Can we train specific muscle fibers to go faster and slow aging?

By Nate Koch/

As we age, our muscles get smaller and lose their ability to move quickly and explosively. No athlete likes that statement! But the good news is that with the right training, you can fight age-related loss of athletic performance. With a basic understanding of muscle fibers and physiology, it’s possible to design an effective training program.

WHAT ARE MUSCLE FIBERS? Without getting into too much human physiology, our muscles are units of fibers stimulated by our nervous system to contract and relax. There are three types of muscle fibers, categorized by how much blood is supplied to the muscle.

Type 1, slow-twitch: These fibers have good blood supply and are small in diameter with a lot of mitochondria (which carry oxygen). They have high aerobic capacity, and can be seen as the “dark meat” or legs on a chicken. They can contract muscles repeatedly for long periods of time at moderate or sub-maximal levels for sustainable and efficient efforts.

Type 2b or x, fast-twitch: These fibers have less blood supply. They are large in diameter with fewer mitochondria and low aerobic capacity, but function well in situations where less oxygen is present (anaerobic). They contract fast and hard but fatigue quickly. These fibers have a faster shortening or contractile velocity and thus produce powerful and explosive movements.

Type 2a: These are a mix of the above two fibers, and have some properties of both. They can be influenced to perform as fast or slow twitch fibers based on training or lack thereof.

The contractile mechanics of the collective fibers and individual fibers, such as velocity, tension and power, are important functions within the fibers that affect athletic performance and muscle function.

The size and fiber type proportions are genetically provided by your parents. Want to know about the makeup of your muscles? There are at least three different techniques that physiology labs use. The most common method referred to in research on muscles is the muscle biopsy, but unless you’re willing become a human pincushion, that’s not a practical scenario.

Different muscles contain varied proportions and sizes of these fibers, based on the body’s requirements for daily living and movement. The core is primarily slow-twitch fibers used to stabilize body position over long periods. The limbs have a mixed fiber composition and tend to have more fast-twitch fibers than the core. Even muscles that perform similar motions and are found in the same area can be vastly different in fiber composition. For example, the calf muscle consists of the soleus, which is primarily type 1, and the gastrocnemius, a mix of type 1 and 2. This is important physiology to note since it can affect the design of a training program. If for instance the goal is to increase hypertrophy, you would choose low load and long duration for the soleus and heavy load and short duration for the gastrocnemius.

How these different types of fibers are recruited in a muscle to complete a movement depends on the muscles involved, genetic makeup, and sport-specific training. An athlete’s distribution of fiber type can be a crucial factor in determining the sport they choose to pursue after early success or failure, and can also determine their subsequent performance in that sport. Multiple studies have shown that endurance athletes tend to have a greater proportion of type 1 muscle fibers.

For example, in 1976 Costill and coworkers obtained biopsy samples from the gastrocnemius (calf) muscle of 40 male and female international-caliber track-and-field athletes. Although the fiber type distribution of those competing in field events was quite unexceptional, the gastrocnemius of the distance (5,000 m to marathon) runners was composed of about 70 percent type 1 and 30 percent type 2 fibers, whereas that of the sprint (100 m) runners was about 25 percent type 1 and 75 percent type 2.

WHAT HAPPENS TO MUSCLE FIBERS AS WE AGE? While the proportions of fibers do not change as we age, the fiber size and percent of space they take up does change. A decrease in fiber cross sectional area can start as early as age 30: if you don’t use it, you’ll lose it. Fast-twitch fibers seem to atrophy to a greater degree than slow-twitch fibers. Remember that fast-twitch fibers are larger in diameter, so when they shrink, the decrease in muscle mass is more pronounced.

While the proportions of fibers to not change as we age, the fiber size and percent of space they take up does change.

Aging also slows contractile velocity and decreases tension in all three fiber types, resulting in a decline in explosive force and power. For example, one study found an 11-percent decrease in vertical jump height per decade.

How does training affect aging muscle fibers?

ENDURANCE TRAINING. Studies have generally demonstrated that continued endurance training maintains the aerobic capacity of muscles, which means it’s maintaining the slow-twitch fibers. But endurance training does not appear to slow down the aging-associated loss of muscle mass, atrophy of type 2 fibers, or diminished force and power production.

RESISTANCE TRAINING. Resistance training increases fiber hypertrophy (cross-sectional area) in all three types of muscle fiber. It’s unclear whether there is an actual shift between fibers in trained athletes. Long-term strength training slows atrophy and helps maintain size and power production within the muscles.

A study by Klitgaard and coworkers found that elderly men with 12–17 years of heavy resistance training had muscle fiber sizes, muscle composition and muscle force characteristics similar to those of young adult control subjects.

