The Effect of Concurrent Training on Fibre Type Conversion
Does this happen?
Concurrent training is when we combine endurance and strength training within the same programme. It is popular with athletes who want to improve multiple fitness qualities, but if programmed inappropriately can create a phenomenon known as the interference effect. This occurs when the adaptations from one training type inhibit the development of the other. When looking specifically at muscle fibre types, concurrent training can influence how these fibres adapt and even shift from one type to another.
Skeletal muscles are made up of slow-twitch (Type I) fibres and fast-twitch fibres. The fast-twitch group includes Type IIa and Type IIx fibres. Type I fibres are highly resistant to fatigue and are suited for endurance work. Type IIa fibres are fast but also more oxidative, making them a middle ground between endurance and strength and power. Type IIx fibres are the most explosive, capable of generating high force at high speeds, but they fatigue quickly and have low oxidative capacity (Schiaffino & Reggiani, 2011).
Research shows that endurance training tends to promote a shift from Type IIx towards Type IIa and sometimes towards Type I (Andersen & Aagaard, 2000). This is partly because endurance work increases mitochondrial biogenesis and oxidative enzymes, making the muscle more resistant to fatigue. In concurrent training, especially when endurance work is frequent or intense, this same shift can occur. One study on concurrent training found that after 12 weeks, participants saw an increase in Type IIa fibres at the expense of Type IIx (Häkkinen et al., 2003). This change was more pronounced in the concurrent training group than in the strength-only group.
The molecular explanation for this lies in gene expression. Endurance training activates pathways such as AMPK and PGC-1α, which encourage the muscle to develop more oxidative properties. This favours fibres like Type IIa and Type I, and as such the gene expression of fast twitch fibres shift in favour of fibres with more oxidative properties. This comes at the expense of type IIx fibres.
This does not mean that concurrent training will change all your explosive fibres. The changes are often subtle and influenced by how you schedule sessions, your training background, and recovery. For athletes who need to preserve Type IIx fibres for explosive performance, careful programming is essential. Strategies include increasing time between endurance and strength sessions, reducing unnecessary endurance volume, and ensuring explosive strength training is performed when fresh.
In short, concurrent training can cause a gradual shift from Type IIx to Type IIa fibres, especially when endurance work is frequent or intense. This shift can improve endurance but may reduce peak explosive potential if not managed carefully.
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References
Andersen, J. L., & Aagaard, P. (2000). Myosin heavy chain IIX overshoot in human skeletal muscle. Muscle & Nerve, 23(7), 1095–1104.
Egan, B., & Zierath, J. R. (2013). Exercise metabolism and the molecular regulation of skeletal muscle adaptation. Cell Metabolism, 17(2), 162–184.
Häkkinen, K., et al. (2003). Neuromuscular adaptations during concurrent strength and endurance training versus strength training. European Journal of Applied Physiology, 89(1), 42–52.
Schiaffino, S., & Reggiani, C. (2011). Fibre types in mammalian skeletal muscles. Physiological Reviews, 91(4), 1447–1531.