Muscles play an obvious role in running, and understanding the different muscle types and recruitment patterns can help us optimize training techniques.
1 Muscle Fiber Types
Skeletal muscle fibers vary in their characteristics, and there are several different ways of categorizing them. While a simple categorization is appealing, in reality fibers tend to vary in multiple ways, so remember that while this is a useful model, All models are wrong. This is a little like classifying cars into different types. You can categorize cars based on the number of doors, how fast they are, what shape the back is, etc. The table below shows several classification systems and the characteristics of the fibers.
|Contraction speed||Myosin ATPase||Myosin heavy chain||Biochemical||Motor Unit Classification||Resistance to fatigue||Force Generated|
|Slow Twitch||Type I||MHCI||Slow Twitch Oxidative (SO)||Slow Twitch Fatigue Resistant||High||Low|
|Fast Twitch||Type IIa||MHCIIa||Fast Twitch Oxidative/Glycolytic (FOG)||Fast Twitch Fatigue Resistant||Medium||Medium|
|Type IIb||MHCIIx/d||Fast Twitch Glycolytic (FG)||Fast Twitch Fatigable||Low||High|
- There is a good correlation between type I and SO fibers, but the correlations between type IIA and FOG and type IIB and FG fibers are not so clear.
- In addition to those shown in the table, mATPase also has IC, IIC, IIAC, IIAB types that are not shown.
- The original Myosin heavy chain classification of MHCIIb in humans now appears to be the MHCIIx/MHCIId type found in small mammals, and humans do not actually have the MHCIIb form.
1.2 Fiber Type Plasticity
There is overwhelming evidence that fibers can change type, with transitions between Type IIa and Type IIb being the most common. Conversion between Type I and Type II has been shown to occur with severe deconditioning, such as spinal injury. The only evidence of transitions from Type II to Type I with training is limited to studies of denervated muscles that were electrically stimulated for weeks. In rats, the transformation occurred sequentially as type IIB/IIX to type IIA to type I, with the type IIB/IIX to type IIA occurring after 2 weeks and the type IIA to type I taking longer than 2 months.
2 Muscle Recruitment
We control our muscles so that we can lift a heavy weight or gently lift a cup of coffee. Our muscles are made up of many small fibers, and when we use less than our full strength we activate just some of those fibers. So lifting the cup of coffee might only use a few of the fibers, but those fibers are fully activated. Each muscle fiber is either generating force or not; there is no in-between. Some muscle fiber types are more easily recruited, and the order is Type I, Type IIa, and finally Type IIb .This can be seen in the patterns of Glycogen depletion. Muscle recruitment has some important implications for training methods. To train all our muscle fibers we either have to generate the maximum force our muscles can produce or we have to exhaust some fibers so that others are activated in their place.
2.1 Endurance Training
Here's an analogy for muscle recruitment patterns while running long distances. Imagine each muscle fiber is a person and the long run is a war. Initially, the highly trained troops are the first to be deployed. When these highly trained troops become fatigued, then the reserve troops are called in to replace them. Eventually even the reserve troops become fatigued, and the draft calls in anyone who is able bodied. If things go on long enough, then the old men and children have to fight. In the same way we rely on a few well-trained muscle fibers early on in a run. As these muscle fibers become fatigued, we call on less well trained fibers. Thus as the training run progresses we work our way through the various muscle fibers. This is why a 15 mile training run does not provide 75% of the endurance benefit of a 20 mile run.
2.2 Maximum Strength Training
Lifting heavy weights requires engaging more of the muscle fibers. Maximum strength training has been shown to improve Running Economy without changing V̇O2max. Improvements in endurance have also been seen with elite level cyclists undergoing maximum strength training (5-6 reps to failure).
Plyometrics use explosive exercises where the full strength of the muscles are engaged for a short period of time. Preceded a muscle contraction with muscle extension under load generates maximum muscle engagement. An example of this can be seen in box jumps, where you jump down from a box and then immediately jump back up. The jumping down extends the muscles under load (eccentric exercise) which then helps generate more force on the jump back up. Like Maximum strength training, plyometrics improve Running Economy and performance without changing V̇O2max. Plyometrics have also been shown to improve neuromuscular control for running that follows cycling, such as occurs during triathlons.
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