SPRINT RUNNING & EXPLOSIVE TRAINING. In multiple studies, masters sprinters demonstrated considerably larger fiber size, intact maximum force normalized to cross-sectional area at the single muscle fiber level, and higher maximal and explosive strength characteristics than untrained older people.

Häkkinen and coworkers compared untrained middle-aged (age 40) and older (age 70) men that were sprinters. The 70-year-old sprinters had on average 65, 69 and 28 percent larger cross-sectional area of type 2, 2a and 2b fibers than age-matched untrained men. In terms of explosive strength and power, the oldest subjects in this study had vertical jump values twice as high as those reported earlier for untrained men aged 71–73.

TEST YOURSELF. Use these exercises to test yourself once or twice a year to see the progression or decline.

1. 60 m sprint

2. vertical jump

3. broad jump

Sprint and explosive training.

4. Wingate test on the bike: 30 seconds max test

Some cycling coaches use formulas to predict fiber type distribution by measuring power and velocity or fatigue resistance, and then use these objective and sport-specific performance measurements to design training programs around the physiology of the athlete. While these tests may not be the most accurate at determining muscle fiber type, they can be functional, reproducible and therefore valuable to training programs.

TRAIN. Review your training plan and goals and decide where you can add other types of training to your schedule. Discuss with your coach.

Resistance training (weight training): Consider the muscle and how it needs to work for your goal. Do you need low load with higher reps, higher load and lower reps, or do you need to increase the speed of your lifts?

Speed work: You can add speed work in all three disciplines: swimming, biking and running.

Plyometric training: Add exercises such as jump rope or box jumps.

Agility training: Use exercises such as ladder drills and shuttles.

Please don’t misinterpret these recommendations; they don’t mean that we should abandon low load, long duration training, but only that we should be add a mix of training to our routines that will fight the effects of aging and the subsequent reduction in explosive speed and power. That is, if you want explosive speed and power!

As a physical therapist, I would be remiss if I did not mention the risk-reward factor. Proper instruction along with a graduated approach is imperative when adding any new training techniques and methods like those listed above. With more power and explosive movements come greater responsibility and potentially more injuries. Muscles need time to adapt to new training stressors, and it’s possible that previous injuries or joint issues may already be imposing a self-governor on higher intensity training.

Also, although there’s extensive research on this topic, the results are not always conclusive. There are still more questions than answers regarding our amazing human physiology and athletic performance.

I’ll finish with some sage advice and practical applications to triathlon training on this topic from two highly respected coaches.

THOUGHTS FROM JOE FRIEL. “Type 2b muscle fibers are well-adapted for producing high levels of power, especially when sprinting or doing brief, highly intense efforts when, for example, in head-to-head competition. Such an effort is not beneficial to non-drafting triathletes. However, when training just below to just above the lactate threshold heart rate (roughly 85–105 percent) or functional threshold power (75–105 percent), type 2b fibers have been shown to take some of the physiological characteristics of type 2a fibers. That means they become better at endurance. This also occurs when training closely on either side of these zones, but the training effect becomes less beneficial. Endurance-adapted 2b fibers enhance endurance performance especially when in or near the above zone percentages. That ‘extra’ endurance pays huge dividends in short-course racing and in long-course triathlons when going up hills or contending for a podium position.”


• Power is the first thing to go as the triathlete ages, while endurance can be developed all the way up to 40 or so. It would make sense that nowhere does the “use it or lose it” principle apply more than here.

• A high volume, low intensity approach without neuromuscular maintenance or development leads to a dramatic sublimation of explosive power.

Resistance training.

• Fast-twitch b’s, which seem best trained by high intensity load (30 seconds) with maximum recovery (4+ minutes), directly affect running economy.

• VO2 kinetics, which is 4:00 to 6:00 maximum intensity work (VO2 max) would also rely on pre-LT metabolism fibers. It’s critical for dealing with uneven effort more effectively.

• Both of the above will play ability/fitness protection roles when considering the pre-loads of the swim for the bike, and especially the bike for the run. It will also protect against pacing errors or adverse conditions like geography and wind.

• Muscle composition seems to be related to an athlete’s fatigue index, that is, how much an athlete slows with each doubling of distance. Fatigue curves for elite triathletes are in the 4–5 percent range. Runners can slow as little 3 percent over the double the distance, although the fatigue indexes of elite sprinters are around 12 percent. Beginner triathletes’ fatigue indexes can be as bad as 20 percent! It would seem that fatigue rates are related to muscle fiber type and training status (information from Championship Triathlon Training, by George Dallam and Steven Jonas).

This article appeared in the August 2016 issue of LAVA Magazine.  LAVA’s core mission is to explore new ideas, technologies and solutions for the age-group triathlete. Please support LAVA with a yearly magazine subscription, starting at $9.95.