Muscle growth and endurance is an adaptation to stress. For example, a sprinter will develop large quadriceps and hamstrings in order to adapt to the stress, while an endurance runner will develop more endurance to efficiently handle the stress.
Type I muscle fibers respond to stress by becoming more efficient and stronger with slight hypertrophy, rather than the extreme hypertrophy seen with Type IIa and IIb muscle fibers. This is the premise behind trainers recommending 6 reps for pure strength/muscle gain and why 10-15 reps are recommended to “tone” a muscle.
Finally, there are four different actions a muscle can perform; isometric, eccentric, concentric, and isotonic. An example of an isometric contraction would be pushing against a wall.
Lifting a dumbbell during a bicep curl is considered the concentric portion while lowering of the weight is called the eccentric portion of the exercise. There are also called the positive and negative portions respectively. And finally, isotonic contractions are those that involve full body actions such as skating or running.
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