Adaptations to Resistance Training
Chapter 10: Adaptations to Resistance Training
Learning Objectives
Examine how strength is gained through resistance training.
Note changes in the muscle structure and in the neural mechanisms controlling the muscle that occur during resistance training.
Defining Muscular Performance
Strength: The maximal force a muscle or muscle group can generate.
Power: The product of strength and the speed of movement (Power = Strength × Speed).
Muscular Endurance: The capacity to sustain repeated muscle actions.
Chief focus of this unit is on strength.
Evaluating Strength
Measuring Strength:
Utilize specialized equipment such as isokinetics (e.g., Biodex) to measure the maximal force.
One-repetition maximum (1RM) is a functional test that determines the maximum weight that can be lifted just one time.
Strength Training Characteristics
Trainability of Strength:
Resistance training programs can produce a 25% to 100% improvement in strength within a timeline of 3 to 6 months.
The mechanisms behind this improvement are significant for understanding training effectiveness.
Muscle Size and Strength
Hypertrophy: Refers to increases in muscle size with a clear relationship between muscle size and strength.
As muscle size increases, strength improves.
Atrophy: Refers to decreases in muscle size often following detraining, leading to weakness.
Traditionally, strength gains were assumed solely from muscle size increases; however, strength encompasses more than just physical size.
Anecdotal Examples of Superhuman Strength
Anecdotes of extraordinary strength include:
Granny lifting a car to save a child.
Medical uses for straitjackets due to unexpected strength in individuals under the influence of certain substances.
Notable athletic feats, like Bob Beamon's long jump in 1968, where he jumped 29’2.5”, breaking the previous world record by 2 feet, a record held until 1991.
Gender and Age Effects on Strength
Women:
Experience strength increases comparable to men when undergoing strength training—same percentage increase but typically lower absolute weights lifted.
Children:
Training can lead to strength improvements without significant hypertrophy, potentially doubling their strength.
Women & Strength
Genetic predispositions and prolonged training (16+ years) can lead to substantial strength capabilities.
Notable example: Bev Francis, a 6-time World Powerlifting Champion, demonstrates the potential strength in trained women.
Increasing Strength without Changing Size
Two Major Factors:
Increased muscle size (hypertrophy).
Alterations in neural control of trained muscle.
Studies show strength gains can be achieved through neural adaptations without concomitant changes in muscle size.
Possible Neural Factors of Strength Gains
Recruitment of additional motor units to generate greater force.
Counteraction of autogenic inhibition, facilitating force production (inhibition from Golgi Tendon Organs and CNS).
Reduction in coactivation of agonist and antagonist muscles.
Changes in discharge rates of motor units, including increased motor neuron firing frequency, known as "rate coding."
Modifications in the neuromuscular junction morphology, improving action potential propagation efficiency.
Muscle Hypertrophy Types
Transient Hypertrophy:
The "pump" experienced during exercise due to fluid accumulation from blood plasma into the muscle interstitial spaces.
Chronic Hypertrophy:
Increase in muscle size over time via either:
Changes in muscle fiber number (fiber hyperplasia).
Increases in muscle fiber size (fiber hypertrophy).
Whole Muscle Hypertrophy Research
Early research suggested muscle fiber number is fixed at birth, implying muscle size growth results from fiber hypertrophy.
Effects include:
Increased myofibrils.
Increased actin and myosin filaments.
Increased sarcoplasm and connective tissue.
Fiber Hypertrophy Mechanisms
mTOR (mechanistic target of rapamycin):
Regulates the balance between protein synthesis and degradation in muscle cells.
Muscle protein synthesis declines during exercise but increases in recovery, leading to net gains in synthesis over time.
Factors like testosterone, hGH, and IGF-1 stimulate mTOR, promoting muscle growth.
Post-exercise intake of carbohydrates and proteins, particularly leucine, enhances mTOR activation and protein synthesis.
