Since resistance-training using conventional loading protocols is thought to lead to greater hypertrophy of type II muscle fiber areas, it might be expected to alter muscle fiber type proportion. But does this actually occur? In this article, Chris Beardsley (@SandCResearch) reviews the literature.
What is the background?
What are muscle fiber types?
In brief, muscle fibers can be classified in three main ways: myosin ATPase histochemical staining, MHC isoform identification, and biochemical identification of metabolic enzymes (Scott, 2001). However, the most common ways involve either ATPase or MHC isoforms. Many studies now use both of these methods. For more background, see my previous article on muscle fiber types in different muscles.
Why assess if resistance-training alters muscle fiber type?
For strength and power athletes
Muscle fiber type is thought to be one of several factors that affect performance in sports that require explosive muscle actions. Having a higher proportion of type II muscle fibers may be beneficial to athletes competing in such sports. Resistance-training might help improve explosive performance in part by changing the proportion of muscle fiber type (although the main factors are of course increases in muscular cross-sectional area, neural drive and neuromuscular co-ordination).
For endurance athletes
In contrast, having a higher proportion of type I muscle fibers is thought to be helpful for athletes competing in endurance sports. Thus, increasing type II muscle fiber proportion might be thought to be an adverse adaptation. However, in contrast to this idea, resistance-training has generally been found to be beneficial for many endurance athletes. It is thought that such improvements in arise as a result of enhancements in work economy. Whether these increases in work economy occur as an effect of muscle fiber type changes is unclear.
What are the selection criteria for this review?
In this review, I have selected long-term trials involving a resistance-training intervention that have measured muscle fiber type using either MHC isoforms or myofibrillar ATPase, or both. Where trials have investigated concurrent training methods, I have only incorporated the group that trained using resistance-training only.
Does resistance-training alter fiber type in untrained subjects?
The studies in the tables below have explored how resistance-training affects muscle fiber type proportion in untrained subjects. The following table shows where studies have reported significant or non-significant increases or decreases in type I and type II muscle fiber distributions:
The following table shows where studies have reported significant or non-significant increases or decreases in type IIa and type IIx muscle fiber distributions:
It is clear that resistance-training in untrained subjects does not lead to a shift in type I to type II muscle fiber distribution.
There is also some evidence that resistance-training causes a shift in muscle fiber distribution within the sub-types of type II muscle fibers, from type IIx muscle fiber type to type IIa muscle fiber type.
Does resistance-training alter fiber type in trained subjects?
The studies in the table below have explored how resistance-training affects muscle fiber type proportion in resistance-trained subjects:
Resistance-training seems to have little effect on muscle fiber type in resistance-trained subjects.
What could explain the lack of changes?
The lack of changes observed in resistance-trained subjects could be because any changes that are likely to occur have already happened by the point at which further training is undertaken. Alternatively, it could be that the changes are very slow in this population, or it could be that the inter-individual variation is very high, which makes detecting a significant difference very difficult (leading to type II error). Further research is clearly needed in this area.
What do we know about these studies?
If you’re curious about the methodology used in the studies cited above (i.e. whether the researchers used histochemical analysis or immunohistochemical methods, and what type of resistance-training was performed) or if you simply just want the references, here are the details and links. They’re in alphabetical order:
Aagard (2001) investigated the changes in muscle fiber type in 11 male subjects who undertook 14 weeks of heavy, lower-body resistance-training. Muscle fiber biopsies were taken from the vastus lateralis before and after the intervention and MHC isoform distribution (type I and II) was assessed.
Adams (1993) investigated the changes in muscle fiber type following 19 weeks of heavy resistance-training. They took muscle biopsies from the vastus lateralis and analyzed them biochemically for MHC composition and histochemically for fiber types with myofibrillar ATPase.
Aniansson and Gustafsson (1981) investigated the effects of resistance-training, 3 times per week for 12 weeks in 12 elderly (69 – 74-years) but otherwise healthy males. The researchers took muscle biopsies from the vastus lateralis muscle for analysis.
Bishop (1999) explored the effects of 12 weeks of lower-body resistance-training in 21 endurance-trained, female cyclists, aged 18 – 42 years. Before and after the intervention, the researchers took a muscle biopsy from the vastus lateralis and analyzed the fiber type percentage and the activities of 2-oxoglutarate dehydrogenase and phosphofructokinase.
Brown (1990) investigated the effects of 12 weeks of both upper- and lower-body resistance-training in 14 elderly males.
Campos (2002) explored the effects of different programs of resistance-training in 32 untrained men over an 8-week intervention. The subjects were divided into four groups: a low repetition group (3 – 5RM for 4 sets), an intermediate repetition group (9 – 11RM for 3 sets), and a high repetition group (20 – 28RM) who all performed 3 exercises (leg press, squat, and knee extension). Before and after the intervention, the researchers took muscle biopsy samples and analyzed fiber-type composition by reference to both ATPase and MHC isoforms.
Carroll (1998) examined the effects of resistance-training of the leg extensor and flexor muscle groups performed 2 – 3 times per week. Before and after the intervention, the researchers measured changes in MHC isoforms in the vastus lateralis.
Charette (1991) explored the effects of a 12-week resistance-training program in 27 healthy, elderly women (age 69 ± 1 years) and took muscle biopsies before and afterwards in order to assess muscle type.
Costill (1979) investigated the effects of 7 weeks of isokinetic resistance-training in 5 males in order to determine the effects on muscle enzyme activities and muscle fiber type. Before and after the intervention, the researchers took muscle biopsies and assessed fiber type using ATPase.
Côté (1988) explored the effects of concentric isokinetic resistance-training protocols separated by a 50-day detraining period on proportion of muscle fiber type and enzyme activities.
