Muscle Fiber Types
Muscle fibers have various different properties that contribute to their differing performance. The muscle fiber types range in a continuum from Fast Glycolytic (FG) to Slow Oxidative (SO) fibers.
Different activities, sports and exercises place different demands on muscles. Activities such as endurance running or cycling use primarily Slow Oxidative (type I) fibers as they can withstand great amounts of stress without fatiguing. SO fibers use the process of aerobic glycolysis to break down glucose and glycogen to produce 36 ATP (the bodies molecule of energy) to be used as fuel to meet demands of the exercise.
Activities such as power lifting or sprinting use primarily Fast Glycolytic fibers as they are able to provide powerful contractions. This, however, is only able to be maintained for short periods of time as FG fibers fatigue fast. Energy for FG fibers comes from anaerobic glycolysis and although the process occurs rapidly, the absence of oxygen causes the muscles to fatigue rapidly. This is the reason why sprinters can only perform ‘all-out’ for a short period of time.
Athletes performing at an elite level will have a majority of one muscle fiber type depending on their sport. For example, sprinters will have a larger percentage of fast twitch (FG) fibers overall.
Fibers contract on a continuum with SO fiber motor units being recruited first, followed by Type IIc, Fast Oxidative Glycolytic and then Fast Glycolytic. As the demands of the strength of contraction increase, the time for which that contraction can be maintained will decrease.
Muscle Phenotype Expression
Changes in muscle phenotype expression play a large role in the training adaptations of an athlete. This is where, due to training, an athlete will have an increased number of fibers due to the type of training done. This has been shown in research into muscle fiber types done by Jansson et al. where subjects ran the equivalent of 5 marathons per week. A significant increase in Slow Oxidative fiber amounts was noted, showing the adaption of muscles, due to nerve activity, to meet the demands of exercise. This was further shown by Green et al. in 15 weeks of intensive training in rats running 3.5 miles daily showing a 3 fold increase in the number of SO fibers in vastus lateralis.
The more active a muscle is, the more likely it is to be slow. If stimulation (or training) is ceased the muscle will revert to back to how it was initially.
Changes in muscle phenotype expression as a result of changes of activity may be mediated by a wide number of effectors. It is likely mediated by Ca2+ ions. Prolonged activation would lead to an increase in Ca2+ ions which would activate the calcineurin pathway. Calcineurin causes the release of NFAT(neural factor of activated t-cells). In this state, NFAT translocates to the nucleus and initiates the activation of slow isoforms via activation of MEF2. Intermittent elevations of Ca2+ ions do not activate the calcineurin pathway so NFAT will remain dephosphorylated.
If FG fibers undergo tension/mechanical loading, they undergo rapid fiber conversion FG-FOG-SO with a 5 fold increase seen in 6 weeks. Normal activity causes active tension production by the muscle. Muscles which are immobilised in a lengthened position will experience a persistent passive tension, although the muscles will not know if the tension is active or passive as EMG signals have been shown to be the same. Continual usage of SO fibers for activities such as postural control means they perform much more work than the high tension producing FG fibers which are rarely used. It could be that SO fibers have the thickest Z lines as they produce the most tension over time. The increase in activity equals the tension experiences possibly.
Muscle Fiber Types and Training
Strength training causes the conversion of fast oxidative glycolytic (FOG) to fast glycolytic (FG) but this involves occasional demand for high level titanic responses that occlude the blood supply. Adult fibers are not fixed entities but can respond to changes in functional demand, including phenotype expression.
When forces that cause the chance in phenotype are removed, the muscle will revert to the old state, for example when someone stops training or stimulation, or returns to earth’s gravity. Muscle fibers become what the nervous system dictates. The genetic component of performance, defined by a person’s inherited distribution of muscle fiber types, ultimately determines the maximum potential and relates to the relative percentage of slow motor neurones verses fast motor neurones that are inherited. An endurance athlete with a high percentage of slow fibers will always do better than a trained athlete with a low percentage of slow oxidative fibers.
End of Muscle Fiber Types