes are worse in females than in males.

Lower limb strength (e.g., hamstrings and quadriceps) and biomechanical asymmetries are common after ACLR. These asymmetries have been associated with the poor outcomes previously mentioned. As a result, restoration of maximal hamstrings and quadriceps strength symmetry is a focus of rehabilitation and criteria for RTS clearance following ACLR. However, there is evidence that explosive quadriceps strength does not recover at the same rate as maximal quadriceps strength during the first year following ACLR. Whether this is also true in the hamstrings is still unknown and previous studies have only explored concurrent recovery of explosive and maximal strength in males.

Given their function in providing dynamic stability and loading on the knee joint, the hamstrings and quadriceps have received much attention during assessment, rehabilitation, and criteria for RTS following ACLR. However, dynamic tasks (e.g., sidestep cutting) commonly performed in team sports require complex activity and coordination of the different lower limb muscles. This has been previously investigated in healthy individuals but to date, it is still unknown how ACLR affects the function of the different lower limb muscles during sidestep cutting. Additionally, reductions in knee joint loading (e.g., contact force) have been reported from 3-9 months and up to 2 years following ACLR. Quadriceps strength deficits have been proposed to be a major factor influencing the reduced knee joint contact forces after ACLR. However, it is still unknown whether knee joint contact forces are reduced after the restoration of quadriceps strength at RTS.

The purpose of this doctoral thesis was two-part. Firstly, to investigate restoration of both explosive and maximal hamstrings and quadriceps strength during early and late rehabilitation following ACLR in males and females. Second, to explore lower limb biomechanics following the restoration of strength following rehabilitation. The knowledge derived from this program of research is aimed at identifying factors that are modifiable during the rehabilitation period after ACLR, information that should help to guide future clinical and research effort.

The first study of this program of research (Chapter 2) was a systematic review and meta-analysis that explored the time-course of hamstrings and quadriceps strength asymmetries during the preoperative period up to six and 12 months following ACLR between males and females. Initial database search retrieved 6,046 articles. After screening for eligibility, 31 studies were included in the systematic review while 13 articles had enough data for meta-analysis. The findings showed that limb symmetry in maximal hamstrings and quadriceps strength are the most commonly used measure of strength following ACLR. Strength asymmetries in the hamstrings and quadriceps were present from preoperative to six and 12 months after ACLR. Despite the proposed importance of explosive strength following ACLR, studies looking at its time-course of recovery are limited. Furthermore, while sex differences in patient outcomes have been previously reported, majority of the data collected were either not stratified and/or dominated by male participants (males = 62%; females = 30%, sex not reported = 8%).

To address gaps in the literature identified in Chapter 2, an observational cohort study was conducted for the second study of this thesis (Chapter 4). This study investigated the maximal and explosive strength recovery of the quadriceps and hamstrings following ACLR. In this study, participants were assessed during the early (3-6 months) and late (7-12 months) stage of rehabilitation following an ACLR with hamstring tendon (HT) autografts. There was a significant influence of time after ACLR on the limb-symmetry index (LSI) for maximal hamstrings (Early: 86 ± 14; Late 92 ± 13; p = 0.005) and quadriceps (Early, 73 ± 15; Late 91 ± 12; p <0.001) strength. Additionally, explosive quadriceps strength LSI showed significant improvements over time (Early: 82 ± 30; Late: 92 ± 25; p = 0.03). However, despite the recovery of maximal hamstring strength there were still significant deficits in explosive hamstring measures later in rehabilitation (Early: 86 ± 46; Late: 83 ± 22; p = 0.75). Additionally, Chapter 4 also investigated whether there were differences in strength recovery between males and females following ACLR. While no differences were found in the rate of explosive and maximal strength recovery between sexes, females had greater quadriceps strength asymmetries (maximal and explosive) compared to males across ACLR rehabilitation.

