The arch of the human foot is unique among hominins as it is compliant at ground-contact but sufficiently stiff to enable push-off. These behaviours are partly facilitated by the ligamentous plantar fascia whose role is central to two mechanisms. The ideal windlass mechanism assumes that the plantar fascia has a nearly constant length to directly couple toe dorsiflexion with a change in arch shape. However, the plantar fascia also stretches and then shortens throughout gait as the arch-spring stores and releases elastic energy. We aimed to understand how the extensible plantar fascia could behave as an ideal windlass when it has been shown to strain throughout gait, potentially compromising the one-to-one coupling between toe arc length and arch length. We measured foot bone motion and plantar fascia elongation using high-speed x-ray during running. We discovered that toe plantarflexion delays plantar fascia stretching at foot-strike, which likely modifies the distribution of the load through other arch tissues. Through a pure windlass effect in propulsion, a quasi-isometric plantar fascia’s shortening is delayed to later in stance. The plantar fascia then shortens concurrently to the windlass mechanism, likely enhancing arch recoil at push-off.

High-speed biplanar videoradiography and markerless tracking algorithms measured the kinematic orientation and position of the bones (calcaneus, tibia, first metatarsal, first proximal phalanx and sesamoids). The provided data contains both filtered and unfiltered data, and is normalized to stance phase (101 points). All methods are explained in the manuscript.

Provided data:

The resultant joint angles, ligament lengths (all participants have plantar fascia length, one participant also has spring ligament, and long/short plantar ligaments), and vector coding results are contained in a structured MATLAB variable "results_norm" which are normalized to 101 points of stance phase. The unfiltered ('crop') and filtered ('filt') data are contained within this structure. 

The structure contains:

- "angles" which has substructures for ZYX Tait Bryan angles of [reference bone]_[bone] i.e. results_norm.angles_filt.tib_cal gives the DP (dorsiflexion +, plantarflexion -), SP (supination +, pronation -) and AD (adduction +, abduction -) of the calcaneus angles relative to the tibia in 12 rows (each participant) and 101 points (1% of stance). 

- "lig" is the length of the plantar fascia (pf) and the length normalized to the maximum length (pf_strain). The participants are depicted in rows, and the there 101 columns for each % of stance phase. The proximal arch ligaments are present for particpant 12: spring ligament (plantar calcaneonavicular ligament - PCNL_med is the medial component, PCNL_inf is the inferior component), short plantar ligament (plantar calcaneocuboid ligament - PCCL), deep fibres of the long plantar ligament (plantar long plantar ligament-  PLPL)

- "kinemat" contains the arch deformation that is not angular; i.e. it is the linear 3D distance between two inferior points, one on the calcaneus and one on the first metatarsal. This is used with the simple windlass model.

- r_sens is the radius of the metatarsal head in mm, multiplied by pi/180 to automatically convert the MTPJ angle into radians when calculating arc length.

There is also the colormap used throughout the paper as "colorblind_map.mat".

Provided code:

To make the figures in the manuscript and understand the provided .mat structure, please open the "makePlots.m". If you run it, it will produce the base graphs of the manuscript figures.

- "codingPFphases.m" produces the colourful graphs, such as Figure 4.

- "makeStatsDotPlot.m" produces the dot plots, such as Figure 5.