Why does one person have a flat foot and another a normal arch? The plantar fascia and the many small and large muscles of the foot all pull across its joints, but until this study, no one had measured how much each individual structure actually contributes to foot posture.
The researchers recruited 111 adults and used the Foot Posture Index to classify them as having a normal foot (68 people) or a flat, pronated foot — pes planus (43 people). Ultrasound was then used to measure the thickness of the plantar fascia at three regions along the arch, and the cross-sectional area (a proxy for size and strength) of six foot muscles, both the intrinsic muscles inside the foot and the extrinsic muscles running down from the lower leg. Statistical modelling then worked out which structures best explained the differences in foot posture.
Foot posture influences loading, injury risk, footwear and orthotic decisions, and which muscles are worth strengthening. Knowing which structures contribute most to a flat foot helps target assessment and treatment rather than guessing.
Thickness was measured at three points along the medial arch: the heel (calcaneal, PF1), the middle (PF2) and the forefoot region near the ball of the foot (metatarsal, PF3). The plantar fascia is the strong band of connective tissue that supports the arch.
The small muscles that sit entirely within the foot — abductor hallucis (AbH), flexor hallucis brevis (FHB) and flexor digitorum brevis (FDB) — measured by cross-sectional area.
Five structures together explained about 69% of the variation in foot posture — and one of them stood out well above the rest.
The forefoot region of the plantar fascia (PF3) was by far the biggest single contributor, explaining 52% of the variation in foot posture on its own — more than all the muscles combined. A thinner fascia there was linked to a flatter foot.
A statistically significant difference in Global Rating of Change (GROC) values favoured the cupping group. Participants felt cupping produced a greater and more noticeable treatment effect than foam rolling.
The top contributors sat on the inner, big-toe side of the foot — the plantar fascia plus the hallux muscles (FHB and FHL). Structures on the outer side contributed far less.
Both intrinsic (FHB, FDB) and extrinsic (FHL, PER) muscles contributed to the model, but each explained far less than the fascia — together rounding the total to 69%.
This was a snapshot in time, so it maps which structures are linked to foot posture — it can’t prove that changing a muscle or the fascia would change the arch.
Foot posture sits underneath a huge range of lower-limb complaints — heel pain, shin pain, knee and hip issues in runners, and more. This study helps practitioners understand which foot structures are doing the most work in shaping the arch, which is useful when assessing a flat-footed client and deciding where to focus.
It also puts numbers to something clinicians see often: the plantar fascia is central to arch support, and the muscles on the inner side of the foot matter more than the outer ones.
Supports paying close attention to the medial foot structures and the plantar fascia when assessing arch-related presentations. The prominence of the hallux muscles is a reason to consider intrinsic foot muscle work as part of a plan — while remembering this study shows association, not proven cause.
Reinforces the rationale behind intrinsic foot muscle and “short foot” exercise programmes, which related research has linked to a higher arch, and highlights the plantar fascia’s central role in arch support and loading.
Foot posture affects how load is distributed in runners and jumpers. Understanding that the plantar fascia and medial foot muscles drive arch behaviour helps when managing overuse injuries and building foot-strength work into training.
A useful anatomical map when treating foot and lower-limb tension, and when deciding whether a client’s foot posture warrants onward referral for orthotic or exercise assessment.
This was a cross-sectional study, so it can only show associations — it cannot prove that changing the plantar fascia or a muscle would change someone’s foot posture. It measured static standing posture rather than the foot in motion, and muscle cross-sectional area is only a proxy for how a muscle actually performs. Only normal and flat feet were included, so high-arched (cavus) feet — the other end of the spectrum — weren’t represented. The authors also note an unequal gender balance, unadjusted age and BMI differences, and that ultrasound measurement is user-dependent.
These points mean the findings are best read as a well-measured map of what’s associated with foot posture, and a strong starting point for future work, rather than a treatment recommendation on their own.
In 111 adults, the plantar fascia and the medial foot structures were the main contributors to static foot posture. The forefoot region of the plantar fascia alone explained 52% of the variation, and five structures together explained 69%. Both intrinsic and extrinsic foot muscles played a part, but each contributed far less than the fascia.
For myotherapists, physiotherapists and remedial massage therapists, this reinforces the central role of the plantar fascia and the value of assessing the medial foot and intrinsic muscles in flat-foot presentations — while keeping in mind that this is an associative study, not proof that targeting these structures will change the arch.
Contributions of Foot Muscles and Plantar Fascia Morphology to Foot Posture and Angin S, Mickle KJ, Nester CJ. Gait & Posture, 2018.
