The impact of Achilles tendon pathology on lower limb joint stiffness regulation during hopping tasks

Achilles tendinopathy (AT) is a debilitating running injury affecting 50% of distance runners over their lifetime. Ultrasound (US) imaging studies have shown that pathological changes are present in 11-52% of asymptomatic individuals. Impairments of the lower leg muscle-tendon function may develop with AT however alteration of lower limb loading strategies has not been examined in runners with asymptomatic Achilles tendon pathology. The primary aim of the thesis was to determine if Achilles tendon pathology changes lower limb loading patterns. It was hypothesised that there would be a different loading pattern between the ankle and knee in runners with Achilles tendon pathology. In order to investigate this primary aim, secondary aims were developed to determine the prevalence of Achilles tendon pathology in an asymptomatic running population and to determine any associated factors of tendon pathology. This study initially assessed the prevalence of tendon pathology and a number of associated risk factors for 37 experienced, high mileage male endurance runners with no history of Achilles tendon pain. The tendon was assessed using US by a musculoskeletal radiologist and classified as either normal or abnormal. Height, body mass, waist circumference, and weight bearing ankle dorsiflexion range of motion (ROM) with the knee in a flexed and extended position were measured. A survey quantified the running history of participants. Following the initial study, 14 runners with no history of Achilles tendon pain and a normal Achilles tendon on US imaging, and 12 runners with asymptomatic Achilles tendon pathology were assessed in a series of further studies. A third group of 12 runners with symptomatic AT were also studied. Each runner completed single leg hopping for both limbs on a level and inclined surface. Embedded in the surface was a force plate (1000Hz) that was synchronised with a three dimensional motion capture system (250Hz). Nonparametric statistics were used to examine the effect of surface angle and group on hopping biomechanics. All results are reported as median and interquartile range (IQR). Almost half (46%) of the asymptomatic distance runners had at least one abnormal tendon. The runners with asymptomatic Achilles tendon pathology had significantly more years of running history (Median 20.0 years, IQR 6.0-25.5, p=0.024) than the runners with no pathology on US (Median 7.0 years, IQR 5.0-15.0). No significant differences between the groups was identified for age, height, mass, waist circumference, ankle ROM, number of weekly running sessions, weekly mileage and number of long distance (marathon and half marathon) running events completed in their lifetime. Symptomatic runners had significantly less active ankle joint stiffness (Level – Median = 8.2 Nm/kg/rad, IQR = 7.7-9.2; Incline - Median= 8.1 Nm/kg/rad, IQR = 7.2-9.7) when compared to the normal group (Level – Median = 9.8 Nm/kg/rad, IQR = 9.0-10.5; Incline - Median= 10.2 Nm/kg/rad, IQR = 8.7-10.4) for both the level (p=0.044) and inclined (p=0.042) surfaces. No differences were identified for leg stiffness between the three groups. Asymptomatic male distance runners had a high incidence of tendon pathology compared to other populations (e.g. soccer players) and cumulative load in running years is an associated factor of tendon change. This thesis identified that the presence of Achilles tendon pathology without a history or presence of pain did not alter lower limb loading strategies. However, runners with symptomatic AT had reduced ankle stiffness. This may explain the recalcitrant and recurrent nature of AT if runners are not able to increase their ankle stiffness following rehabilitation and recovery after injury.