Endurance training improves oxygen uptake and mitochondrial function. Interval training is often promoted as a time-efficient alternative to steady-state cardio, and some claim it might even stimulate greater muscle growth.
We compared High-Intensity Interval Training (HIIT) and Moderate Intensity Continuous Training (MICT) over two weeks. Myofibrillar protein synthesis rates were assessed, reflecting muscle contractile proteins. Each of the ten subjects trained one leg with HIIT and the other with MICT, with both protocols matched for total volume and duration.
Key findings:
- Single-leg maximal workload and VO2peak improved with both HIIT and MICT.
- Myofibrillar protein synthesis did not differ between HIIT, MICT, or the control period.
Interpretation
Even though HIIT had a higher exercise intensity, it did not increase muscle protein synthesis rates more than MICT. In fact, neither training mode increased muscle protein synthesis rates compared to the control period, during which participants performed daily physical activity.
These findings suggest that even higher-intensity endurance exercise is not a strong stimulus for muscle protein synthesis. This aligns with long-term studies showing minimal impact of endurance exercise intensity on gains in fat-free mass.
A major strength was the one-leg within-subject design with volume- and duration-matched protocols, which allowed a clean comparison of exercise intensity. Limitations included the fact that single-leg cycling does not reflect real-world training and that habitual physical activity was not controlled.
Link to our open-access study:
Trommelen et al., Daily Myofibrillar Protein Synthesis Rates Do Not Differ During Interval Compared to Continuous Exercise Training Matched for Duration and Work in Healthy Young Men. IJSNEM, 2026.
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