Bold Mass The third type of critical muscle fiber is a slow-twitch muscle family. These are called the marathon muscles. The characteristics of these muscles are extended durations and medium to lower intensity activity.These slow-twitch muscles contain a very high ability to avoid fatigue. They also use large amounts of oxygen to power these muscles.Unfortunately, these muscles are slower to contract. The forces obtained in these muscles are not as powerful as the other muscles. Therefore, these muscles are not used in workouts that require a large degree of power moves.

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