Coyle (1981) investigated the effects of an intervention involving maximal two-legged isokinetic knee extensions performed 3 times per week for 6 weeks at either 60 degrees/s or 300 degrees/s or both 60 and 300 degrees/s in college-aged males. Muscle fiber type was assessed before and afterwards using ATPase.
De Souza (2014) compared the effects of 8 weeks of concurrent, strength-only and interval training on muscle fiber type in 37 physically active males.
Farup (2014) compared the effects of 10 weeks of either resistance-training or endurance cycling on muscle fiber type. The researchers took muscle biopsies from the vastus lateralis to quantify fiber phenotype.
Häkkinen (2001) explored the effects of a 6-month resistance-training program (2 days per week) on muscle fiber proportion of the vastus lateralis in 10 middle-aged men, 11 middle-aged women, 11 elderly men and 10 elderly women.
Häkkinen (2003) compared the effects of 21 weeks of either concurrent strength and endurance training vs. resistance-training only. The researchers assessed muscle fiber proportion in the vastus lateralis using ATPase.
Hather (1991) investigated the changes in muscle fiber type following 19 weeks of heavy resistance-training involving either concentric-only or eccentric-only muscle actions in the leg press and leg extension exercises, performed 2 days per week. The researchers took muscle biopsies from the vastus lateralis and analyzed them histochemically for fiber types with myofibrillar ATPase.
Jackson (1990) assessed muscle fiber area changes from two opposing resistance-exercise training regimes in the quadriceps muscle group in 12 college-age men. One program was focused on strength (high-resistance, low-repetition) and the other on muscular endurance (low-resistance, high-repetition). The researchers took muscle biopsies from the vastus lateralis to assess muscle fiber type proportion changes.
Karavirta (2011) assessed the interference effect of combined strength and endurance training in 96 previously untrained 40 – 67-year-old men over a 21-week training period.
Kraemer (1995) compared the effects of different types of training across four training groups that performed either high-intensity strength and endurance training, upper-body only high-intensity strength and endurance training, high-intensity endurance training, or high-intensity strength training. Muscle fiber type proportion was assessed using ATPase.
Malisoux (2006) assessed the effects of plyometric training in 8 males. They took muscle biopsies from the vastus lateralis before and after the intervention and analyzed muscle fiber type according to MHC isoforms.
McCall (1996) investigated the effects of 12 weeks of intensified resistance-training (3 sessions per week, 8 exercises per session, 3 sets per exercise, 10RM per set) in 12 male subjects with recreational resistance training experience. The researchers took muscle biopsies of the biceps brachii and analyzed them using ATPase.
Netreba (2013) explored the effects of an 8-week period of leg press resistance-training on the muscle fiber type of the vastus lateralis in 30 male subjects. There were 3 different groups, who trained using 25, 65 and 85% of 1RM.
Putman (2004) investigated the effect of strength training, endurance training, and concurrent training on muscle fibre type transitions in 40 subjects using the vastus lateralis muscle. MHC isoforms were assessed in order to determine muscle fiber type.
Pyka (1994) explored the effects of a resistance-training program in 8 male and 17 female elderly subjects over a 1-year period. The researchers took muscle biopsies at baseline and after 15 and 30 weeks. The program comprised a 12-exercise circuit (3 sets of 8 repetitions at 75% of 1RM), 3 times a week.
Roman (1993) investigated changes in the structural characteristics of the elbow flexors in 5 elderly males following 12 weeks of heavy-resistance training. The researchers took muscle biopsies of the biceps brachii muscle and assessed muscle percent fiber distribution histologically with ATPase.
Scheunke (2012) assessed the effects of different types of resistance-training program in 34 untrained women in a 6-week program. All subjects performed the leg press, squats, and knee extensions 2 – 3 days per week with either 6 – 10RM for each set or 20 – 30RM for each set. Additionally, the 6 – 10RM group was subdivided into groups that performed very slow repetitions or normal speed repetitions. The researchers collected muscle biopsies and analyzed them by reference to both ATPase and MHC isoforms.
Thorstensson (1976) assessed the effects of a lower-body resistance-training program performed 3 times a week for 8 weeks by 14 male students. The researchers took muscle biopsies from the vastus lateralis for muscle fiber analysis using ATPase.
Trappe (2000) examined the effects of 12 weeks of progressive knee extensor resistance training in 7 older men who trained 3 days per week at 80% of 1RM. Before and after the intervention, the researchers took muscle biopsy samples from the vastus lateralis and analyzed the MHC isoforms.
Wang (1993) the researchers collected muscle biopsies from the vastus lateralis muscle before and after 18 weeks of resistance-training and performed fiber typing using ATPase.
Widrick (2002) assessed the effects of a 12-week period of lower-body resistance-training in 6 young male subjects. The researchers took vastus lateralis muscle biopsies and analyzed the MHC isoforms.
Williamson (2000) took muscle biopsies from the vastus lateralis examined MHC isoforms following 12 weeks of progressive knee extensor resistance-training in 7 healthy men.
What are the practical implications?
For trained individuals
Resistance-training does not change muscle fiber type in resistance-trained individuals, whether between type I and II or between types IIa and IIx. Training protocols should not be designed to alter muscle fiber type but should take into account the existence of the various muscle fiber types present in the muscles.
For untrained individuals
Resistance-training does not change muscle fiber type in untrained individuals between type I and II muscle fibers. Training protocols should not attempt to alter muscle fiber type between type I and II muscle fibers but should take into account the existence of the various muscle fiber types present in the muscles.
Resistance-training does change muscle fiber type in untrained individuals between type IIa and IIx muscle fibers. However, the percentage of type IIx muscle fibers is small and whether such shifts can be prevented (if they are in fact undesirable) is unclear.