What is the role of joint coupling variability and joint stiffness in lower limb injury?
Netball requires direction changes whilst sprinting, which predisposes netballers to risk of injury. The main question this thesis addressed was “What is the role of joint coupling variability and joint stiffness during change of direction sprints and the association with lower limb injury?”. Specifically, differences in joint coupling variability and joint stiffness between elite and non-elite netballers, between male and female netballers, between dominant and non-dominant lower limbs and between injured and non-injured netballers post six months of prospective monitoring were ascertained. Before these study aims could be addressed the reliability of joint coupling variability and joint stiffness measures during unanticipated straight run and unanticipated 180° turning tasks performed by netballers required examination. Twelve elite female netballers, 12 male non-elite netballers and 12 non elite female netballers performed unanticipated straight sprints and sprints with a 180° turn on their dominant and non-dominant legs in a motion analysis laboratory from a 10 m approach. Measurement reliability and measurement variability of joint coupling and joint stiffness were measured within players across testing sessions. Joint coupling variability for rearfoot and knee motion were most reliable for most unanticipated tasks whereas ankle frontal plane stiffness was most reliable for all four unanticipated tasks. Consequently, these measures were used throughout the thesis research for comparisons between groups. Female netballers, in particular at the elite level, that demonstrated low levels of lower extremity joint coupling variability (JCV) in their dominant leg during unanticipated straight sprint tasks were susceptible to injury (r = 0.66). Conversly, male netballers that demonstrated high joint coupling variability in their dominant leg during unanticipated sprint tasks were likely to sustain injury (Rearfoot(eversion/inversion)Knee(flexion/extension) injury r = -0.46: Rearfoot(eversion/inversion)–Knee(rotation) injury r = -0.35: Tibial(rotation)–Knee(flexion/extension) injury r = -0.54). Associations with injury occurrence were not as prevalent or strong for joint stiffness (JS) as those found with joint coupling variability (JCV 7 out of 12, r = -0.54 to 0.66; JS 7 out of 12, r = -0.55 to 0.33). During ground contact for an unanticipated turn, injury was more likely for elite level female netballers if they had low levels of ankle joint stiffness (r = 0.33), and for non-elite female netballers if they had high knee joint stiffness (r = -0.51). In contrast male netballers with high levels of ankle joint stiffness during ground contact of an unanticipated straight sprint were likely to be injured (r = -0.55). Therefore, the role of joint coupling variability and joint stiffness during change of direction sprints and the association with lower limb injury was gender and movement specific. The thesis findings support low joint coupling variability associations with injury and injury occurrence associations with both high and low joint stiffness theories proposed in the literature. Further investigations related to context specificity of joint coupling variability or joint stiffness between genders, limbs and other athletic populations is required. Additionally research involving training interventions to optimise joint coupling variability and joint stiffness may benefit the performance and welfare of netballers.