Responses and Adaptations to Resistance Training

stress responses

The musculoskeletal, endocrine, immune, and cardiorespiratory , , after each training bout better prepare the body for subsequent training bouts.

progressive overload

involves manipulating training volume, intensity, or frequency in order to apply continued stress to the body.

prolonged soreness, nagging muscle or joint pain, a loss of enthusiasm for training, or plateaus in strength

In fact, great personal trainers constantly observe clients during training sessions and interpret cues such as , , to indicate that the training program is too aggressive or that a deload period (e.g., a recovery week) is needed.

within the first month

The initial and obvious adaptation to resistance training is an increase in strength and is occurs

one to two months

Visual changes in muscle mass or tone occur as soon as , , after initiating a regimented program.

several months to years

Discernable improvements in bone mineral density occur over

manage expectations

Personal trainers who understand the time course of each adaptation can clearly communicate this information to clients in order to properly

muscle growth responses to training

Researchers have observed that people exhibit high, average, and low , ,

10% to 20%

To be certain, researchers posit that only , , of individuals are “true” low muscle growth responders,

Acute responses

are the changes that occur in the body during and shortly after an exercise bout. An example is the depletion of fuel substrates in muscle, such as creatine phosphate (CP), during a short high-intensity exercise bout.

chronic adaptations

are changes in the body that occur after repeated training bouts and that persist long after a training session is over. For example, long-term resistance training leads to increases in muscle mass, which play a role in increasing a muscle’s force production capability.

interspersed recovery blocks (deload periods).

Thus, it is crucial that a training program entail a progressive overload scheme; to best promote optimal training adaptations, programming should not have clients perform lifts to failure during every set, and the program should contain

initial phases

The rapidity with which overload increases the capacity for muscle to handle heavier loads at the start of a training program suggests there is a dramatic increase in the activation of motor units during the , , of resistance training.

neurological adaptations

in strength associated with the first couple of months of resistance training are primarily due to , ,

rapid and forceful contractile

In addition, during this time the quality of muscle protein (e.g., myosin heavy chains and myosin adenosine triphosphatase [ATPase]) is also altered to allow for more , , capabilities

5%

A low responder may experience little to no change in whole-body muscle mass or a , , (marginal) increase in leg muscle thickness in response to 12 weeks of resistance training.

30%

On the other hand, a high responder may experience a 10-pound (4.5 kg) increase in whole-body muscle mass, or a , , increase in leg muscle thickness over this same time course.

until four to eight weeks

it is notable that muscle hypertrophy (i.e., muscle growth) is usually not measurable (although it is still occurring) , , after the initiation of a resistance training program

minimize the consequences of aging

Nevertheless, given that strength decrements later in life are a primary risk factor for frailty, continued training over a client’s lifetime helps improve quality of life and

anaerobic enzyme quantity, changes in stored energy substrates, increased contractile or myofibrillar protein content, and increased noncontractile muscle proteins.

A variety of cellular adaptations occur with resistance training programs. These include changes in

during and immediately after

the acute responses to each training stress that occur in the neuromuscular system , , a training session likely drive many of the chronic adaptations.

bioenergetically prepared

although the depletion of CP or muscle glycogen during a training bout may seem like a physiological detail that is too nuanced to influence practice, these responses cue muscles to increase the synthesis of enzymes critical for CP and glycogen resynthesis so that the client is more , , to lift weights as training ensues over weeks to months.

metabolic conditioning

Furthermore, different training paradigms differentially affect these responses, which has practical implications as it relates to clients’

motor unit

These fibers contracting together and the innervating neuron are called a

electromyography (EMG)

The action potential is manifested as a voltage change on the sarcolemma that can be recorded with either surface or intramuscular electrodes. The technique of recording these electrical events is referred to as

motor unit recruitment and rate coding

Control of muscle force is accomplished by the interplay of two primary factors:

Motor unit recruitment

refers to the process in which tasks that require more force involve the activation of more motor units.

Rate coding

refers to control of motor unit firing rate (number of action potentials per unit of time). The faster the firing rate, the more force is produced from the unit.

50% of maximum

As a general rule, small muscles (like those in the hands) that require very precise motor control achieve full recruitment from the available units at relatively low percentages of maximum force output , , and after this point they depend entirely on firing rate to increase force production.

