The Size Principle of Muscle Fibres
How to Utilize Fast Twitch Fibres for Muscle Mass and Strength
Feb 8, 2009
When you pick something up off the floor, a pencil, a shopping bag or 200lbs worth of weight, your muscles contract in a series of stages depending on how heavy the weight is or how fast you pick it up. In short, how much power you use to carry out the movement. These stages represent the different numbers and types of muscle fibres used, starting off with Slow Twitch muscle fibres and finishing off with Fast Twitch muscle fibres.
The Size Principle
Your body is an efficient, well-oiled machine which will only allow you to use as much energy as is needed for the operation at hand. It is based on this principle that the order of muscle fibre contraction is decided. Slow Twitch muscle fibres have a large oxidative capacity, meaning they take longer to fatigue than other fibres. Because of this they're used in most of the body's daily activities, from getting up in the morning, to walking around during the day.
However, once the body is placed under a certain amount of stress, stronger and larger muscle fibres are called upon to aid the Slow Twitch fibres in dealing with the task. The Fast Twitch muscle fibres have little oxidative capacity and fatigue quickly; despite this, they have more potential for strength and size so they are suited to dealing with short term stresses such as jumping, sprinting or lifting heavy objects.
In The Gym
It stands to reason that utilizing this physiological response to stresses would prove beneficial in trying to increase muscle mass and strength.
It's clear that either increasing the weight or moving the weight at a faster tempo will produce the desired effects. However the technique and safety requirements of the lift must still be adhered to. Therefore, consideration must be given with respect to the speed at which you lower the weight through the eccentric portion and the speed used during the concentric portion of lift, to make sure the weight is under control.
When considering which exercises should be used, thinking about which types of exercises will put the body under the most amount of stress will be beneficial. Exercises that require full body movement or, at least, more than one joint (compound exercises) should be used over smaller exercises as they will not elicit the required response from the Central Nervous System, allowing fast twitch fibres to be used.
Once the lifter and/or coach is happy with the technique of the lift then the tempo/weight can be exploited to produce different affects.
To improve starting strength, the weight can be lowered down and then held for a brief pause to off-set any elastic energy build up due to the stretch shortening cycle of the muscle. From there, lift explosively through the concentric portion of the lift.
To improve reactive strength, the weight can be lowered to the end of the range of motion and then immediately explode into the concentric portion of the lift as quick as possible, using the stretch shortening cycle of the muscle.
The weight on the bar must also be considered even when lifting at speed, for if the load is too light then there will not be enough stress on the body for it to utilize the necessary fibre types. Loads upward of 60% of 1 repetition maximum (load that can only be lifted for one repetition) are normally used, depending on the training goal.
Consideration must also be given to the recovery periods when using these methods. The increased weight/speed of the movement will also put stress on the Central Nervous System, which takes longer to recover than muscle tissue. At least 72 hours should be given between training sessions that use these principles, however less intense exercises can be done in between as a form of active recovery.
Sources
Exercise Physiology – Energy, Nutrition & Human Performance 6th Edition, McArdle, Katch
Muscle Revolution – Chad Waterbury
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