005 MUSCLE FIBRE TYPE AND EXERCISE-INTENSITY

Slight exercise, such as walking, easy bike-riding or jogging can be maintained for many hours. In that case energy-supply is completely aerobic via fat-oxidation in the type I fibres. The amount of fat within the body is practically inexhaustible.

When doing moderate exercise, such as running and bike-riding all the type I-fibres will soon be used. Beside fat-oxidation the oxidation of carbohydrates will increase. Energy-supply is still fully aerobic. Well-trained athletes can maintain this level of exercise for 60 – 90 minutes because the store of carbohydrates, if not refilled in time, will be exhausted in one to one and a half hour during maximum aerobic exercise.

When the level of exercise intensity is further increased, such as a time trial or during a 10K running race, type II-A fibres begin to support the aerobic energy supply. The complete aerobic system is in action, but the anaerobic system also contributes. This anaerobic energy supply results in the formation of lactate. Up to a certain level there is an equilibrium between the formation and the neutralization of lactate; this prevents the accumulation of lactate. The body is still capable to neutralize this lactate at the same speed as its formation.

When the level of exercise intensity is even more increased, such as in a highly intensive interval training, the body calls for an increasing share of anaerobic energy supply. At a certain level the body will no longer be able to neutralize all the lactate that is formed. This resulting in high lactate levels. All the type II-B fibres are in function during these intensive workouts. In this case energy supply is exclusively anaerobic using carbohydrates as a fuel. These workouts show high lactate levels resulting in a short period of time to maintain this high level.

In sprint workouts the exercise intensity is even higher, but now the energy is supplied by the phosphate battery, which functions anaerobically and alactically, meaning that no lactate will be formed. This is only possible when the recovery periods after the intensive sprints are long enough. A minimum of 3 to 5 minutes or even more if necessary. This is the time required by the phosphate battery to reload. When the recovery periods after the sprints are too short there will be an accumulation of lactate. The sprint workout will no longer be an effective workout; it has become a lactate tolerance training.

Daphne Schippers
Gatlin and Bolt
Muscle fibre type Duration in maximum workload Energy system Sports activities
FT fibres Type II-B Shorter than 15 seconds Phosphate battery Throwing and jumping, 100 meter sprint, tennis, baseball, short breakaway in cycling races
FT and ST fibres Type I and II B 15 – 90 seconds Phosphate battery + Lactate system 200 – 400 meter sprint, 500 -1200 meter cycling
ST – and FT – fibres Type I and II–A and II-B 1.5 to 3 minutes Lactate system + oxygen system Boxing 3 minutes per round. Intervaltraining cycling. Wrestling.
ST – fibres Type I Longer than 3 minutes Oxygen system Marathon, cross-country running, cycling.

The relation between the muscle fibre type, the duration in maximum effort and the dominating energy system in various sports activities.

Type I Type II - A Type II - B
Energy supply aerobic Anaerobic+aerobic Anaerobic
Fuel Fats Carbohydrates+fats Carbohydrates
Exercise intensity light medium heavy
Duration Long (hours) Middle (1-2 hours) short
Lactate production None medium high
Speed slow fast high/maximum

Characteristics of Type I, Type II-A and Type II-B muscle fibres

Exercise intensity Active muscle fibres Fuel
Light Type I Fat + carbohydrates
Mudium Type 1 + II - A Carbohydrates + fat
Heavy, high intensity Type I + II - A + II - B Carbohydrates

Muscle fibre type in relation to the exercise intensity.

Summary:

The distribution of the various types of fibres is genetically determined. Training may bring about some slight changes. Sprinters may improve their endurance capacity by a specific training programme. This training programme changes the white fibres into red fibres. This harms their sprinting capacity.

Real endurance athletes do not develop more white fibres by special sprint workouts. So an endurance athlete will never become a sprinter.

With growing age sprinting capacity will weaken faster than endurance capacity. This is because of a decrease of the FT fibres. Endurance capacity may be maintained up to a high age.

Every type of muscle fibre requires specific training. This should always be taken into consideration when making a training programme.

DIFFERENCES IN SKELETAL MUSCLE FIBER TYPES
Type I fibres Type II a fibres Type II x fibres Type II b fibres
Contraction time Slow Moderately fast Fast Very fast
Resistance to fatigue High Fairly high Moderate Low
Activity used for Aerobic activity Long term anaerobic activity Short term anaerobic activity Short term anaerobic activity
Maximum duration of use Hours Less than 30 minutes Less than 5 minutes Less than 1 minute
Power produced Low Medium High Very High
Mitochondrial density Very high High Medium Low
Capillary density High Intermediate Low Low
Oxidative capacity High High Moderate Low
Major storage fuel Triglycerides Creatine phosphate, glucose ATP, Creatine phosphate, glycogen (little) ATP, creatine phosphate
Properties Consumes lactic acid Produces lactic acid and creatine phosphate Consume Creatine phosphate Consume Creatine phosphate