Muscle Fiber Types
There are two main types of muscle fibers: slow-twitch fibers and fast-twitch fibers. Within the fast-twitch fibers there are two sub-types: Type IIx and Type IIa. Below are the difference and what you need to know about each.
Type I Slow-Twitch Fibers
*Slow Oxidative (or aerobic)
*Resistant to fatigue and capable of sustaining aerobic metabolism (endurance fibers)
*Smaller than fast-twitch fibers
*Contract more slowly than fast-twitch fibers
Type IIx Fast-Twitch Fibers
*Limited capacity for aerobic metabolism
*Have high number of glycolytic enzymes which provides a considerable anaerobic capacity
*Fatigue more easily than slow-twitch fibers & cannot sustain their efforts for more than a few
seconds (under two minutes at most)
*Largest and fastest muscle fibers and capable of the most force
Type IIa Fast-Twitch Fibers
*Intermediate or fast-oxidative glycolytic fibers
*Possess speed fatigue and force-production capabilities somewhere between slow-twitch and Type
IIx fast-twitch fibers (up to three minutes)
*Used for strength and power activities
*Are highly adaptable and are capable of increasing their oxidative capacity to levels similar to
those in slow-twitch fibers
"A muscle's fiber-type composition is typically an equal mixture of both fast- and slow-twitch fibers, though some muscle groups are known to be made up of primarily fast- or slow-twitch fibers. The percentage of specific fiber types contained in skeletal muscle may be influenced by genetics, hormones, and the activity and exercise habits of the individual." (ACE's Essentials of Exercise Science for Fitness Professionals)
Even though people respond to specific exercise programs based on their own individual genetic composition, studies have shown that a combination of high-intensity resistance training and short-interval speed work can cause the conversion of Type I fibers into Type IIa fibers (Anderson et al., 2000; Staron et al., 1990) Therefore, muscle-fiber composition is only one variable that determines success in overall physical performance.
Type I Slow-Twitch Fibers
*Slow Oxidative (or aerobic)
*Resistant to fatigue and capable of sustaining aerobic metabolism (endurance fibers)
*Smaller than fast-twitch fibers
*Contract more slowly than fast-twitch fibers
Type IIx Fast-Twitch Fibers
*Limited capacity for aerobic metabolism
*Have high number of glycolytic enzymes which provides a considerable anaerobic capacity
*Fatigue more easily than slow-twitch fibers & cannot sustain their efforts for more than a few
seconds (under two minutes at most)
*Largest and fastest muscle fibers and capable of the most force
Type IIa Fast-Twitch Fibers
*Intermediate or fast-oxidative glycolytic fibers
*Possess speed fatigue and force-production capabilities somewhere between slow-twitch and Type
IIx fast-twitch fibers (up to three minutes)
*Used for strength and power activities
*Are highly adaptable and are capable of increasing their oxidative capacity to levels similar to
those in slow-twitch fibers
"A muscle's fiber-type composition is typically an equal mixture of both fast- and slow-twitch fibers, though some muscle groups are known to be made up of primarily fast- or slow-twitch fibers. The percentage of specific fiber types contained in skeletal muscle may be influenced by genetics, hormones, and the activity and exercise habits of the individual." (ACE's Essentials of Exercise Science for Fitness Professionals)
Even though people respond to specific exercise programs based on their own individual genetic composition, studies have shown that a combination of high-intensity resistance training and short-interval speed work can cause the conversion of Type I fibers into Type IIa fibers (Anderson et al., 2000; Staron et al., 1990) Therefore, muscle-fiber composition is only one variable that determines success in overall physical performance.
Greater Muscle Mass = Higher Metabolism
First, what is metabolism? Metabolism is the process by which the body generates the energy it needs for maintenance, repair, growth of tissues, and for muscles contraction and movement. The energy is obtained from the foods we eat. However, the chemical energy stored in the food that we eat is not directly used to fuel our bodies, but rather it is used to generate adenosine triphosphate ("ATP"). Only a small amount of ATP is stored in the muscle cells. The body must regenerate ATP once it is used through the process of metabolism.
