Do the anatomical and physiological properties of muscle determine its adaptive response to different loading protocols?

Key points

1. 01Light and heavy loads produced similar muscle hypertrophy in all three calf muscles over 8 weeks
2. 02The predominantly slow-twitch soleus grew similarly to the mixed-fiber medial gastrocnemius (mean increase ~7.8% vs ~8.9%)
3. 03The lateral gastrocnemius showed the greatest growth (13.7%), more than the medial gastrocnemius and soleus
4. 04Isometric plantarflexion strength increased similarly under both loading conditions (~16-18% torque increase)
5. 05Low-load training is a viable strategy for building muscle in the lower leg, equivalent to high-load training when sets are taken to failure

How it was conducted

Design

Randomized within-subject controlled trial; each participant trained one leg with light load and the contralateral leg with heavy load simultaneously

Participants

26 untrained young men (mean age 22.5 years, weight 77.3 kg) who completed the 8-week protocol (30 enrolled, 4 dropped out)

Intervention

8 weeks of supervised plantarflexion exercise (seated and standing calf raises), 4 sets per exercise, 2 sessions per week, sets taken to momentary concentric failure

Groups

LIGHT: 20-30 repetition maximum; HEAVY: 6-10 repetition maximum

Primary outcome

Muscle thickness of soleus, medial gastrocnemius, and lateral gastrocnemius via B-mode ultrasound

Secondary outcome

Maximal isometric plantarflexion strength via isokinetic dynamometry

What they found

• Soleus muscle thickness increased by 1.3 mm (heavy) and 1.5 mm (light); between-condition difference was 0.2 mm (90% CI: -0.3 to 0.7 mm) favoring light
• Medial gastrocnemius thickness increased by 1.5 mm (heavy) and 1.8 mm (light); between-condition difference was 0.2 mm (90% CI: -0.2 to 0.8 mm) favoring light
• Lateral gastrocnemius thickness increased by 2.1 mm (heavy) and 2.3 mm (light); between-condition difference was 0.2 mm (90% CI: -0.5 to 0.8 mm) favoring light
• Differential growth between MG and soleus across loading conditions was 0.03 SDs greater in MG (90% CI: -0.17 to 0.24 z-scores); differential for LG vs soleus was 0.02 SDs (90% CI: -0.24 to 0.28 z-scores)
• Lateral gastrocnemius grew by 0.63 SDs (90% CI: 0.52 to 1.00), more than MG at 0.44 SDs (90% CI: 0.30 to 0.65) and soleus at 0.33 SDs (90% CI: 0.25 to 0.44)
• Isometric plantarflexion strength increased by 15 N·m in both conditions; between-condition difference was -1.2 N·m (90% CI: -7.4 to 4.5 N·m) favoring heavy
• Overall calf hypertrophy was 9.2% (heavy) vs. 10.7% (light) across all triceps surae muscles

Limitations

• Results are specific to the triceps surae; generalizability to other muscle groups is uncertain
• Sample was limited to untrained young men; findings may not apply to women, older adults, or trained individuals
• No muscle biopsies were performed, so individual fiber type differences between muscles could not be directly confirmed
• Muscle thickness was measured at a single site, so non-uniform hypertrophy along the muscle length would not be detected

Why it matters

For patients

People who cannot lift heavy weights due to injury or preference can still expect similar muscle gains from lighter, high-repetition training done to effort.

For clinicians

Prescribing load based on a muscle's fiber type composition is not supported by this evidence; effort level (training to failure) appears more important than load magnitude for hypertrophy outcomes.

For readers

The popular idea that slow-twitch muscles need light loads and fast-twitch muscles need heavy loads is not confirmed in this human study; both calf muscles grew equally well with either approach.