The ability to perform dynamic tasks (e.g., sidestep cutting) is one of the major determinants of an ACLR individual’s readiness to RTS. Sidestep cutting tasks, in particular, are common in change-of-direction sports. It is also during these tasks that ACL injuries frequently occur. Previous studies found kinematic and kinetic impairments during sidestep cutting performance in ACLR individuals. However, these studies have been joint level analysis of lower limb biomechanics. Given the complex coordination of the different lower limb muscles during the performance of a sidestep cut, the third study of this thesis (Chapter 5) explored the lower limb muscle contributions to ground reaction forces during vertical support, deceleration, propulsion, and redirection of forces during a sidestep cut in ACLR limbs (who had a quadriceps strength LSI ≥ 90%) and compared them to healthy limbs. Chapter 5 found that muscle function during a sidestep cut is significantly different in the ACLR limb when compared to the contralateral and control limbs. There were less contributions to vertical support (contralateral mean difference = -0.040 BW.s, 95%CI = -0.049 to -0.031, p < 0.001; control mean difference = -0.042 BW.s, 95%CI = -0.061 to -0.022, p < 0.001), braking (contralateral mean difference = 0.020 BW.s, 95%CI = 0.014 to 0.027, p < 0.001; control mean difference = 0.029 BW.s, 95%CI = 0.017 to 0.041), and medial redirection (contralateral mean difference = -0.006 BW.s, 95%CI = -0.01 to -0.001, p = 0.011) GRFs from the quadriceps of the ACLR limb when compared to the contralateral uninjured limb. Alterations in gluteus maximus, gastrocnemius, soleus, hamstrings, and dorsiflexors muscle function were also found when comparing the ACLR and contralateral uninjured limbs. Despite resolution of quadriceps strength asymmetry following ACLR rehabilitation, the quadriceps’ role in contributing forces for the execution of a sidestep cut is significantly impaired. Furthermore, muscle contributions from other major lower limb muscles are also altered following RTS.

Given the alterations in the ability of the quadriceps to modulate GRFs despite restoration of isokinetic strength symmetry, the final study of this thesis (Chapter 6) was conducted with the aims of investigating patellofemoral (PFJ) contact forces in the ACLR limb when compared to healthy limbs at time of RTS. Chapter 6 demonstrated that ACLR limbs have lower PFJ contact forces compared to the contralateral (mean difference = 5.89 BW, 95%CI = 4.7 to 7.1, p < 0.001) and control limbs (mean difference = 4.44 BW, SE = 2.1 to 6.8, p = < 0.001). Additionally, the ACLR limb possessed smaller knee flexion angles (contralateral mean difference = 4.88°, 95%CI = 3.0 to 6.7, p < 0.001; control mean difference = 6.01°, 95%CI = 2.0 to 10.0, p < 0.002) as well as lower knee extension moment and quadriceps force (contralateral mean difference = 4.14 BW, 95%CI = 3.4 to 4.9, p < 0.001; control mean difference = 2.83 BW, 95%CI = 1.4 to 4.3, p < 0.001). These findings suggest that PFJ loading can still be impaired despite the restoration of quadriceps strength symmetry which could have potential implications for PFJ osteoarthritis.

In conclusion, this program of research showed that explosive and maximal quadriceps strength asymmetries resolve during ACLR rehabilitation. Hamstrings maximal strength also restores during the same time; however, explosive hamstrings strength did not. While it was also found that sex does not influence strength recovery, females did have larger maximal and explosive quadriceps strength asymmetries compared to males following ACLR. Finally, impairments in lower limb biomechanics (less quadriceps muscle contributions to vertical support, deceleration, and medial redirection, lower PFJ contact force and quadriceps force, and smaller knee flexion angle) are still present in the ACLR limb compared to the healthy limbs during the performance of a sidestep cut. These deficits still exist, despite the recovery of maximal quadriceps strength following ACLR and provides evidence for the assessment of lower limb muscle function during dynamic movements as part of the RTS criteria.