Can toe curls weaken feet ?

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Are toe curls primarly an intrinsic or extrinsic foot muscle exercise and why does it matter?

Research shows that toe curls activate primarily the long toe flexor muscles, flexor digitorum longus and hallucis longus, whilst another recent study found that, in the group studied ,extrinsic foot muscle exercise conducted before intrinsic exercise resulted in reduced intrinsic muscle size.

Differential Effects of Intrinsic- Versus Extrinsic-First Corrective Exercise Programs on Morphometric Outcomes and Navicular Drop in Pediatric Pes Planus
Jafar Ketabchi, Foad Seidi, Shila Haghighat, Sue Falsone, Yousef Moghadas-Tabrizi, and 1 more
This is a preprint; it has not been peer reviewed by a journal.
https://doi.org/10.21203/rs.3.rs-3974670/v1
This work is licensed under a CC BY 4.0 License
Abstract
Although the connection between muscular strength and flatfoot condition is well-established, the impact of corrective exercises on these muscles remains inadequately explored. This study aimed to assess the impact of intrinsic- versus extrinsic-first corrective exercise programs on muscle morphometry and navicular drop in boys with flexible flatfoot. Twenty-five boys aged 10–12 with flexible flatfoot participated, undergoing a 12-week corrective exercise program, with a shift in focus at six weeks. Ultrasound imaging measured muscle thickness and cross-sectional area (CSA), and the navicular drop test assessed flatfoot severity. The results demonstrated a significant interaction between exercise type and sequencing on muscle morphometry. Initiating with intrinsic exercises led to sustained improvement, even after transitioning to extrinsic exercises, while extrinsic-first exercises caused deterioration in intrinsic muscle morphology, which was recovered after transitioning to intrinsic exercises. Statistical analysis revealed significant improvements in muscle thickness and CSA over time, particularly when initiating intrinsic exercises first. The intrinsic-first group also exhibited a more pronounced reduction in navicular drop. In conclusion, initiating corrective exercises with intrinsic muscles proved more effective in improving foot muscle morphometry and reducing navicular drop in boys with flatfoot. Therefore, commencing correction with intrinsic muscle exercises is recommended before progressing to extrinsic muscle exercises.

 

From Ketabchi et al above , it looks like doing extrinsic foot muscle strengthening without intrinsic muscle strengthening will weaken the existing intrinsic muscles ,at least in flat footed boys 10-12.

Calf raises was one of the exercises used and that is worth noting since from the research this exercise done alone as an intrinsic strengthening exercise may actually have the opposite effect.

Kebatchi et al used a number of intrinsic specific exercises which included the short foot exercise and toe curls . But are toe curls actually an intrinsic muscle exercise?

Research indicates that toe curls are primarily about the long toe flexors and this is particularly true of the hallux . You can curl your big toe backwards against an immovable steel ring just as hard as you can and activity in the abductor hallucis will still be small. Likely the same will be true of the flexor hallucis brevis.

If you want to strengthen the intrinsic plantar muscles then you need to be moving the toes around the MTPJ with no flexion in the PIP joints ie straight toes.

How do you strengthen muscles that extended the PIP joints( lumbricals and interossei ) with an exercise that flexes them ?

Based on Ketabchi, and I admit it's a bit of a stretch, I would not be at all surprised to find that a person with a "normal" musculature for a shod foot demonstrates muscle atrophy in abductor hallucis after of program of toe curls. Or even toe curls and calf raises combined.
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Here is a very recent paper that makes it clear that toe curls, or even toes press down exercises that involve flexion of the great toe at the PIP joint ,target the flexor hallucis longus( extrinsic ) rather than the abductor hallucis ( intrinsic) , and that toe press down exercises that do not involve flexion at the PIP joint target the abductor hallucis more than the extrinsic toe flexors.

This is probably why a recent expert consensus paper left toe curls out of their recommended exercises for intrinsic foot muscle strengthening.

Ketabachi et al use toe curls as an intrinsic foot muscle exercise but they also used other more appropriate exercises such as the short foot exercise. Based on the evidence and IMO, the belief that toe curls are primally about the intrinsics is an error and a potentially very important one.

The Foot

Volume 60, September 2024, 102111

Differences in muscle activity of extrinsic and intrinsic foot muscles in toe grip and push-down movements of the great toe

Author links open overlay panelYuki Ogawa a b, Toshihiro Maemichi c, Ryusei Yamaguchi a, Takumi Okunuki a d, Osamu Kinoshita e, Hideaki Nagamoto a, Tsukasa Kumai c
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Highlights

• •
  IPJ flexion of the great toe activates the FHL. Toe push-down activates AbdH.
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  The presence or absence of IPJ flexion affects the AbdH/FHL muscle activity ratio.
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  TP with IPJ extension position primarily activates AbdH.
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  Toe flexion method selection is essential for muscle strength measuring and training.

Abstract

Toe flexor strength is generated primarily by the flexor hallucis longus (FHL) of the extrinsic foot muscles (EFMs) and the plantar intrinsic foot muscles (PIFMs) of the great toe. Toe flexion methods can be broadly classified into toe grip (TG) and toe push-down (TP). Additionally, TP's interphalangeal joint (IPJ) position may influence the FHL and PIFMs activity ratios. This study aimed to elucidate the differences in the muscle activity and muscle activity ratios of the FHL and AbdH during TG, TP with IPJ flexion (TPIF), and TP with IPJ extension (TPIE).
Surface electromyography and a custom-made instrument were used to measure the FHL and AbdH muscle activity during TG, TPIF, and TPIE of the great toe in 28 healthy men. The muscle activity and AbdH/FHL muscle activity ratio in the three conditions were statistically compared.
The FHL activity was significantly higher during TG and TPIF than during TPIE. The AbdH muscle activity was significantly higher during TPIF and TPIE than that during TG. The AbdH/FHL muscle activity ratio was significantly higher for TPIE, TPIF, and TG in that order.
This study showed that the FHL and AbdH muscle activity differed depending on the TG and TP of the great toe, and that the AbdH/FHL muscle activity ratio was different in the IPJ position. These results suggest that selecting a toe flexion method according to the target muscle when measuring and training the great toe flexor strength is important.