Achilles tendon injuries were identified as problematic for New Zealand High Performance athletes by Triathlon New Zealand. Analysis of six years of injury data for New Zealand high performance triathletes indicated that Achilles tendon injuries (17%) were among the most common overuse lower limb injuries together with calf injuries (17%). The majority of injuries were attributed to running. These results correspond to injury analysis of British elite triathletes with reported high prevalence of Achilles tendon injuries in Olympic distance athletes. The exact mechanism of injury and what causes the pain is still unclear. However the reoccurring nature of the injury and often prolonged recovery times signal a need for the development of preventative interventions.

The main aim of the thesis was to determine whether a single measure can be used to identify individuals at risk of Achilles tendon injury. This was achieved through a series of specific questions which followed the Van Mechelen and Finch models for injury prevention research. The thesis was able to address, the extent of the problem, what is understood about the problem and make a unique contribution to understanding the mechanism of injury.

Individual risk factor analysis for Achilles tendon injuries via a systematic literature review resulted in only five risk factors that were clearly associated with injury. Increased braking force was associated with increased injury risk, while increasing surface stiffness, high arch height, large propulsive force and large vertical force were associated with decreased injury risk. Various other risk factors were also found that did not show clear effects. The slow progressive nature of overuse injuries suggests that the changes in loading to the tendon are subtle and therefore, individual risk factor analysis is not likely to clearly determine the causative factors. Therefore, investigation of measurements that measure changes in movement patterns may provide greater insight. Stiffness was found to be increased in the leg and decreased in the ankle for athletes who had prior Achilles tendon injuries compared to uninjured controls. Lower limb stiffness is a measure of how the joints work in relation to one another to absorb impact upon contact and reflect the synergistic activity of muscles, tendons and ligaments. Stiffness therefore may provide a useful measure of looking at the landing movement as a whole and provide information regarding injury risk.

The influence of the different risk factors, that were identified to have a definite or possible role in Achilles tendon injury risk, on lower limb stiffness were therefore investigated via a systematic review. The majority of Achilles injury risk factors were associated with increases in lower limb stiffness measures, however the results were unclear. Based on the evidence, it was considered that stiffness was a potentially useful measure for analysing Achilles tendon injury risk and should be investigated further.

In a reliability laboratory study of 12 male triathletes, vertical and leg stiffness had good reliability as did ankle stiffness. Knee and hip stiffness reliability were moderate to poor, however combining the knee and ankle improved the reliability. Knee and ankle stiffness appeared to have the greatest contribution to leg stiffness.

The effect of different training conditions on lower body stiffness measures were investigated in 75 triathletes. The effect of increasing running pace on the different lower limb stiffness measures was dependent on the magnitude of the increase in velocity and the starting pace. Vertical, knee and ankle stiffness increased with increasing running pace. Leg and hip stiffness were largely unaffected by changes in pace. It was hypothesised that the different joints had varying levels of importance in modifying stiffness to stabilise gait depending on whether pace was increased by increasing stride length or decreasing contact time. Further investigation of how changes in the temporal and spatial parameters of an athlete’s running gait influence lower limb stiffness may provide insight into both injury risk and how stiffness can be modified in at risk individuals.

A useful screening tool would be adaptable to the requirements of the athlete. Therefore, stable reliability over a range of paces would be ideal. Reliability of vertical and leg stiffness remained unchanged over the pace ranges measured. Combined knee and ankle stiffness had acceptable reliability over all paces. Between subject variability was largely unaffected by increasing running pace suggesting a similar pattern of stiffness adjustment within the group. Outliers from this pattern of adjustment may provide insight into injury risk. Within subject variability for the knee and ankle demonstrated a slight ‘U-shaped’ pattern highlighting the need for athletes to utilise a wide range of running paces during training in order to maximise gait variability for injury prevention. Extremely high variability may indicate a pace at which coordination is unable to be maintained and prolonged periods at this pace should therefore be established gradually.

In triathlon, the cycle to run transition is widely thought to be associated with increased injury risk. Running following a 30 minute self-paced cycle did not appear to alter stiffness substantially in a laboratory based study of 34 triathletes. Leg and ankle stiffness showed small increases in the first minute of running after cycling compared to isolated running. Individual responses were apparent within the group, with the most notable being either an increase in ankle stiffness but a decrease in knee stiffness in the first four minutes of running. An opposite change in stiffness between the knee and ankle may indicate an uncoupling of normal gait coordination which could be related to an increased risk in injury. Further research into the relationship between gait parameters, running economy and stiffness is required in order to understand the differences between responding and non-responding athletes.

After baseline screening for stiffness measures, 75 triathletes were followed for a year via on-line weekly reporting of training and injury information. Eight Achilles tendon injuries were reported during this time. Using an intention to treat analysis, individuals who had experienced an Achilles tendon injury either prior to the study or during the surveillance period had reduced ankle stiffness compared to uninjured triathletes. Leg stiffness was increased in triathletes who developed an Achilles tendon injury during the surveillance period, and knee stiffness was increased in triathletes who developed their first Achilles tendon injury during the study, compared to uninjured athletes. A new measurement, the knee-ankle stiffness ratio, was developed which may be able to predict triathletes who will go on to develop an Achilles tendon injury. Athletes at risk of developing Achilles tendon injuries had a higher knee stiffness compared to ankle stiffness, while those who did not develop Achilles injuries had similar stiffness at the knee and ankle during running.

The knee-ankle stiffness ratio needs to be further investigated to determine the mechanism of the imbalance and what interventions can be used to reduce the stiffness difference. The knee-ankle stiffness ratio may provide a useful measurement for identifying triathletes and perhaps other running athletes who are at risk of developing Achilles tendon injuries in the future.