Muscle co-activation did not appear to increase loading of the ACL

Results from the studies presented in chapters two and three showed that vertical stiffness was related to hamstring-quadriceps co-activation and pre-activation, and that vertical stiffness was not linked to muscle strain injury. The next step to this body of research was to establish if vertical stiffness could be linked to non-contact ACL injury. However, because the volume of ACL injuries

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which occur per professional football club per season is far less than the number of muscle strain injuries (Opar and Serpell, 2014) a study similar to that described in chapter three using non-contact ACL injury as an outcome variable could not be conducted. Consequently, the studies conducted as part of this body or research after chapter three explored the relationship between vertical stiffness and measures known to load the ACL or measures which represent loading of the ACL; namely anterior tibial translation and ACL elongation respectively. This was done across two studies, the first of those, presented in chapter four, aimed to determine the effect of hamstring-quadriceps co- activation on knee joint motion and ACL elongation. As will be discussed in this section,

hamstring-quadriceps co-activation constrained excessive joint motion and ACL elongation. Therefore, the second study, presented in chapter five, examined the relationship between vertical stiffness with anterior tibial translation and ACL elongation.

The study presented in chapter four was a pilot study which used novel, high-tech, medical imaging technology to measure knee joint motion on a basic step-up task while concurrently measuring muscle activity of the vastus medialis, vastus lateralis, semimembranosus and biceps femoris using EMG. Knee joint kinematics were measured from a 4-D model which was created for each participant from image registration of CT with fluoroscopy frame by frame. The

technology used in this study differs from other kinematic measurement/motion analysis systems in that this technology measures knee joint motion directly from bone. Other motion analysis systems typically use skin marker sets with biomechanical models to predict translations and rotations. Skin markers are limited by the effect of wobbling mass (i.e. skeletal muscle and subcutaneous adipose tissue) (Reinschmidt et al., 1997, Begg et al., 1989, Windolf et al., 2008). The technology used for this project has known precision at the knee joint of only 0.38 mm error for in-plane translation and 0.42 degrees for rotation (Scarvell et al., 2010). In this study the attachments of the ACL were mapped to the 4-D model in accordance with methods described elsewhere (Grood and Suntay, 1983) for each frame. ACL elongation was considered the difference between maximum and

minimum ACL length throughout the step-up. To this point in time no other study had measured ACL elongation dynamically in-vivo, or anterior tibial translation with such precision in-vivo.

In this study it was hypothesised that with deliberate co-activation of the hamstrings and quadriceps muscles knee joint motion would be constrained, including anterior tibial translation, and ACL elongation would not be as great. It should be acknowledged, however, that this study revealed a strong inverse relationship between ACL elongation and medial hamstring-quadriceps co-activation, but positive relationship between ACL elongation and lateral hamstring-quadriceps co-activation. This may be concerning given the study presented in chapter two revealed greater activity of the lateral hamstring and quadriceps muscles on a hopping task which simulated the change of direction manoeuvre seen when non-contact ACL typically occurs. Furthermore, a strong positive relationship was observed between lateral hamstring-quadriceps co-activation index

calculated and vertical stiffness in that study. Thus, evidence from this body of research to this point leans toward vertical stiffness being related to ACL elongation and therefore a risk factor for non-contact ACL injury. However, the studies presented in chapter two and in section 6.1.1. have shown that the relationship between hamstring function and vertical stiffness or hamstring function and ACL elongation is likely to be a general one; muscles do not operate in isolation, rather synergistically, and therefore this data should be interpreted with caution.

Careful thought has been given to differences in the methods used in chapter two, compared with chapter four, hence the call for caution when considering anterior tibial translation and ACL elongation with lateral hamstring-quadriceps co-activation. In the study presented in chapter four co-activation index was calculated from peak activation, not mean activation as was the case in chapter two. Worth noting also was that in the study presented in chapter two a strong positive relationship was seen for vertical stiffness with peak medial hamstring activation and timing of peak medial hamstring activation. Finally, in the present study a co-activation index of the medial and

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lateral hamstring muscles was calculated from peak semimembranosus activation divided by peak biceps femoris activation; for flexion and extension semimembranosus and biceps femoris muscles are agonists but for knee rotation and for knee adduction/abduction they are antagonists. Therefore, it was expected that knee abduction and external rotation would not be as great when medial-lateral hamstring co-activation index was ‘smaller’, and this would manifest as an inverse relationship between ACL elongation and medial-lateral hamstring co-activation index. Indeed, as the ACL elongated the medial-lateral hamstring co-activation index reduced. When the findings of the four studies in this thesis are seen together, the data support the notion that hamstring and quadriceps function is synergistic in dynamic hopping and landing tasks. Therefore, the relationships for lateral hamstring and quadriceps activation, or for medial hamstring and quadriceps activation, with vertical stiffness and ACL elongation are not closed relationships. Rather, the relationships are co- dependent; it is the combined actions of the hamstrings and quadriceps which influence dynamic knee joint stability. This in turn will affect vertical stiffness (see section 1.5. of the introduction and figure 2).

Another important finding from the study in chapter four, was the strong positive relationship between anterior tibial translation and ACL elongation. Some limitations exist surrounding how ACL length and subsequent ACL elongation were measured in this study. These limitations were explored in the discussion section of the chapter. However, at the very least, it should be noted that anterior tibial translation is a measure known to load the ACL (Butler et al., 1980) and it is likely that a manifestation of this is ACL elongation. The anterior tibial translation and ACL elongation relationship provides validation of the findings of this chapter. Therefore, the strong positive relationship between the two measures adds strength to the argument that net co- activation of the hamstring and quadriceps muscles reduces ACL elongation.

Although the study presented in chapter four was just a pilot study, combined with other research (Isaac et al., 2005, MacWilliams et al., 1999), and read in context with studies presented in this body of research prior to this, it appears that net hamstrings-quadriceps co-activation constrains anterior tibial translation and subsequently reduces ACL elongation; a measure which represents loading of the ACL. Given that net hamstring-quadriceps co-activation is also related to vertical stiffness, a story begins to emerge from chapters two to four that vertical stiffness may not increase ACL injury risk.