A Tale of ageing muscle fibers

Author: Arpit Kushwaha, Assessment Division, INFS

      Key Points:

  • It has been seen that with an increase in age there are some changes in muscle fibers, and a decrease can be noticed in muscle mass and strength.
  • These changes in muscle fibers were found to be fiber-specific as well. Type II muscle fibers size decrease with age, while type I remains unaffected resulting in increasing percentage of Type I muscle fibers area proportion in elderly’s body.
  • Progressive resistance training is a well-established strategy to counteract the age-related decline in muscle mass and strength.

Key Terms: Aging, muscle loss, muscle gain, muscle fiber, strength, resistance training.


Aging is associated with loss of strength and muscle mass, which is also known as sarcopenia. This slight decrease in muscle mass is observed after 30 years of age and becomes more drastic as we age further. This reduction in muscle mass is associated with loss of strength and power, which may also lead to hampering proper muscle function and making individuals more prone to sudden falls or injuries. There is a conflicting data on the effect of aging on muscle fibers. In this article, we will understand how aging affects the muscle fiber’s number, size, and proportion.

Ageing- The villain:

With the increasing age, our body undergoes several metabolic changes, which includes progressive insulin resistance and a decrease in muscle protein synthesis (MPS) along with some age-related changes in muscle fibers. Type I muscle fibers also known as “slow twitch” or slow contracting fibers have many mitochondria and enzymes for energy production. They have more resistance towards fatigue and can metabolize intra-muscular triglycerides (IMTGs) for energy requirements. While type II fibers are categorized as a fast twitch or fast contracting and they are more easily fatigued. It is observed that with aging, there is a decrease in the total number of muscle fibers and their size. Interestingly, some studies have shown that this reduction in fiber size and number is fiber-specific. The major decrease has been observed in the size and number of type II muscle fibers, while type I remains largely unaffected, resulting in an overall increase in the percentage of type I muscle fibers. The size of type II muscle fibers has been reported to be 10-40% smaller in older individuals as compared to young people. Therefore, it is believed that one of the main contributing factors for the reduction in strength in elderly people can be this reduction in number and size of fast twitch muscle fiber. However, several studies have reported no change in the relative proportion of type I and II muscle fibers and thus, it remains unclear whether aging affects the fiber-type proportion but the overall conclusion in terms of a decrease in type II fiber size is very consistent.

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Figure 1: The graph shows the overall area proportion of Types I, IIA, and IIB muscle fibers and connective tissues in younger (age < 65 years) and older (age 65) subjects. subjects older than 65 years had more Type I muscle fibers and connective tissues but relatively less Type II muscle fibers.

Strength training-The Hero:

One of the major consequence of inactivity is a loss of strength, power and muscle mass. This waste of muscle can be contributed to the inactivity induced reduction in protein synthesis. This age-related inactivity also disturbs the proportion of myosin filament to the actin filament. Plasticity is an amazing quality of skeletal muscles, they can easily adapt to the needs they are exposed to. Like in adults, researchers have observed that in older people too we can see some muscle gain if proper stimulation of muscles is backed by adequate nutrition. Thus, progressive resistance training works as a savior from this aging and loss of muscles. This is also evident if you have a look at the MRIs below and compare the cross-sectional areas of the thighs muscles of two male triathletes aged 40 and 70 with those of a sedentary male at 74 years of age. Note how similar the muscle mass of the triathletes is, regardless of the age. So, exercising can actually aid you to keep your muscles young.

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Figure 2: The figure shows the cross-sectional area of 3 adult males.


With aging there occur a lot of morphological, physiological and hormonal changes in our body and the most important of them is a loss of muscle mass. This age-related decline in muscle mass can be attributed to change a reduction in muscle fiber number and size. Since type I fibers almost remains unaffected, type II seems to play an important role in the aging process of human muscles. These type II fibers possess the ability to generate high tension, these changes, thus, not only make us vulnerable to loss of strength but also hamper the quality of life in various aspects. Therefore, to counter these problems related to change in muscle fibers and to prevent them from wasting, elderly people should focus on getting involved more and more in physical activity.


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