Muscle is very active tissue and requires continuous energy supplies for ongoing cellular processes such as protein synthesis, maintenance, and building. It is estimated that a pound of muscle uses between 30-50 calories a day at rest to meet its metabolic requirements. That's one pound a day at rest. How many pounds of skeletal muscle do you have? How active are you? Exactly! When muscle mass is increased so too is your metabolic rate increased, both during activity and at rest. Conversely, when muscle mass is decreased, your metabolic rate is decreased as well at activity and at rest. If you are not performing regular strength exercise, muscle mass decreases with age at about one-half pound of muscle every year of inactivity after the age of 25. It is this reduction in muscle mass that research reveals that resting metabolism decreases approximately half a percent every year after age 25. Endurance exercises increase metabolic rate only during the activity session and shortly following; however, strength exercises increase metabolic rate during and for a relatively long period following the workout. No worries if inactivity has been your life thus far, as strength gains have shown to be made even in your 90s...so get moving!
Make sense why Eating Supportively is so important?
Training Guidelines
Muscle is very active tissue and requires continuous energy supplies for ongoing cellular processes such as protein synthesis, maintenance, and building. It is estimated that a pound of muscle uses between 30-50 calories a day at rest to meet its metabolic requirements. That's one pound a day at rest. How many pounds of skeletal muscle do you have? How active are you? Exactly! When muscle mass is increased so too is your metabolic rate increased, both during activity and at rest. Conversely, when muscle mass is decreased, your metabolic rate is decreased as well at activity and at rest. If you are not performing regular strength exercise, muscle mass decreases with age at about one-half pound of muscle every year of inactivity after the age of 25. It is this reduction in muscle mass that research reveals that resting metabolism decreases approximately half a percent every year after age 25. Endurance exercises increase metabolic rate only during the activity session and shortly following; however, strength exercises increase metabolic rate during and for a relatively long period following the workout. No worries if inactivity has been your life thus far, as strength gains have shown to be made even in your 90s...so get moving!
Make sense why Eating Supportively is so important?
Training Guidelines
Below are some general guidelines from the ACE Fourth Edition Personal Trainer Manual.
Recommended Training Volumes
General Muscle Fitness: 1-2 sets of 8-15 reps
Muscular Endurance: 2-3 sets of greater than or equal to 12 reps
Muscular Hypertrophy: 3-6 sets of 6-12 reps
Muscular Strength: 2-6 sets of less than or equal to 6 reps
Power:
Single-Effort Events: 3-5 sets of 1-2 reps
Multiple-Effort Events: 3-5 sets of 3-5 reps
Rest Intervals Between Sets
General Fitness: 30-90 seconds
Muscular Endurance: less than or equal to 30 seconds
Muscle Hypertrophy: 30-90 seconds
Muscular Strength: 2-5 minutes
Power:
Single-Effort Events: 2-5 minutes
Multiple-Effort Events: 2-5 minutes
*A high-effort set of resistance exercise reduces the muscle's internal energy stores of creatine phosphate (phosphagen energy system). Replenishment of these local energy substrates is relatively rapid, with 50% renewal within the first 30 seconds, 75% renewal within the first minute, and 95% renewal within the first two minutes. For most practical purposes and general muscular conditioning, one-minute rest intervals between successive exercise sets are sufficient.
Recommended Training Volumes
General Muscle Fitness: 1-2 sets of 8-15 reps
Muscular Endurance: 2-3 sets of greater than or equal to 12 reps
Muscular Hypertrophy: 3-6 sets of 6-12 reps
Muscular Strength: 2-6 sets of less than or equal to 6 reps
Power:
Single-Effort Events: 3-5 sets of 1-2 reps
Multiple-Effort Events: 3-5 sets of 3-5 reps
Rest Intervals Between Sets
General Fitness: 30-90 seconds
Muscular Endurance: less than or equal to 30 seconds
Muscle Hypertrophy: 30-90 seconds
Muscular Strength: 2-5 minutes
Power:
Single-Effort Events: 2-5 minutes
Multiple-Effort Events: 2-5 minutes
*A high-effort set of resistance exercise reduces the muscle's internal energy stores of creatine phosphate (phosphagen energy system). Replenishment of these local energy substrates is relatively rapid, with 50% renewal within the first 30 seconds, 75% renewal within the first minute, and 95% renewal within the first two minutes. For most practical purposes and general muscular conditioning, one-minute rest intervals between successive exercise sets are sufficient.
Note that we will review energy systems next week......
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