9

Adaptations to Resistance Training

chapter

Learning Objectives

• Discover how strength is gained through resistance training

• Note changes in the muscles and in the neural mechanisms controlling them that occur as a result of resistance training

• Learn what causes muscle soreness and how to prevent it

Resistance Training and Gainsin Muscular Fitness

Muscle is very plastic, increasing in size and strength with training and decreasing with immobilization

© BananaStock

One-Repetition Maximum (1RM)

The maximal weight an individual can lift just once

World Records for the Snatch, Clean and Jerk, and Total Weight for Men and Women

Neural Control of Strength Gains

• Recruitment of motor units – Increased number of motor units recruited from

increased neural drive– Synchronicity of motor unit recruitment is improved

• Increased frequency of discharge from the -motor neuron

• Decrease in autogenic inhibition• Reduction in the coactivation of agonist and antagonist

muscles• Morphological changes in the neuromuscular junction

Muscle Hypertrophy

Transient hypertrophy is the increase in muscle size that develops during and immediately following a single exercise bout

– Fluid accumulation in the interstitial and intracellular space from the blood plasma

Chronic hypertrophy is the increase in muscle size after long-term resistance training

– Changes in both the size of muscle fibers (fiber hypertrophy) and the number of muscle fibers (fiber hyperplasia)

Microscopic Views of Muscle Cross Sections Before and After Training

Photos courtesy of Dr. Michael Deschene's laboratory.

Fiber Hypertrophy

• Net increase in muscle protein synthesis—possibly increasing the number of actin and myosin filaments, and increasing the number of myofibrils

• Facilitated by postexercise nutrition• Testosterone plays a role in promoting muscle growth

Fiber Hyperplasia

• Muscle fibers can split in half with intense weight training (cat research)

• Each half then increases to the size of the parent fiber• Conflicting study results may be due to differences in

the training load or mode• Satellite cells may also be involved in the generation of

new skeletal muscle fibers• Hyperplasia has been clearly shown to occur in animal

models; only a few studies suggest this occurs in humans too

Heavy Resistance Training in Cats

Muscle Fiber Splitting

The Satellite Cell Responseto Muscle Injury

Reprinted, by permission, from T.J. Hawke and D.J. Garry, 2001, “Myogenic satellite cells: Physiology to molecular biology,” Journal of Applied Physiology 91: 534-551.

Integration of Neural Activationand Fiber Hypertrophy

• Early gains in strength appear to be more influenced by neural factors

• Long-term strength increases are largely the result of muscle fiber hypertrophy

Resistance Training

Key Points• Neural adaptations always accompany strength gains• Neural mechanisms leading to strength gains include:– Increased frequency of stimulation – Recruiting more motor units– More synchronous recruitment – Decreased autogenic inhibition

• Transient muscle hypertrophy results from edema

(continued)

Resistance Training (continued)

Key Points• Chronic muscle hypertrophy reflects actual structural

changes in the muscle• Muscle hypertrophy results from an increase in the

size of the individual muscle fibers and maybe an increase in the number of muscle fibers

Muscle Atrophy and Decreased Strength With Inactivity

Immobilization• Decreased rate of protein synthesis• Decreased strength• Decreased cross-sectional area• Decreased neuromuscular activity• Affects both type I and type II fibers, with a greater

effect in type I fibers• Muscles can recover when activity is resumed

Muscle Atrophy and Decreased Strength With Inactivity

Cessation of Training• Decreased strength• Little change in fiber cross-sectional area (type II fiber

areas tend to decrease)• Maintenance training is important to prevent strength

losses

Changes in Muscle Strength With Resistance Training in Women

Adapted, by permission, from R.S. Staron et al., 1991, “Strength and skeletal muscle adaptations in heavy-resistance-trained women after detraining and retraining,” Journal of Applied Physiology 70: 631-640.

Changes in Mean Cross-Sectional Areas for the Major Fiber Types With Resistance

Training in Women

Fiber Type AlterationsWith Resistance Training

• Transition of type IIx to type IIa• Results from cross-innervation or chronic stimulation

Muscle Atrophy and Fiber Type Alterations

Key Points• Occurs when the muscle becomes inactive, as with

injury, immobilization, or cessation of training• Maintenance programs can prevent atrophy or loss of

strength• There is a transition of type IIx to type IIa fibers • One fiber type can be converted to the other fiber type

as a result of cross-innervation or chronic stimulation and possibly with training

Acute Muscle Soreness

• Results from an accumulation of the end products of exercise in the muscles or edema

• Usually disappears within minutes or hours after exercise

Delayed-Onset MuscleSoreness (DOMS)

• Soreness is felt 12 to 48 hours after a strenuous bout of exercise• Results primarily from eccentric muscle activity (e.g., downhill

running)• Is associated with:– Structural damage– Impaired calcium homeostasis leading to necrosis– Accumulation of irritants– Increased macrophage activity

• May be caused by inflammatory reaction inside damaged muscles

Electron Micrograph of a Muscle Sample Taken Immediately After a Marathon

From R.C. Hagerman et al., 1984, "Muscle damage in marathon runners," Physician and Sportsmedicine 12: 39-48.

Electron Micrograph Showing Normal Arrangement of Actin and Myosin Filaments and Z-disk Before and

Immediately After a Marathon

From R.C. Hagerman et al., 1984, "Muscle damage in marathon runners," Physician and Sportsmedicine 12: 39-48.

Armstrong’s Sequenceof Events in DOMS

1. Structural damage to the muscle cell and cell membrane

2. Impaired calcium availability, leading to necrosis3. Increased microphage activity and the accumulation of

irritants inside the cell, which stimulate free (pain) nerve endings

DOMS and Performance

• Maximal force-generating capacity is diminished but gradually returns

• Loss of strength is due to:– Physical disruption in the muscle– Failure within the excitation–contraction process– Loss of contractile proteins

Estimated Contributions of Physical Disruption, Contractile Protein Loss, and Excitation–Contraction Coupling Failure

to the Loss of Strength Following Muscle Injury

Reprinted, by permission, from G.L. Warren et al., 2001, “Excitation-contraction uncoupling: Major role in contraction-induced muscle injury,” Exercise and Sport Sciences Reviews 29: 82-87.

The Delayed Response to Eccentric Exercise of Various Physiological Markers

Adapted, by permission, from W.J. Evans and J.G. Cannon, 1991, “The metabolic effects of exercise induced muscle damage,” Exercise and Sport Sciences Reviews 19: 99-125.

Reducing Muscle Soreness

1. Reduce the eccentric component of muscle action during early training

2. Start training at a low intensity and gradually increase it

3. Begin with a high-intensity, exhaustive bout of eccentric-action exercise, which will cause much soreness initially but will decrease future pain

Muscle SorenessKey Points• Acute muscle soreness occurs late in an exercise

bout and during the immediate recovery period after an exercise bout

• Delayed-onset muscle soreness (DOMS) occurs 12 to 48 hours after exercise

• Occurs mostly with eccentric muscle action• Causes include structural damage to muscle cells

and inflammatory reactions within the muscles• Muscle soreness may be an important part of

maximizing the resistance training response

Resistance Trainingin Special Populations

Key Points• Resistance training can benefit almost everyone,

regardless of his or her sex, age, or athletic involvement

• Most athletes in most sports can benefit from resistance training