Foot stiffening during the push-off phase of human walking is linked to active muscle contraction, and not the windlass mechanism

Key points

1. 01Increasing soleus (plantar flexor) activation compressed the foot arch more and resisted toe extension, the opposite of windlass-driven stiffening
2. 02Plantar intrinsic muscles (abductor hallucis and flexor digitorum brevis) increased activation to stiffen the metatarsophalangeal joint during push-off
3. 03Plantar aponeurosis length did not change significantly with increasing load, showing no passive windlass stretch in proportion to push-off effort
4. 04In the walking experiment, a forceful push-off doubled the metatarsophalangeal joint moment via greater muscle activation without increased toe extension or aponeurosis strain
5. 05The study challenges the long-standing assumption that toe extension passively tensions the plantar aponeurosis and rigidifies the foot

How it was conducted

Design

Two controlled in-vivo experiments: Experiment 1 -- seated isometric loading with mechanically forced toe extension (n=10); Experiment 2 -- overground walking comparing normal vs forceful push-off (n=9)

Participants

Experiment 1: 10 healthy adults (9 men, 1 woman), no leg or foot injuries; Experiment 2: 9 healthy adults (5 men, 4 women), University of Exeter ethics approval

Interventions

Experiment 1: actuator loads of 0.5, 1.0, and 1.5 body weights applied to thigh with simultaneous forced metatarsophalangeal joint extension; Experiment 2: self-selected walking vs voluntarily maximized push-off

Measurements

Surface and fine-wire EMG of soleus, abductor hallucis, and flexor digitorum brevis; 3D motion capture of foot segments; ground reaction forces; estimated plantar aponeurosis length via geometric model

Primary outcome

Metatarsophalangeal joint quasi-stiffness, plantar aponeurosis length change, and integrated EMG across loading and walking conditions

Statistics

Linear mixed models with maximum-likelihood ratio tests (Experiment 1); paired t-tests (Experiment 2); alpha = 0.05

What they found

• Medial longitudinal arch compression increased significantly with higher actuator loads (low = mean angle low, mid, high = progression upward, MLRT p = 0.01)
• Arch rise during toe extension was significantly reduced at higher soleus activation (MLRT p = 0.03), showing the windlass mechanism was opposed by muscle activity
• Intrinsic muscle integrated EMG significantly increased quasi-stiffness of the metatarsophalangeal joint (MLRT p = 0.04)
• Plantar aponeurosis final length was not significantly different between loading conditions despite increasing load and muscle activation
• In Experiment 2, peak anterior ground reaction force was significantly higher in PUSH than control (17 +/- N versus lower value, t-test p < 0.01)
• Flexor digitorum brevis integrated EMG: CON = 0.0X +/- 0.05, PUSH = 0.0X +/- 0.05, t-test p = 0.036; abductor hallucis: CON = 0.X +/- 0.02, PUSH = 0.1 +/- 0.03, t-test p < 0.001
• Peak metatarsophalangeal joint moment doubled from control to push condition (CON = 4.3 +/- Nm, PUSH = 8.9 +/- Nm, t-test p = 0.004)
• Plantar aponeurosis length changes were similar between CON and PUSH walking conditions despite the large difference in push-off force

Limitations

• Small sample sizes (n=10 and n=9) limit statistical power and generalizability
• Extrinsic toe flexor muscles (flexor hallucis longus and flexor digitorum longus) were not measured with EMG, so their contribution to metatarsophalangeal joint mechanics is unknown
• Plantar aponeurosis length was estimated with a simple geometric model rather than directly measured, and true strain or force contribution could not be quantified
• Poor calcaneus marker data in Experiment 2 prevented modelling of plantar aponeurosis length during early-mid stance, restricting analysis to the second half of stance

Why it matters

For patients

People with plantar fasciitis may benefit from intrinsic foot muscle strengthening, as stronger muscles could shield the plantar aponeurosis from repeated high strains thought to contribute to this common painful condition.

For clinicians

The windlass clinical test (passive manual toe extension to load the plantar fascia) may evoke an active muscular response, particularly under weight-bearing, which could confound interpretation of a supposedly passive test.

For readers

The traditional model that toe extension passively tensions the foot like a windlass is mechanistically incomplete; active muscular control is the primary regulator of foot stiffness and push-off mechanics during walking.