90% of maximum

In contrast, large muscles like those in the quadriceps employ recruitment up to , , or more, and maximum firing rates tend to be lower than for the small muscles.

heavily on recruitment

Therefore, small muscles typically depend more heavily on firing rate to control force output, while large muscles tend to depend more

increases

Motor unit recruitment , , with fatigue to compensate for the loss in force production capability of the previously activated motor units

fire at higher rates (rate coding)

In addition, motor units that were firing at low rates at the start of the set may have , , as the set progresses in response to the fatigue associated with the task.

metabolites

Although fatigue is a highly complex phenomenon, it is clear that the acute changes in muscle cells include an accumulation of , , (i.e., substances [such as lactate] formed during metabolic reactions) and depletion of fuel substrates

increases muscle cell hypertrophy

For instance, petri dish and rodent experiments have shown that chronic lactate administration , , in the absence of exercise

cellular process of autophagy

For instance, petri dish and rodent experiments have shown that phosphate administration increases the , , which can potentially be leveraged to remove cellular structures affected by exercise-induced muscle damage

depleted

Also noted earlier, CP can become , , during resistance exercise, reflecting the reliance on the phosphagen system during typical resistance training.

80%

In fact, it has been estimated that over , , of the ATP production during bodybuilding-type resistance training comes from glycolysis.

carbohydrate

glycogen levels decrease in response to high-intensity resistance training, and this points to the importance of adequate dietary , , for those who perform intensive resistance training.

Hormones

are blood-borne molecules that are produced in the endocrine glands.

protein and peptide hormones and steroid hormones.

There are two primary types of hormones:

growth hormone and insulin

Two examples of protein and peptide hormones are

Steroid hormones

are all derived from a common precursor (cholesterol) and include hormones such as testosterone (the primary male sex hormone) and estrogen (the primary female sex hormone).

growth or the degradation

Many hormones have effects on either the , , of tissue such as skeletal muscle.

Anabolic hormones

such as testosterone, growth hormone (GH), insulin, and insulin-like growth factor-1 (IGF-1) stimulate growth processes,

catabolic hormones

such as cortisol promote protein degradation to help maintain blood glucose homeostasis.

hormone-specific receptor

To facilitate a biological response, hormones must bind to a , , in a target tissue.

androgen receptors

For example, testosterone affects muscle cells at the molecular level by binding to

muscle protein synthesis

a process critical for skeletal muscle hypertrophy

noncontractile protein and myofibrillar protein

Muscle protein synthesis rates are determined by the sum of , , synthesis rates.

net protein balance

is the sum of protein synthesis and protein breakdown rates.

concomitant increases

However, research suggests that the initial phases of training (i.e., first three or four weeks) involve , , in muscle protein synthesis and breakdown rates after each exercise bout, and increases in breakdown rates after each bout become more subdued as an individual continues training.

fat and carbohydrate breakdown

Epinephrine binds to adrenergic receptors to increase , , by the cell so that more ATP will be available for muscle contraction.

Testosterone and GH concentrations

are also transiently elevated in males during and after a bout of resistance exercise and both hormones stimulate increases in skeletal muscle protein synthesis.

muscle fiber hypertrophy

Scientists have determined that postexercise increases in blood testosterone, GH, or IGF-1 concentrations within 60 minutes after the first session of a 16-week resistance training intervention were not associated with

resistance training adaptations

research has suggested that intrinsic mechanotransduction signaling in skeletal muscle primarily facilitates

mechanotransduction

is the process where protein signals in muscle increase in response to a resistance exercise bout

rapamycin

protein signals in muscle ultimately converge to activate the mammalian target of , , signaling complex 1 (mTORc1) to increase muscle protein synthesis.

androgen receptor (AR) concentrations

The aforementioned 16-week training study revealed that pre- to postintervention increases in muscle , , were significantly associated with increases in muscle fiber size.

characteristics

The hormonal response to resistance exercise is dependent on the , , of the training bout.

stronger endocrine responses

As a general rule, bouts that have higher volume and shorter rest periods elicit , , than do bouts with lower volume and longer rest periods, although the differences between protocols may diminish with prolonged training.

greater increases

large muscle mass exercises promote , , increases in anabolic hormones than do small muscle mass exercises

pulsatile postexercise

However, again, these , , increases in hormones appear to play a limited role in the adaptive response to training.

intrinsic adaptations

Alternatively stated, higher-volume exercise may promote greater skeletal muscle growth than higher-load, lower-volume resistance training because of , , that occur in skeletal muscle fibers.

