I want an easy way to explain to clients which muscle fibers are activated and used under what circumstances. The fast twitch and the slow twitch are each activated during different exercises/drills. Can you make this difference easy to remember and to explain?
Great question! I will keep it as simple as possible, yet in my experience, nothing about neuromuscular physiology is simple! To begin with, I will address the first part of your question: "Which muscle fibers are activated and used under what circumstances?"
The pattern of recruitment of muscle fiber types has been shown to follow a fixed sequence. When the force required/generated is LIGHT, only SLOW- TWITCH muscle fibers are recruited. As the force desired increased to moderate levels, FAST-TWITCH-A are additionally recruited, and as the force increased towards maximum FAST-TWITCH-B fibers are eventually recruited. This pattern of recruitment means that SLOW-TWITCH muscle fibers are the most frequently used for long periods.
So basically what's being stated here is that it's NOT the activity/exercise that is determining fiber type being used; more importantly, it's the INTENSITY of the activity/exercise.
- Slow Twitch/Type I
- Simply stated, these fiber types are the primary workers in AEROBIC based activities/sports (group fitness classes, distance running, etc.).
- Generally fatigue resistant and have a high capacity for aerobic energy supply (with oxygen), but they have limited potential for rapid force development.
- It has been said that many of the body's postural/stability muscles (i.e. the CORE), are SLOW-TWITCH/TYPE-1 so as to maintain optimal posture and fight gravity throughout the day.
- Fast Twitch/Type II A&B
- Simply stated, these fiber types are stimulated, along with type I, during high intensity ANAEROBIC activities/sports (weights, sprinting, etc.).
- Essentially the opposite, characterized by fatigability, low aerobic power, rapid force development, high ANAEROBIC power (without oxygen).
- Type IIa have grater capacity for aerobic metabolism and more capillaries surrounding them than type IIb and therefore show greater resistance to fatigue.
I hope this helps.
- Milner-Brown, Alexander PhD. "Neuromuscular Physiology." National Academy of Sports Medicine, 2001, 40-41.
- Baechle, TR., Roger, WE. "Essentials of strength Training and Conditioning." National Strength and Conditioning Association, 2000, 17-18.