Muscle force contributions to knee joint loading

Anterior cruciate ligament (ACL) injuries are one of the most common knee injuries suffered by athletic populations. ACL injuries are particularly burdensome due to potential surgical requirements, extensive rehabilitation time and associated financial costs for the individual and the community. Additionally, ACL injuries are associated with increased risk of early onset knee osteoarthritis. As such, ACL injury preventative and rehabilitative strategies are of paramount importance.

ACL injuries typically occur during non-contact dynamic tasks, such as unanticipated sidestep cutting. At the time of injury, the knee joint experiences relatively large degrees of knee valgus and rotation (either internal or external) and high mechanical loads. These loading patterns, along with the anterior shear force, are known to increase loads on the ACL, especially in combination with each other. Muscles produce forces that can cause and oppose these knee joint loads, and therefore play a critical role in dictating the size and the nature of the loads experienced by the ACL. Prior research has investigated the role of muscle force in ACL load development, and has indicated that the hamstrings are most capable of reducing ACL loads. Subsequently, any pathology that may influence hamstring function may increase the risk of ACL injury.

Some studies have shown that participants with a history of hamstring strain injury (HSI) have lower knee flexor strength and hamstring muscle activation compared to healthy legs. Consequently, a relationship between prior HSI and ACL injury could exist. However, establishing this relationship is difficult due to the relatively low incidence of ACL injury. Subsequently, prospective studies aiming to investigate this relationship would be very costly, due to the requirement of very large sample sizes and long follow-up periods. Additionally, such a relationship would depend on the functional role of the hamstring muscle group during potentially ACL-injurious manoeuvres such as sidestep cutting, which has not been fully elucidated. Furthermore, given the multi-planar demands of tasks that place the ACL at risk of injury, better understanding the contribution of the individual hamstring muscles to knee joint loading relative to the other lower-limb muscles is imperative.

Musculoskeletal simulation offers the ability to analyse cause-effect relationships between muscle force development and joint loading whilst accounting for whole body kinematics. This analysis could not only reveal the true potential of the hamstring muscles in protecting the ACL, but could also elucidate the role of other muscles which have been less studied.

The purpose of this doctoral thesis was to explore the relationship between muscle forces in the development of knee joint loading during potentially injurious manoeuvres, as this knowledge may be used to inform interventions that aim to reduce ACL injury risk. Given recent hypotheses suggesting a possible association between prior injury to the hamstrings and an increased risk of ACL injury and based on the current literature, which indicates that the hamstrings are one of the most important muscle groups for unloading the ACL, the focus of the first study (Chapter 4) was to determine the impact of HSI on hamstring function. Specifically, a systematic review and meta-analysis was used to compare knee flexor strength and flexibility in previously injured legs to the uninjured contralateral leg. It was found that deficits in concentric and eccentric strength (and associated hamstring to quadriceps strength ratios) were present at and after return to play. Isometric strength deficits were also present after HSI, but these recovered within 20-30 days. Hamstring flexibility deficits were also found after HSI, but these recovered within 40-50 days post injury. A secondary aim of this study was to document the totality of measures reported in the literature that have been taken in previously injured hamstrings. The review revealed that knee flexor and extensor strength were the most commonly assessed variables in participants with previously injured hamstrings and that there are few studies which examine the function of other lower-limb muscles. Furthermore, there was limited information examining multi-planar movements. The findings of the review highlighted the need to better understand how the hamstrings contribute to knee joint loading, relative to the contribution of other lower-limb muscles, to better guide future work examining the link between prior HSI and future ACL injury.

The conclusions obtained from Chapter 4 informed the direction of the three subsequent chapters. The focus of the second study (Chapter 5) was to investigate the contribution of the hamstrings to ACL loading during the weight acceptance phase of an unanticipated sidestep cut relative to other lower-limb muscles. A musculoskeletal modelling approach was used to determine how different lower-limb muscles contribute to the key markers of ACL loading, namely the anteroposterior tibiofemoral shear force, and the valgus and rotation reaction moments. It was found that the hamstrings and gluteal muscles play a dominant role in protecting the ACL, by opposing the anterior shear force and valgus reaction moment, respectively. These same muscle groups were found to oppose each other in the transverse plane, thus limiting knee rotation loading.

