Tibialis posterior (TP), plays a substantial role in supporting the arch of the foot. Tibialis posterior tendon dysfunction (TPTD) involves pain and swelling on the inner side of the foot and often leads to an acquired flat foot deformity. However, there is also a tendency for foot width to increase and arch height to decrease with age. Greater concurrent activation of muscles that perform opposing movements has been shown with age. However it is unknown whether greater co-activation of TP and its opposing peroneal muscles increase with age. There is uncertainty regarding the management of TPTD, recently highlighted by the James Lind Alliance as a research priority.

To improve understanding of TP function we can study the electrical signal when TP activates using electromyography (EMG). Foot arch height has been shown to influence the pattern of TP activation in young-middle aged adults during walking. However, to enable comparisons between groups of participants (e.g. young vs. old, or patients vs. healthy controls) it is necessary to perform a normalization process to account for individual differences e.g. in orientation of muscle fibres. Recent expert consensus advocated normalising to a maximum voluntary contraction (MVC) when comparing groups. Previous studies have reported poor between-day reliability of TP EMG when normalising to an MVC. In the latter studies, the MVC contraction was performed against manual resistance, potentially reducing the reliability of the measurement due to variability in the resistance from the tester. Reliability of the MVC may be improved by using a controlled means of providing resistance to muscle contraction such as an isokinetic dynamometer, or a ForceFrame (VALD Health). Establishing a reliable method to record EMG from TP will enable future work investigating the mechanisms leading to TPTD and explore the potential for EMG to tailor treatment plans.

Due to the depth of TP, it is necessary to use fine-wire electrodes, inserted under the skin to record EMG. Fine-wire insertions will be performed by trained co-worker Dr Jo Reeves using ultrasound guidance and verified with electrical stimulation. Surface sensors will be placed on additional muscles (peroneals, tibialis anterior and gastrocnemii). With the electrodes in place the participants will lie on their backs and perform three maximum contractions of ankle inversion against resistance. The EMG signal from the contraction with the highest force value will be used to normalise subsequent EMG during walking. Participants will perform six walks on a standard walkway.

A target of 30 participants will be recruited (15 younger adults and 15 older adults). A subset of 20 participants will repeat the above procedure in a second session, separated by at least a day. Previous work on the between-day reliability of EMG recording from soleus during walking demonstrated comparable reliability using fine-wire electrodes to surface electrodes with 18 participants (Bogey et al., 2003). Reliability will be established with the standard error of measurement, the coefficient of variation, the intra-class correlation coefficient and the coefficient of multiple correlation.