Fiber Hyperplasia Mechanisms
Proposed that muscle fibers may split during intense training, with each half increasing to the size of the original fiber.
Satellite cells are implicated in muscle fiber regeneration.
Documented occurrence in animal models; limited evidence exists for humans.
Resistance Training Studies in Animals
In a study with cats subjected to 101 weeks of training:
Muscle weight increased by 11%, and fiber number increased by 9%.
This confirms the occurrence of hyperplasia as individual muscle fibers were counted.
Hypertrophy versus Hyperplasia
Most research indicates that individual fiber hypertrophy accounts for over 95% of muscle growth.
Studies of bodybuilders show potential for hyperplasia, with evidence of fiber sizes that match untrained individuals but with significantly larger overall muscle bulk.
Mechanism of Hyperplasia Occurrence
Muscle fibers can sustain damage during eccentric contractions.
Activated satellite cells may combine to repair damaged fibers or generate new fibers.
Satellite Cell Activation Process
Activation of quiescent satellite cells follows muscle injury.
Satellite cells proliferate and migrate to injured fibers.
Fusion with damaged myofibers contributes to hypertrophy or the production of new myofibers (hyperplasia).
Neural Activation and Fiber Hypertrophy
Strength gain mechanisms vary with training duration:
Early strength increases are influenced by neural factors.
Long-term strength increases result predominantly from muscle fiber hypertrophy.
Model of Neural and Hypertrophic Factors
Contribution to strength gains varies with time:
Short-term studies identify hypertrophic contributions.
Long-term studies highlight neural factors.
There is a potential reversal of strength gain trends as hypertrophy ceilings are reached.
Effects of Muscular Inactivity
Muscular atrophy involves a decrease in muscle size and generally affects slow-twitch fibers.
Reduction in:
Myofibrils
Mitochondrial density
Muscle protein synthesis within 6 hours of immobilization.
Predictable strength loss of approximately 3-4% per day.
Complete Immobilization (e.g., casting): Recovery typically exceeds the duration spent immobilized but is less than the original training volume.
Cessation of Training
Detraining: Results in a decline in strength without complete immobilization.
Maintains fiber area with a generally shorter recovery period than the detraining period.
Strength Changes in Women
Post-6 weeks retraining following a 30-32 week layoff can allow women to regain strength levels achieved from 20 weeks of consistent training.
Strength Maintenance Strategies
After strength development, reductions in training frequency, intensity, or duration can help maintain strength gains.
A study indicated that reducing training to one day a week maintained gains from a prior 18-week training period, contrasting with a 21.4% strength decrease observed in a detraining group.
Changes in Major Fiber Types
Discussions about fiber type changes remain complex, with some studies indicating no change in basic fiber type while others highlight characteristic changes.
Cross-innervation and chronic stimulation studies reveal that changes can occur, demonstrating possible transformations under specific conditions.
Diet and Resistance Training
Maintaining positive protein balance is crucial for optimal hypertrophy:
Recommended intake of protein is approximately 1.6-1.7 g/kg body weight per day.
Commonly cited “bro” intake of 1g/lb checks out to about 2.2g/kg, which is approximately 38% more than necessary, indicating room for dietary optimization.
Timing of protein intake is less critical than overall caloric intake for weight gain.
Resistance Training in Special Populations
Age and sex differences influence strength development:
Women can achieve strength gains similar to men but average maximum strength is lower.
Children can safely increase strength and endurance if appropriate precautions are in place.
Elderly individuals can also gain strength and muscle mass with increased protein intake, resulting in improved functional ability and reduced fall risk.
Resistance Training for Sport
Strength is a critical factor for athletic performance:
Resistance training should tailor to increase strength and power for optimal sport performance.
Research is still required to determine the extent of how resistance exercises can enhance athletic capabilities.
Increased muscular endurance correlates with reduced risk of injuries, highlighting the intricate relationship between training, fatigue, and injury risk.