Assessment of plantarflexor function during a stretch-shortening cycle task

The plantarflexors are important due to their role in locomotion and stiffness control, high prevalence of injury, link to knee stability and anterior cruciate ligament injury. Current methods of assessing the plantarflexors are limited primarily by issues of validity and reliability. The aims of this thesis were to develop a method of measuring plantarflexor function in a dynamic yet controlled stretch-shortening cycle (SSC) task and to assess plantarflexor muscle-tendon unit (MTU) function in healthy young adults using this method. Use of an adapted force sledge was found to be a highly reliable method to determine plantarflexor force, force development and reactive strength capabilities during a fast SSC task (single ICC >0.85, average ICC >0.95). Using this method, moderate between-limb differences in measures of force, force development and reactive strength index of up to 44.6% were observed in 21 young healthy adults. These differences are much larger than the 15% upper limit previously suggested for healthy limbs. Differences in plantarflexor force, force development and mechanical behaviour of the plantarflexors were observed due to gender in a group of 34 age and training-matched individuals. While some differences were removed with normalisation to muscle volume, differences in peak force and power remained, which suggests underlying differences in the MTU which may affect injury risk or training response. To provide further insight into underlying muscle-tendon behaviour, a motion analysis-based method of determining centre of pressure position was developed. This was shown to be reasonably accurate in location of the centre of pressure during two-legged hopping at frequencies greater than 1.5 Hz. Incorporating this method with the adapted force sledge and ultrasound, the main determining factors of stiffness during this task were found to be the rate of peak force development, rate of force development in the first 30 ms and contact time. Measures previously suggested to be associated with tendon stiffness, namely force development time and elastic charge time, were found to have no relationship with Achilles tendon stiffness during the stretch-shortening cycle task. Similarly, no relationship was shown between Achilles tendon stiffness and reactive strength index. Instead, reactive strength index was determined by normalised peak eccentric power, concentric work and peak concentric ankle joint power during the impact. In summary, an adapted force sledge can be used to reliably assess plantarflexor MTU function during a SSC task and investigate several research questions related to the function of this particular muscle group. It can also be combined with motion analysis to determine joint kinetics. The results of this research are important for both practitioners and researchers for understanding healthy plantarflexor function during a SSC task. This work has implications for the design and selection of appropriate test protocols and understanding what is being measured, both for establishing normative data in a valid setting and for understanding the range of measures that may be observed in healthy young active adults.