The focus of the third study (Chapter 6) was to determine the contribution of the hamstrings to the medial and lateral tibiofemoral compartment contact force during unanticipated sidestep cutting relative to other lower-limb muscles. This was because ACL injuries rarely occur in isolation, and are associated with long-term degeneration of articular knee cartilage. A custom musculoskeletal model was created with a modified knee joint mechanism, which permitted the computation of tibiofemoral compartment contact forces via a dynamic equilibrium approach. It was found that medial tibiofemoral contact loading was primarily produced by the vasti, gluteus medius and gluteus maximus and the medial gastrocnemius, whilst lateral tibiofemoral loading was produced primarily by the vasti, soleus, and the medial and lateral gastrocnemius. The medial hamstrings tended to load both compartments, whilst the biceps femoris long head loaded the lateral compartment and induced a relatively small decompression impulse in the medial compartment. Additionally, it was found that most muscles tended to compress both compartments, whilst other muscles had the ability to compress one compartment and decompress the other.

The focus of the fourth study (Chapter 7) was to determine how the hamstrings contribute to coordinating the stance phase of an unanticipated sidestep cut. A musculoskeletal modelling approach was used to estimate lower-limb muscle forces, and a ground reaction force (GRF) decomposition method was used to determine how muscles contributed to the GRFs. It was found that bodyweight support is primarily modulated by the vasti, gluteus maximus, soleus, and gastrocnemius. These same muscles, along with the hamstrings, were also the primary modulators of anteroposterior progression. By contributing to the medial GRF, the vasti, gluteus maximus and gluteus medius were primarily responsible for redirecting the centre-of-mass toward the cutting direction.

This program of research has identified the contribution of the hamstrings, as well as other lower-limb muscles, to knee joint loading and performance during a change-of-direction task. The first study synthesised the retrospective evidence base investigating hamstring strength and flexibility in participants with a history of HSI. This study also identified that assessments of function post HSI tend to focus mostly on the hamstrings during isolated strength assessments, neglecting other lower-limbs muscles. This highlighted the need to better understand the hamstrings role in potentially ACL injurious tasks, relative to other lower-limb muscles. In these investigations the hamstrings were found to be an important muscle group to oppose anterior shear forces during unanticipated sidestep cutting, whilst other non-knee-spanning muscles were found to have a substantial role in developing and opposing other surrogate markers of ACL loading. Similarly, both knee-spanning and non-knee-spanning muscles were found to play a substantial role in compressive loading of the medial and lateral tibiofemoral compartments. Additionally this program of research developed a greater understanding of the contribution of the hamstrings, and other lower-limb muscles, to the coordination of a sidestep cut. The hamstrings played a key role in maintaining anterior propulsion during early stance, although the majority of the demands of sidestep cutting (bodyweight support, propulsion and redirection) were provided by the vasti, gluteus maximus, soleus and gastrocnemius.

The data from this program of research will inform ACL injury rehabilitation and injury prevention practices which should consider not only targeting the hamstrings but also other non-knee-spanning muscles for loading and unloading the knee during sidestep cutting. Additionally, this thesis provides data that may inform strategies aiming to modulate muscle forces to alter tibiofemoral compressive forces, which may be involved in ACL injury and concomitant meniscal and articular cartilage injury. Finally, this thesis provides further data informing how these muscles contribute to the performance of sidestep cut, in order to achieve optimal balance between performance and injury risk considerations. The findings from this thesis also dictates that future investigations that aim to examine the link between prior HSI and increased knee joint loading need to broaden the scope of such work to consider the influence of other lower-limb muscles as well as multi-planar movements.