Plantar intrinsic foot muscle activation during functional exercises compared to isolated exercises

Key points

1. 01Functional exercises matched or exceeded isolated foot exercise activation in 100% of comparisons; in 3 of 12 comparisons they were significantly higher.
2. 02Hopping produced significantly greater activation of flexor hallucis brevis (median +48%) and abductor hallucis (median +108%) than the best isolated exercise.
3. 03Toe stance on a compliant surface also produced significantly greater flexor hallucis brevis activation than its isolated counterpart.
4. 04Integrated muscle activation (contraction duration x amplitude) was lower for dynamic exercises, but only 1-3 toe-walking steps or 3-9 hops were needed to match a 3-second isolated contraction.
5. 05Isometric isolated exercises activated intrinsic foot muscles more than concentric exercises, but still did not outperform functional exercises.

How it was conducted

Design

Crossover experiment; all exercises performed in a single laboratory session

Participants

29 healthy younger adults (21 women, 8 men); mean age 23.3 years (SD 3.8)

Muscles measured

Abductor hallucis, flexor digitorum brevis, flexor hallucis brevis (plantar intrinsic); flexor hallucis longus (extrinsic, secondary)

Exercises compared

4 functional (toe stance, toe stance on compliant surface, toe walking, hopping) vs. the best muscle-specific isolated foot exercise for each of 3 muscles (12 comparisons total)

Outcome measure

Surface EMG mean amplitude (%) and integrated EMG over time (iEMG, %*s), normalised to a reference exercise

Statistics

Wilcoxon signed-rank tests (one-tailed, alpha=0.05) with Holm-Bonferroni correction for multiple testing

What they found

• 3 of 12 mean EMG amplitude comparisons showed significantly greater activation for functional exercises; 9 of 12 showed no significant difference.
• Hopping vs. hallux grip (FHB): median difference +48%, IQR 4% to 80%, p<0.05.
• Hopping vs. toe spread-out (AbH): median difference +108%, IQR 79% to 153%, p<0.05.
• Toe stance on compliant surface vs. hallux grip (FHB): median difference +29%, IQR -9% to 54%, p<0.05.
• iEMG was significantly smaller for functional exercises in 5 of 6 dynamic comparisons.
• Toe walking iEMG was 2.8 times smaller than the isolated exercise for FHB (median difference -103%*s, IQR -215 to -66%*s, p<0.05) and 2.9 times smaller for FDB (median difference -99%*s, IQR -248 to -30%*s, p<0.05).
• Hopping iEMG was 9.1 times smaller for FHB (median difference -157%*s, IQR -255 to -111%*s, p<0.05), 7.7 times smaller for FDB (median difference -160%*s, IQR -424 to -47%*s, p<0.05), and 2.4 times smaller for AbH (median difference -33%*s, IQR -327 to -15%*s, p<0.05).
• Isometric exercises produced higher mean EMG than concentric exercises for FHB (median -14%, IQR -33 to -5%, p<0.05) and AbH (median -8%, IQR -72 to -4%, p<0.05), but concentric produced higher activation for FDB (median +23%, IQR -13% to 84%, p<0.05) and FHL (median +48%, IQR 24% to 86%, p<0.05).
• Forward lean during toe stance did not significantly change PIFM activation (FHB p=0.05, FDB p=0.05, AbH p=0.22).

Limitations

• All participants were healthy young adults; results may not apply to older adults, people with diabetic neuropathy, or those with PIFM dysfunction.
• Surface EMG cannot fully isolate intrinsic from adjacent or deeper muscles, though prior literature suggests crosstalk remains within reasonable limits.
• No objective performance measures (force plates, motion capture) were used, so suboptimal exercise execution cannot be ruled out.
• Cross-sectional EMG data cannot predict long-term structural or functional adaptations from training.

Why it matters

For patients

Exercises like standing on your toes or toe walking may be just as good as, or better than, specialised foot exercises at working the small muscles of the foot, and are generally easier and more comfortable to perform.

For clinicians

Functional weight-bearing exercises in toe stance (toe walking, single-leg toe stance, hopping) can produce equivalent or superior intrinsic foot muscle activation compared to the best isolated foot exercises, and their ease of execution and adherence advantages make them a strong option for PIFM rehabilitation.

For readers

This is the first study to directly compare functional and isolated foot exercises for intrinsic foot muscle activation; findings support a shift toward functional training but require confirmation in clinical populations through longitudinal intervention studies.