This study showed that arch height tended to decrease after running a half marathon although there were differences between the two measures used. Age, gender, BMI, race time did not predict the change in arch height. The change in arch height was larger in people with a higher pre-race navicular height.
A significant drop was seen in the left FPI only. This may reflect the influence of leg dominance resulting in differences in lower limb kinematics and kinetics between the two sides. However, this is speculative and leg dominance was not recorded in the current study. A more likely explanation is that this reflects the limited sample size; to detect an effect size of 0.38 on the right would require a sample size of 137 participants (power = 0.85 α = 0.05).
The definition of foot pronation by Root et al.  describes the arch lowering through subtalar joint pronation as one of the observable clinical signs. More recently Nester  reviewed experimental evidence which challenged Root’s explanation of the arch lowering primarily through subtalar joint pronation. Nester indicated that arch lowering through sagittal plane plantarflexion and frontal plane eversion is found variably between individuals to be comprised of movement primarily at the ankle and talonavicular joint and to a lesser extent through the other tarsal joints. It is difficult, therefore, to infer which soft tissue structures may be yielding in any one individual to lower the arch under weightbearing conditions following a prolonged run. It is possible that creep of passive soft tissues upholding the medial longitudinal arch may have occurred and also muscle fatigue in anti-pronatory muscles. The arch profile frequently changes upon weightbearing and so the navicular moves as a result of whichever combination of joint movements has occurred, and can be used as a measure of the weightbearing response of the foot which is known clinically as pronation and supination .
Another effect of exercise, however, is the perfusion of blood to the muscles of the foot which results in engorgement and an increase in volume. The muscles closest to the skin in the medial longitudinal arch contribute to the maintenance of arch height  and so would likely be placed under considerable work during a half marathon with significant engorgement occurring during a race. Since the increase in volume would fill the medial longitudinal arch space this would give the impression of lowering of the arch irrespective of navicular height. Indeed some feet appeared to have increased in volume at the medial midfoot perhaps resulting from increased perfusion to the abductor hallucis and other local muscles. Engorgement and an increase in blood volume could potentially affect the interpretation of the FPI, which is done visually, whilst the measure of navicular height to a bony landmark should be unaffected.
Anecdotally the testers noticed an apparent abduction of the forefoot relative to the rearfoot post-race which may result from fatigue of the medial foot soft tissues which are the stabilisers of the tarsus in the transverse plane. This could even be a distortion of the shape of the foot due to superficial muscular engorgement both medially and laterally. It would be useful to investigate the transverse plane response of the foot to prolonged running and walking in future studies.
The navicular height pre-race was ~ 5 mm higher than that recorded by Swedler et al.  in large scale study of army recruits. This may reflect the difference in the points of measurement; the inferior border of the navicular bone in the study by Swedler et al.  and the navicular tuberosity in the current study. One of the drawbacks with this study, however, is that we did not normalise navicular height to the length of the foot. People with larger feet would tend to have a higher navicular height and show a larger change in height after running. This may explain why a significant relationship was seen between the change in navicular height and the pre-race navicular height. However, the FPI-6 also showed an increase in pronated position and this tool does not normalise for foot length. The fact that the degree of navicular drop was not related to the pre-race FPI score whilst this was related to pre-race absolute navicular height suggests that the degree of arch lowering is the same regardless of foot length.
Time to return to baseline FPI-6 score and navicular height was not recorded for the participants and further research into this might be useful. The cause of the changes in foot posture seen in this study are not clear and could be the result of damage to soft tissues or yielding within elastic limits in the soft tissues due to neuromuscular and mechanical fatigue. The impact of these changes on foot and ankle function is also unknown nor the effect of running a longer distance than a 13.1 mile half marathon. The changes may indeed be clinically significant enough to predispose the bones, joints and soft tissues to damages if running were to continue after the changes have taken place, for example in a full marathon. The levels of pain anecdotally reported after the race by the runners were not considered to indicate significant injury but this may be erroneous due to general systemic fatigue and raised endorphin levels which have been reported in trained athletes during sporting activity  The perception of pain and function may be potentially important in modification of activity after clinically significant changes have occurred and should be investigated further.
We did not control for footwear in this study although discussion about footwear was offered in the pre-race data collection sessions and advertised as an incentivisation for the study. Participants were advised to continue with their planned footwear for the race and consider any advice given by the researcher EC when only buying new training footwear in future. Participants were advised on lacing techniques, however for the race to ensure that shoes fitted well to reduce the risk of skin erosions.
Biomechanical studies of low and high arched runners have, demonstrated an effect of cushioning and motion control sports shoes on the biomechanics of the lower limb during treadmill running . They found that low arched runners have more internal tibial rotation when wearing cushion shoes and less when wearing motion control shoes whilst no effect on internal tibial rotation was seen in high arched runners. As internal rotation is associated with foot pronation, this suggests that the type of shoe may be a factor in determining changes in foot posture with running.