Evaluation of skeletal muscle activity during foot training exercises using positron emission tomography

Key points

1. 01Rock-paper-scissors activated 4 muscles significantly: flexor digitorum longus, extensor hallucis brevis, extensor digitorum brevis, and quadratus plantae (vs. the resting control limb).
2. 02Towel gathering activated 4 muscles significantly: tibialis anterior, extensor digitorum longus, peroneus brevis, and abductor hallucis (vs. the resting control limb).
3. 03Extensor hallucis brevis showed significant activation in both exercises; extensor hallucis longus showed significantly greater activation during towel gathering than rock-paper-scissors (p=0.046).
4. 04FDG-PET measures glucose uptake as a proxy for muscle activity, allowing comprehensive mapping of all 25 lower-limb muscles without electrode interference.
5. 05This is the first study to use FDG-PET to comprehensively evaluate lower-limb skeletal muscle activity during intrinsic foot muscle training exercises.

How it was conducted

Design

Prospective randomized controlled experiment with within-person (training vs. control limb) comparisons

Participants

8 healthy university athletes (4 per group); age 20.5+/-0.6 years in RPS group, 20.8+/-1.0 years in TG group

Groups

Rock-paper-scissors (RPS) group and towel-gathering (TG) group; right foot trained, left foot served as control

Intervention

Continuous exercise for a set period with intravenous injection of 3 MBq FDG; PET-CT imaging performed after training and a 45-minute rest

Primary outcome

Standardized uptake value (SUV) of FDG in 25 skeletal muscles of the lower limb, compared between training and control limbs

What they found

• In the RPS group, flexor digitorum longus showed significantly higher SUV in the training limb vs. control (0.66+/-0.11 vs. 0.60+/-0.13, p=0.012).
• In the RPS group, extensor hallucis brevis showed significantly higher SUV in the training limb vs. control (0.96+/-0.34 vs. 0.65+/-0.22, p=0.022).
• In the RPS group, extensor digitorum brevis showed significantly higher SUV in the training limb vs. control (1.77+/-0.9 vs. 0.60+/-0.18, p=0.049).
• In the RPS group, quadratus plantae showed significantly higher SUV in the training limb vs. control (0.99+/-0.32 vs. 0.71+/-0.18, p=0.033).
• In the TG group, tibialis anterior showed significantly higher SUV in the training limb vs. control (0.79+/-0.03 vs. 0.64+/-0.13, p=0.026).
• In the TG group, extensor digitorum longus showed significantly higher SUV in the training limb vs. control (0.72+/-0.11 vs. 0.55+/-0.04, p=0.048).
• In the TG group, peroneus brevis showed significantly higher SUV in the training limb vs. control (0.72+/-0.05 vs. 0.57+/-0.08, p=0.016).
• In the TG group, extensor hallucis brevis showed significantly higher SUV in the training limb vs. control (0.94+/-0.11 vs. 0.79+/-0.08, p=0.042).
• In the TG group, abductor hallucis showed significantly higher SUV in the training limb vs. control (0.94+/-0.12 vs. 0.68+/-0.09, p=0.007).
• Only extensor hallucis longus showed a significant difference between the RPS training and TG training groups (p=0.046), with greater activity during towel gathering.
• No significant differences were found between the RPS control and TG control limbs for any of the 25 muscles, confirming baseline comparability.

Limitations

• Very small sample size (4 participants per group), limiting statistical power and generalizability.
• Study conducted on trained athletes only; results may not apply to patients with foot or lower-extremity disorders.
• FDG-PET captures only glucose uptake and may not fully reflect all skeletal muscle activity, particularly in muscles with higher type II fiber composition.
• Only a single training session was assessed; long-term effects on muscle strength, arch support, and static balance remain unknown.

Why it matters

For patients

People doing foot exercises for arch support or toe strength can understand that rock-paper-scissors and towel gathering target different muscles and serve different rehabilitation goals.

For clinicians

Clinicians can use this evidence to select between foot rock-paper-scissors (medial longitudinal arch) and towel gathering (toe grip and ankle stabilizers) based on the specific muscle groups they want to activate in rehab programs.

For readers

This is the first FDG-PET study to comprehensively map lower-limb muscle activation during common intrinsic foot exercises, providing an objective imaging basis for exercise prescription that goes beyond surface EMG.