The influence of body posture and added mass on intrinsic and extrinsic foot muscle activation...

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What am I missing here ? My understanding was that during quiet standing tension in the plantar fascia will generate a degree of digital purchase in the toes( torque around the MTPJs).

If you stand on one leg and lean forwards you will generate greater tension in the plantar fascia and hence ,via the plantar plates, greater digital purchase ( greater MTPJ torque).

In the study linked to below, surely leaning forwards will increase the tension in the plantar fascia and hence the torque around the MTPs compared to not leaning forwards, right across the range of exercises tested.

The influence of body posture and added mass on intrinsic and extrinsic foot muscle activation and force output during common foot strengthening exercises

John W A Osborne 1, Hylton B Menz 2, Karl B Landorf 2, Glen A Whittaker 2, Matthew Cotchett 2, Luke A Kelly 3
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• PMID: 41412495
  
• DOI: 10.1016/j.jshs.2025.101110

Free article
Abstract

Background: Intrinsic and extrinsic foot muscles contribute to foot and lower limb function. This knowledge provides opportunities to target these muscles in exercise and rehabilitation. However, a barrier is the limited understanding of how prescribed exercises affect muscle activation and force output. This study examined muscle activation and torque production about the metatarsophalangeal joints of intrinsic and extrinsic foot muscles during common foot strengthening exercises. The secondary aim was to determine whether the addition of a postural change (leaning forward) or addition of mass can further increase muscle activation and torque.
Methods: Fifteen healthy participants (nine males, six females; age = 36.5 ± 8 years, mean ± SD) participated. Muscle activation was measured using fine-wire electromyography (EMG) electrodes for two intrinsic and two extrinsic muscles, and surface EMG electrodes for four superficial muscles. Metatarsophalangeal joint torque was measured using a custom toe dynamometer. Participants attended one 90-min session and performed 16 exercises. Five included the addition of leaning forward and three the addition of mass (20% of participant's mass) to compare their effects on muscle activity and torque.
Results: Muscle activation and metatarsophalangeal joint torque varied considerably across exercises. Both the addition of leaning forward and the addition of mass increased muscle activation and metatarsophalangeal joint torque, with the addition of leaning forward producing a 35% median increase in torque about the metatarsophalangeal joints.
Conclusion: Many common exercises, despite high muscle activation, produce relatively low metatarsophalangeal joint torque, raising questions about their clinical value. Adding a forward lean significantly increases toe flexor muscle activity and torque, reaching levels greater than added mass and comparable to walking.

 

The reverse windlass mechanism has been discussed before on podiatry arena .Here is a quote from Craig Payne -

"On weightbearing, the windlass unwinds as the arch flattens and the foot elongates. This is seen to place a compressive force upon the posterior and anterior struts of the ‘truss’ structure of the foot, which produces tension in the plantar aponeurosis ‘tie-rod’. This causes a powerful plantarflexory force to the proximal phalanges, as the insertions of the aponeurosis’ digital slips is wound around the plantar surface of each metatarsophalangeal joint to attach to the base of the proximal phalanges. "

In Osborne et al 2025 https://doi.org/10.1016/j.jshs.2025.101110 , the authors have considered the effects of adding mass and a forward lean, both of which will act to increase tension in the plantar fascia producing an increase in the plantarflexory force to the proximal phalanges( torque at the MTPJs ) .

When considering torque the authors have omitted the effects of the reverse windlass first suggested by Wicks 70 odd years ago, indeed the plantar fascia ,a key player in foot mechanics, is not mentioned in the paper at all. What has happened here?

 

A major problem when measuring toe flexor torque /power around the MTPJs is taking into account what the toe interproximal joints are doing.

If we press our toes down hard, and the lumbricals and interossei are weak, then the toes will adopt a hammertoe configuration. The mechanics of this configuration are very different than if the IP joints did not buckle under load especially with regard to tension in the plantar plates.

I have taken to saying that toes which buckle under load but which are not in a deformed configuration when not under load show " functional lesser toe deformity" . Functional lesser toe deformity is not the same as flexible lesser toe deformity which may well be one stop further down the line.

Susan Mayes has long advocated calf raises as a rehab/ prehab exercise but with an emphasis on keeping the toes straight at the IP joints.

 

Susan Mayes was head physio at Australian Ballet and was conspicuously successful when it came to helping her dancers avoid or recover from injury . She used calf raises to strengthen the ankle planter flexors and separate intrinsic foot muscle exercises for these muscles. My understanding is that she has recently started using banded flexion exercises for the hallux and 5th toe in preference to exercises like the short foot exercise.

Elsewhere, Torstein E. Dæhlina et al 2023, studied weighted calf raises performed over 11 weeks and found that "No changes were observed in the ankle plantar flexor or II-V digit flexor strength (n > 0.05). "

Osborne et al found that "The exercises with the highest mean EMG RMS amplitude were: single leg heel raise for AH (57.6% ± 19.7%), hallux flexion for FHB (62.5% ± 24.7%), and the single leg heel raise for FDB (58.1% ± 20.6%)."

So adding a forward lean to the single leg calf raise does not increase mean muscle activation in the AH or FDB. One might speculate that given weighted calf raises do not increase 2-5 digit flexor strength ( Daehlin ) and leaning calf raises activate the FDB to a lesser extent than calf raises without a lean (Osborne), calf raises of any type are not a good way of strengthening the lesser toes.