Chronic adaptations

are long-term changes in the structure and function of the body as a consequence of exercise training.

strength and muscle mass

With respect to resistance training, the general adaptations that one experiences after prolonged resistance training are increases in

enzyme and substrate concentrations

Increases in strength are influenced by changes in neurological function as well as changes in muscle mass. In addition, changes in muscle , , may influence muscular endurance.

neural drive

several studies indicate that strength increases consequent to resistance training are influenced by increases in

EMG amplitude measured

The assumption that neural factors are involved is based not only on the discrepancy between hypertrophic and strength increases early in a training program but also on increases in , , during maximal contractions

neural sequencing

In addition, not all studies show an increase in EMG amplitude following resistance training programs, indicating increased strength may be a result of improvement in

Cocontraction or coactivation

refers to the simultaneous activation of an agonist and an antagonist during a motor task.

isometric and isokinetic

Several studies have shown significant cocontraction during , , actions of the knee joint, primarily to provide joint stabilization.

antagonist torque

Decreased cocontraction would reduce the , , that must be overcome by the agonist during a muscle action, thus enhancing the expression of strength.

joint stabilization

There appear to be decreases in cocontraction after isometric resistance training without compromised , , through improved skill of the exercise and neurological recruitment patterns

unilateral resistance training

The contribution of neural factors to strength improvement has also been inferred from observations that , , improves strength in the untrained limb (i.e., cross-education effect)

trained angle

from observations that isometric resistance training at one joint angle results in strength increases that are larger at the , , than at other joint angles

hypertrophy

The most obvious adaptation in skeletal muscle in the form of increased muscle size (cross-sectional area and volume).

greater degree

However, type II fibers typically show a , , of hypertrophy/atrophy compared with type I fibers

overload-induced hypertrophy

Although definitive reasons for this are currently unknown, emerging evidence suggests that , , in rodents elicits greater type II versus type I fiber hypertrophy owing to more robust increases in muscle protein synthesis, greater decrements in muscle proteolysis, and greater increases in muscle ribosomes, which are the macromolecules that facilitate muscle protein synthesis

proportional increase

Given that muscle cells volumetrically contain ~70% of contractile protein in the form of myofibrils, it has been speculated that the increase in muscle fiber cross-sectional area is due to a , , in the number of myofibrils within a given muscle fiber.

30%

if an individual experiences a 30% increase in muscle fiber size with resistance training, this is likely due to a , , increase in the number of myofibrils within the hypertrophied muscle fibers.

protein synthesis rates

The increase in myofibril number is likely caused by the pulsatile increases in myofibrillar , , stimulated by each training bout as well as the possible splitting of existing myofibrils into separate “daughter” myofibrils

Hyperplasia

or the increase in number of muscle fibers, has not been definitively shown to occur in humans, but there is evidence that it occurs in animals that undergo extreme loading models to induce supraphysiological hypertrophy

force and power production

The net result of an increase in muscle cross-sectional area, and the associated increase in myofibrils containing actin and myosin filaments, is an increase in the , , capability of the muscle.

fiber subtype shift

With respect to muscle fiber types, resistance training induces a , , from type IIx to type IIa muscle fibers

myosin heavy chain composition

These subtype shifts are observable after just a few training sessions and likely reflect a change in the , , of the muscle cell.

quality of

Therefore, resistance training alters not only the quantity of muscle tissue (hypertrophy) but also the , , contractile proteins within individual fibers.

promote the transition

some researchers have posited that years of resistance training may eventually , , of some type I fibers to type II fibers.

long-term aerobic endurance

several human biopsy studies have revealed that type IIa and IIx fibers are more prominent in individuals who engage in long-term resistance training, whereas type I fibers are prominent in individuals who engage in , , training

the magnitude or volume of loading

it is likely that the , , rather than eccentric muscle damage, is a more important determinant of muscle growth with resistance training.

cytoskeletal and structural proteins

In addition to increasing the contractile protein content of skeletal muscle, resistance training appears to increase , , These proteins help give skeletal muscle cells shape and structural integrity.

extracellular matrix

cytoskeletal and structural proteins are also involved in force transmission from the myofibrils to the , , and in the storage of elastic energy as occurs in stretch–shortening cycle activities.

protein titin

Strength and power athletes have also been shown to have higher levels of the , , a large elastic structural protein, which may enhance elastic energy storage.

dystrophin

Interestingly, resistance training has not been shown to affect expression of the key protein , , which suggests that the synthesis of specific structural proteins is stimulated by resistance training.

alive

However, bone tissue is dynamic and very much

movement and protection

In addition to its role in , , bone serves as a depot for important minerals, most notably calcium.

Osteoporosis

is the consequence of long-term net demineralization of bone.

strain

Bone tissue is significantly influenced by , , that is, deformation (bending) of bone rapidly stimulates bone cells to begin activities that stimulate bone formation.

postmenopausal women

Because osteoporosis is mainly, though not exclusively, a condition associated with , , most research has focused on women.

decline

Menopause is particularly critical in the development of osteoporosis because hormones like estrogen, which facilitate bone formation, markedly , , after menopause.