Fast-Twitch vs Slow-twitch
Muscle fiber types and importance to designing a proper training program

Boxer’s get their punching power from leg extension and push-off, rotation of the torso, and arm action. As a result, boxers need total body strength, muscle balance and muscle endurance. Specifically this means leg strength for power of push-off on a punch and establishment of a strong sound base of support for balance; torso strength for rotation and absorption of punches; shoulder strength to hold the gloves up, and back and chest strength to throw punches and pull the arms back quicker.

Many boxers want to gain muscular strength, speed, power and endurance but at the same time do not want to gain weight. This is possible thru good eating habits and a proper resistive exercise program.

One of the most critical factors in improving muscular size and strength is the ratio of slow-twitch versus fast-twitch muscle fibers available. Three main types of muscle fibers have been documented: slow twitch (red), intermediate (pink) and fast-twitch (white). Obviously the key for a successful boxer is “fast-twitch” muscle fiber. All muscles contain a mixture of fibers, some containing more of one type of fiber depending upon their function.

Your amount of fast-twitch and slow-twitch fiber has nothing to do with your training; it is a genetic factor and cannot be changed. However, both fast-twitch and slow-twitch fibers have the potential for growth and fast-twitch fibers have a greater capacity to grow and produce force than do slow-twitch fibers. The boxer who has a greater amount of fast-twitch muscle fiber has a much better chance to make significant improvements in muscular size and force production.

Muscular endurance, another key factor in boxing, is also based upon your inherited fiber-type ratio. The more slow-twitch fibers you have, the more muscular endurance you potentially have. It is therefore important for the boxer to develop full potential from both fast-twitch fibers for speed and slow-twitch fibers for muscle endurance if peak performance is to be obtained.

What is Your Fiber type?

Most athletes don’t know their muscle fiber type and ratio. One method of finding out is to have a muscle biopsy done. A needle is inserted into a muscle and a small plug of tissue is removed and examined under the microscope to get a fiber-type percentage. This examination can be done at sports-medicine facilities and universities that conduct research in exercise science. However, the accuracy of this type of testing has been questioned.

There are other types of tests that are more convenient, practical and less painful:

Muscular Endurance Test: this usually involves finding a one repetition maximum with controlled speed of movement on some type of machine rather than with free weights. In-other-words, for each exercise you need to find out how much weight you can put on the machine so you can do one repetition only.
It is recommended that this test be done with single-joint movement which involves less risk of injury and isolates one muscle making it easier to evaluate endurance. Do not use supportive gear such as wrist straps or weightlifting belts because these items could distort results.
Example: Let’s assume that your one repetition maximum on a leg extension is 150 pounds. To test your muscular endurance, use 80 percent of your I-RM which would be 120 pounds. If you can do more than 15 repetitions at this weight before you fatigue, you can assume that your quadriceps (thigh muscle) is composed primarily of slow-twitch fibers. The more repetitions you can do, the higher the number of slow twitch fibers. Conversely, the fewer repetitions you do, the higher the number of fast-twitch fibers.
If you fatigue before you do about 10 repetitions you probably have a higher number of fast-twitch fibers. If you fatigue somewhere between 10 and 15 repetitions you probably have about a 50/50 split of fast-twitch and slow-twitch fibers.

In an endurance test for muscles of the upper body, doing more than about 12 repetitions with 80% of a one repetition maximum means you have a high percentage of slow-twitch fibers; fewer than 8repetitions means you probably have more fast-twitch fibers.

For boxing you would want to do this test with leg extension (quadriceps), leg curl (hamstring), calf raise (calves), bench press (pectorals), pull-over (upper back or lats), bicep curl (biceps) and tricep extension (triceps). These are all single joint movements.

Another Possibility: Fast-twitch muscle fibers have a greater capacity for hypertrophy (growth) so another way to evaluate your fiber type would be to inspect your general physical appearance. If you have a well-developed physique you probably have more fast-twitch muscle fibers; on the other hand, if you have slight muscular development you probably have a higher percentage of slow-twitch fibers.

Designing Repetition Ranges:

Once you know your muscle-fiber type and where they are located you can customize your resistive training program to achieve maximum potential. Those athletes who have predominantly slow-twitch muscle fibers can do a higher percentage of their one repetition maximum and can do more repetitions, making significant gains in strength. The key here is the amount of time the muscle is actually loaded and not the number of repetitions. The optimum time for someone with a 50/50 ratio of fast-twitch to slow-twitch fiber is 90 to 120 seconds for hips/gluteals, 60-90 seconds for legs and 40-70 seconds for the upper torso. The muscles of the lower body need to be trained longer because of their larger size and greater work capacity.
Someone with a high percentage of fast-twitch fiber should exercise for a shorter duration: 60-90 seconds for hips/gluteals, 50-70 seconds for legs, and 40-50 seconds for the upper torso. Since it isdifficult to perform sets in a precise time period, each repetition should be around 6 seconds in length: about 2 seconds in raising the weight, and about 4 seconds lowering it.

Repetition ranges based upon fiber types would then be:

• 50/50% mix of fast-twitch to slow-twitch = 15-20 repetitions for hips/gluteals, 10-15 for the legs, and 6-12 for the upper torso.
• Slow-twitch dominant = 20-30 repetitions for hips/gluteals, 15-20 for the legs, and 10-15 for the upper torso.
• Fast-twitch dominant = 10-15 repetitions for hips/gluteals, 8-12 for the legs , and 6-8 for the upper torso.

Remember, each individual has a unique blend of muscle fibers so there may be individual variation within your particular program.

Be aware that there is no scientific research indicating that an increase in the actual number of muscle fibers is possible. Nor can you convert one type of muscle fiber to another. The success of your resistive exercise training will be based upon your already established predominant muscle-fiber type and the discipline of your training.