Updated: Oct 21, 2020
Resistance training can transition fiber composition from slow-twitch to fast-twitch fibers. More importantly, several studies have shown that neural adaptations to resistance training can switch the fast-twitch type IIx fibers to type IIa fibers (Andersen & Aagaard, 2010; Andersen, Schjerling, & Saltin, 2000; Liu et al., 2003; Wilson et al., 2012). The adaptation from type IIx fibers to type IIa fibers could imply that hypertrophy is occurring in the muscle cross-sectional area. This theory is further supported by Andersen, Schjerling, & Saltin (2000), as these researchers noted that type IIa fibers produce more protein, have a positive correlation with physical activity, and have thicker muscle fibers than the type IIx fibers. In fact, type IIx fibers have been listed as default force-producing fibers in sedentary people. Moreover, there is a higher concentration of type IIx muscle fibers in inactive people than there are in physically active individuals (Andersen, Schjerling, & Saltin, 2000). This can explain the conversion that occurs when inactive individuals participate in chronic resistance training for more than a month.
To better substantiate these prior claims, Liu et al. (2003) tested the impact resistance training has on fiber switching in a 6 week, 3 days per week training protocol. The study matched 12 healthy subjects in a maximum strength group, against another 12 subjects in a maximum strength “combo” group which added ballistic and stretch-shortening movements. The results illustrated that the maximum strength group decreased their type IIx fibers by 13.9% (33.4-19.5%). This was matched by an increase of 21.8% (44.9 to 66.7%) in the concentration of type IIa fibers. However, no significant change was observed in the type I fibers. The combo group demonstrated similar results for type IIa fibers as it increased by 15% (47.7% to 62.7%). This was not matched by similar inverse findings in type IIx fibers, as these fibers remained relatively unchanged. The inverse change for the combo group occurred in the type I fibers as these fibers decreased by 9% (18.2 to 9.2%). The results from this study indicated that training at a higher velocity increases the concentration of type II fibers by maintaining the number of type IIx fibers, and converting a significant percent of type I fibers to type IIa fibers (Liu et al., 2003). This would appear to be an ideal conversion for high performing power athletes in football, and athletes competing in short distance racing, namely track and field sports.
Andersen, J. L., & Aagaard, P. (2010). Effects of strength training on muscle fiber types and size;
consequences for athletes training for high-intensity sport. Scandinavian journal of medicine & science in sports, 20 Suppl 2, 32–38. https://doi.org/10.1111/j.1600-0838.2010.01196.x
Andersen, J. L., Schjerling, P., & Saltin, B. (2000). Muscle, genes and athletic performance.
Scientific American, 283(3), 48–55. https://doi.org/10.1038/scientificamerican0900-48
Liu, Y., Schlumberger, A., Wirth, K., Schmidtbleicher, D., & Steinacker, J. M. (2003). Different effects on human skeletal myosin heavy chain isoform expression: strength vs. combination training. Journal of applied physiology (Bethesda, Md. : 1985), 94(6), 2282–2288. https://doi.org/10.1152/japplphysiol.00830.2002
Wilson, J. M., Loenneke, J. P., Jo, E., Wilson, G. J., Zourdos, M. C., & Kim, J. S. (2012). The effects of endurance, strength, and power training on muscle fiber type shifting. Journal of strength and conditioning research, 26(6), 1724–1729. https://doi.org/10.1519/JSC.0b013e318234eb6f