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Dominant Muscle Fiber Types

Hi All

Could someone help me out. I was wondering which muscles are Type I dominant and which are Type II dominant.

Thanks

Mark

found out so far

Dominant Type I muscles

  • soleus

Dominant Type II muscles

  • Gastocnemius
  • Hamstrings
  • Triceps
  • Abs

[quote]markandspike wrote:
Hi All

Could someone help me out. I was wondering which muscles are Type I dominant and which are Type II dominant.

Thanks

Mark
[/quote]
The following are the percentage of fiber type 1 in different muscle groups and which are fiber type is predominant.

Deltoid 57.1% (tonic muscle)
Erector spinae 56.4% (tonic muscle)
Supraspinatus 59.3% (tonic muscle)
Trapezius 53.7% (tonic muscle)
Latissimus dorsi 50.5% (tonic muscle)
Gluteus maximus 52.4% (tonic muscle)
Infraspinatus 45.3% (phasic muscle)
Rectus abdominis 46.1% (phasic muscle)
Biceps brachii 46.5% (phasic muscle)
Triceps 32.6% (phasic muscle)
Adductor magnus 58.2% (tonic muscle)
Biceps femoris 66.9% (tonic muscle)
Soleus 87.7% (tonic muscle)
Tibialis anterior 73.0% (tonic muscle)
Vastus medialis oblique 52.1 (tonic muscle)
Vastus lateralis 42.3 (phasic muscle)
Gastrocnemius (lateral head) 50.5 (tonic muscle)
Gastrocnemius (medial head) 43.5 (phasic muscle)
Rectus femoris 35.4 (phasic muscle)

Source: R. Colling (1997), Distribution of muscle fibre type. Ex Physiol 552

It’s hard to say because it is different for everyone. This is why some people do very little and gain so much and the next person does everything and gains so little. Genetics play a big role and your lifestyle may also play another role in determining how the percentages of your muscle fibre types may breakdown.

An indirect method that can be used in the weight room to determine the fiber composition of a muscle group is to initially establish the 1RM (the greatest weight you can lift just once). Then perform as many repetitions at 80% of your 1RM as you can. If you do fewer than seven repetitions, then the muscle group is likely composed of more than 50% FT fibers(Type II’s).

If you can perform 12 or more repetitions, then the muscle group has more than 50% ST fibers(Type I’s). If you can do between 7 and 12 repetitions, then the muscle group probably has an equal proportion of fibers.

Training a FT -fibered muscle for endurance will not increase the number of ST fibers, nor will training a ST-fibered muscle for strength and power increase the number of FT fibers. With the proper training, FT -B fibers can take on some of the endurance characteristics of FT -A fibers and FT -A fibers can take on some of the strength and power qualities of FT-B fibers. However, there is no inter-conversion of fibers. FT fibers cannot become ST fibers, or vice versa. What you’re born with is what you must live with.

Although the type of fiber cannot be changed from one to another , training can change the amount of area taken up by the fiber type in the muscle. In other words, there can be a selective hypertrophy of fibers based on the type of training.

For example, you may have a 50/50 mix of FT/ST fibers in a muscle, but since FT fibers normally have a larger cross-sectional area than ST fibers, 65% of that muscle’s area may be FT and 35% may be ST. Following a strength training program for improvement in muscular strength, the number of FT and ST fibers will remain the same (still 50/50), however the cross-sectional area will change. This happens because the ST fibers will atrophy (get smaller) while the FT fibers will hypertrophy (get larger).

Depending on the specific intensity used in training, the muscle may change to a 75% FT area and a 25% ST area. The change in area will lead to greater strength but decreased endurance capabilities. In addition, since the mass of FT fibers are greater than that of ST fibers, you will gain mass, as measured by the circumference of the muscle.

Conversely, if you train for muscular endurance, the FT fibers will atrophy while the ST fibers hypertrophy, causing a greater area of ST fibers. The area of the muscle, which began at 65% FT and 35% ST before training, may change to 50% FT and 50% ST following training, The endurance capabilities of the muscle will increase while its strength will decrease, and you will lose some muscle mass, again because ST fibers are lower in mass than FT fibers. The decrease in mass may be observed by a smaller circumference of the muscle.

Hope this helps.

merlin