Muscle Fiber Types, Properties, and Adaptations in Exercise Science

Type I Muscle Fibers

Red, slow twitch fibers that are aerobic and fatigue resistant.

Type II Muscle Fibers

White, anaerobic, fast twitch fibers.

Type IIa Muscle Fibers

Fast oxidative glycolytic fibers with moderate endurance and high power.

Type IIx Muscle Fibers

Fast glycolytic fibers with low endurance and the highest power.

Fiber type composition

Influenced by genetics, blood levels of hormones, individual exercise habits, and training status.

Contractile properties

Primarily determines fiber type.

MHC isoforms

Identifying these helps determine fiber type.

Oxidative capacity

Determined by the number of capillaries, mitochondria, and amount of myoglobin.

Specific force

Tension defined as Force/Fiber CSA.

Maximal force production

The maximum force a muscle can produce.

Speed of contraction (vmax)

Regulated by myosin ATPase activity and measured by maximal shortening velocity.

Maximal power output

Calculated as Power = Force x Velocity.

Muscle fiber efficiency

Defined as Efficiency = Force Produced / Energy Cost.

Slow Twitch Muscle Fibers

Characterized by high oxidative capacity, low anaerobic capacity, slow contractile speed, small motor unit size, low motor unit strength, and small fiber size.

Fast Twitch (Type IIa) Muscle Fibers

Characterized by moderate oxidative capacity, high anaerobic capacity, fast contractile speed, large motor unit size, high motor unit strength, and large fiber size.

Fast Twitch (Type IIx) Muscle Fibers

Characterized by low oxidative capacity, high anaerobic capacity, fast contractile speed, large motor unit size, high motor unit strength, and large fiber size.

Motor unit

A motor neuron and the muscle fibers it innervates.

All-or-none principle

A muscle fiber either contracts completely or not at all.

Henneman's Size Principle

The basis for the orderly recruitment of specific motor units to produce a smooth muscle action.

Electromyography

Study of muscle function from the detection of the electrical activity during the depolarization of nerves and muscle membranes that accompany contraction.

Graded Contractions

MVC

Interpolated twitch

Used commonly in rehab to gauge a patient's muscle activation.

Asynchronous firing

Pattern for submaximal contractions 1-2-3-4-1-2-3-4-1.......

Synchronous firing

During maximal contractions motor units summate twitches and increase firing rate.

Muscle Spasm

Fatigue of motor unit(s) lead to involuntary contractions.

Control of Muscle Force Production

Affected by; Muscle size / Structure (CSA), Type of Contraction, Recruitment / Activation, Contraction Velocity, Muscle Length, Fiber Type.

Short/Thick Muscle

Built for force; Increased CSA → increased force.

Long/slender Muscle

Built for speed; Increased length → increased contractile velocity.

Muscle Architecture

Arrangement of muscle fibers relative to the axis of force generation.

Pennation angle

Angle of pull at which the muscle fibers attach relative to the tendon.

Agonist

Prime movers; responsible for the movement.

Antagonists

Oppose the agonists to prevent overstretching of the agonist.

Synergists

Assist the agonist and sometimes fine-tune the direction of movement.

Concentric

Muscle shortening, lowest force generation, uses the most energy.

Isometric

Muscle length unchanged, more force capacity than concentric, uses less energy than concentric.

Eccentric

Muscle lengthens, generates the greatest force, uses the least amount of energy.

Size Principle of Motor Unit Recruitment

Small units → Large units; Slow Units → Fast units.

Force-Velocity Relationship

The maximum shortening velocity is greatest at the lowest force.

Neural adaptation

Increases ability to recruit muscle fibers.

Contractile protein adaptation

Increases in the amount of protein; Hypertrophy - increase in muscle size (protein synthesis).

Time course of Neural Adaptations

Occur quickly (perhaps even after one bout) and prior to any increase in muscle CSA.

Actin and Myosin

Proteins that increase in quantity and size during muscle hypertrophy.

Sarcomeres

Structural units of a muscle fiber that increase in number and size with training.

Myofibrils

Components of muscle fibers that increase in size and number due to training.

Fiber Size (CSA)

Cross-sectional area of muscle fibers that increases in size and strength with training.

Time Course of Muscle Adaptation

Changes in myosin can be seen quickly; ~16 workouts for measurable CSA increase, ~8 weeks for noticeable contractile protein increases.

Myostatin

A myokine that inhibits muscle growth by acting on muscle cells.

Protein Gain Limitations

Most individuals can see a 15-40% increase in muscle CSA, affected by genetics and drugs.

Fiber Type Conversion

Fibers do not truly convert but take on characteristics of other fibers based on training stimulus.

Transient Hypertrophy

Temporary increase in muscle size due to edema immediately following a workout.

Myofibrillar Hypertrophy

Increase in size and number of myofibrils within muscle fibers proportional to sarcoplasm size increases.

Sarcoplasmic Hypertrophy

Controversial increase in sarcoplasmic proteins related to myofibril size increase assisting anaerobic metabolism.

Hyperplasia

Splitting of muscle fibers leading to an increase in the number of muscle fibers.

Exercise-induced Muscle Cramps

Painful, involuntary contractions of skeletal muscle during exercise.

Electrolyte Depletion Theory

Proposes cramps occur due to electrolyte imbalances from heavy sweating.

Altered Neuromuscular Control Theory

Suggests muscle fatigue increases spindle activity and decreases GTO activity, leading to cramps.

Overload Principle

Training must exceed accustomed levels to elicit an adaptive response.

Specificity in Training

Training adaptations are specific to the muscles or systems involved in the activity.

Work Equation

Work = force x distance x time, used to quantify exercise intensity.

Progressive Overload

Achieved by manipulating intensity, volume, density, frequency, duration, and distance.

2 for 2 Rule

If 2 or more reps above the goal are completed in the final set for 2 consecutive sessions, increase weight.

Upper Body Weight Increase

Increase in 1.25 - 5 lb. increments for upper body exercises.

Lower Body Weight Increase

Increase in 5-15 lb. increments for lower body exercises.

General Adaptation Syndrome (GAS)

3 Phases of physiological response to training.

Shock or Alarm Phase

Initial stress causes soreness, stiffness, and temporary performance drop.

Resistance Phase

Body adapts at the neurological level and improves performance; also called 'supercompensation'.

Periodization of Training

Training is based around the phases of GAS to optimize performance gains.

Exhaustive Phase

Similar stresses seen in alarm phase persist over a long period, leading to overtraining.

Deload

An 'easy' week planned to avoid the exhaustive phase.

Principles of Weight Training

Includes specificity, progressive overload, and exercise order.

Exercise Order

Most technical/exhausting exercises should be performed first.

Compound Exercises

Multi-joint movements should be performed before isolation exercises.

Exercise Selection

Assess goals and capabilities, ensuring safety and enjoyment.

Proper Warm-up

Increases adequate blood flow and contraction speed.

Valsalva Maneuver

Holding breath against a closed glottis increases intra-abdominal pressure and blood pressure.

Weightlifting Belt

Provides a brace to increase intra-abdominal pressure, decreasing compressive forces on the spine.

Adaptation

What happens when exercise is performed correctly.

Protein Balance

Muscle proteins are in a constant state of flux; regulated by hormones.

Atrophy

Negative protein balance leading to muscle loss.

Anabolic Hormones

Hormones like testosterone and GH that promote muscle growth.

Catabolic Hormones

Hormones like glucagon and cortisol that break down tissue.

Hormone Release Stimuli

Muscle stretch and force generation stimulate the release of anabolic hormones.

Muscle/Metabolic Adaptations: Aerobic Metabolism

Includes increases in mitochondria size, number, and capillary density.

Law of Diffusion

Decreases diffusion distance, allowing for better gas exchange and removal of metabolites.

Phosphocreatine depletion

Causes fatigue during sprinting and other maximal speed movements.

Glycogen depletion

Occurs during long duration events from both the muscle and liver. If blood glucose falls too much, the brain will stop exercise.

Central Fatigue

Fatigue associated with alterations in central nervous system function rather than a dysfunction within the muscle.

Reduced brain signaling

May be related to levels of serotonin and dopamine in the brain and blood.

Inhibition from muscles

Sensory feedback hypothesis which signals from pain, temperature, etc. signal back to CNS and inhibit force production.

Overtraining

An accumulation of training stress that impairs an athlete's ability to perform training sessions and results in long-term decrements of performance.

Common symptoms of overtraining

Chronic fatigue, mood disturbances, hormonal imbalances, frequent overuse injuries, impaired immunity.

Absolute strength

Higher for men; upper body strength is 50% higher and lower body strength is 30% higher than women.

Muscular hypertrophy

Men exhibit a greater degree of muscular hypertrophy than women due to testosterone levels that are 20 - 30 times higher.

Delayed Onset Muscle Soreness (DOMS)

Result of tissue injury caused by excessive mechanical force exerted upon muscle and connective tissue, especially during eccentric movements.

Max strength

Typically peaks between 25 and 35 years.

Sarcopenia

Loss of muscle mass, particularly Type II, associated with aging.

Strength in Children

Strength is mainly a function of height and the maturity of the CNS for children aged 7-17.

Youth Resistance Training

Participation can improve motor skills, enhance bone mineral density, and reduce injuries in sport and recreational activities.

Neuromuscular

The relationship between nerve and muscle, particularly to the motor innervation to the skeletal muscles and its pathology.

Adaptations to Exercise

With exercise, adaptations to the neuromuscular function improve performance through changes in rate coding, increased motor unit recruitment, and synchronization.