Maximizing Strength: The Stimuli and Mediators of Strength Gains

Brief Review: Maximizing Strength

Introduction

  • The U.S. Army is interested in improving soldiers' physical performance using ethical interventions.
  • Traditional heavy resistance exercise (RE) training is effective for improving physical performance.
  • Maximizing strength gains is important for soldiers and civilians.
  • Scenarios exist where traditional heavy RE is not possible (e.g., limited equipment, injury).
  • Developing interventions for maintaining physical performance when heavy RE is not feasible is valuable.
  • The benefits of RE are partly due to improvements in maximal strength.
  • The purpose of this review is to:
    • Identify stimuli that trigger RE-induced strength gains.
    • Identify factors that mediate the effectiveness of these stimuli.
    • Discuss potential directions for maximizing strength gains.
    • Discuss practical applications for increasing or maintaining strength in special scenarios.

Definitions

  • Traditional heavy RE: Lifting and lowering an external load through a full range of motion using free weights or exercise machines.
  • Heavy: External load allowing 1-5 repetitions per set with maximal effort.
  • Strength: Maximal level of volitional force or torque generated during a single attempt of simple and complex tasks.
  • 1RM (one repetition maximum): The most weight that can be safely lifted through a full range of motion with correct technique, used to quantify strength.
  • Other methods include isometric strength (maximal force against an immovable object) and isokinetic strength (maximal force at a fixed velocity).
  • This review focuses on strength, not muscular power, endurance, or muscle size (hypertrophy).

Scope, Style, and Limitations

  • Focus is on internal stimuli that link resistance exercise to strength gains.
  • The goal is to identify factors that allow for creative research and manipulation of stimuli to maximize strength gains.
  • Preference is given to recent systematic reviews.
  • The primary goal is to identify potential opportunities for future exploration.
  • Narrative format may introduce study selection bias.
  • Broader themes might help convey potential theories and future opportunities.

Stimuli of Strength Gains

  • Strength gains are stimulated through a sequence of events:
    • Maximal mental effort.
    • Maximal neural activation of muscle to produce forceful contractions.
    • Lifting and lowering movements (concentric-eccentric muscle actions).
    • Training through a full range of motion.
    • (Potentially) inducing muscular metabolic stress.

Maximal Mental Effort

  • The signal to perform RE originates in the motor cortices of the brain.
  • RE training produces adaptations in the central nervous system, contributing to strength gains.
  • Imagined forceful muscle contractions (motor imagery training) increase strength over time.
  • Improvements are accompanied by physiological changes (central neural adaptations).
  • Greater intensity of mental effort during motor imagery training produces greater strength gains.
  • Greater mental effort during training produces greater strength gains over time, despite the same external force output and duration of muscle contraction.

Maximal Neural Activation of Muscle to Produce Forceful Contractions

  • Maximal mental effort leads to maximal neural activation of muscle, producing forceful muscle contractions.
  • This interaction causes adaptations in corticospinal and reticulospinal tracts and in the muscle.
  • Gains in strength are attributable to a higher instantaneous discharge rate of motor units and a higher incidence of doublet discharges.
  • The muscle adapts to repeated contractions, including increases in contractility and in the number of force-generating contractile proteins.
  • Evidence supporting forceful muscle contractions independently stimulate strength gains:
    • Strength gains are greater after traditional physical contractions compared to imagined contractions.
    • Electrically evoked contractions increase strength over time.
    • Training with increasingly heavier loads produces progressively greater strength gains.

Involving Lifting and Lowering Movements (Concentric-Eccentric Muscle Actions)

  • Traditional RE involves lifting (concentric) and lowering (eccentric) muscle actions.
  • Concentric-eccentric training produces greater improvements in strength than concentric-only training.
  • Combined concentric-eccentric actions are likely not an independent stimulus for strength gains but are effective in the context of forceful muscle contractions or substantial metabolic stress.

Training Through a Full Range of Motion

  • Heavy RE with concentric and eccentric muscle actions leads to forceful muscle actions through a given range of motion (ROM).
  • Strength gains tend to be specific to the ROM used during training.
  • Training with a full ROM generally maximizes strength gains across most contexts.
  • ROM is interrelated to concentric-eccentric actions and forceful muscle contractions.

Inducing Muscular Metabolic Stress

  • Muscular contractions increase metabolic reactions, leading to metabolic stress (depletion of energy substrates and accumulation of by-products).
  • Conflicting evidence exists whether metabolic stress stimulates muscle growth and strength gains.
  • It's difficult to determine if metabolic stress is an independent stimulus or has a synergistic effect with muscle contraction.
  • Blood flow restriction (BFR) in combination with low-load RE training increases intramuscular metabolic stress and enhances long-term strength gains.
  • Low-load BFR training might cause comparable improvements in muscle strength as traditional heavy-load RE.

Mediators of Strength Gains

  • Factors that mediate the long-term effectiveness of RE stimuli:
    • Optimizing the dose of RE within a session.
    • Beginning each set of RE in a minimally fatigued state.
    • Optimizing recovery between training sessions.
    • (Potentially) periodizing the training stimulus over time.

Optimizing the Dose of Resistance Exercise Within a Session

  • The optimal dose of RE is quantified by intensity (external load) and volume (repetitions per set and number of sets).
  • Higher training loads produce greater subsequent gains in strength.
  • Heavier loads that allow approximately 1-5 repetitions per set maximize strength gains over time.
  • As little as one set per exercise can increase strength over time.
  • 2-3 sets per exercise stimulates greater strength gains than 1 set per exercise.
  • Using relatively heavy loads (1-5 repetitions per set) and approximately 2-3 sets per exercise is the optimal dose of RE to maximize strength gains.

Beginning Each Set of Resistance Exercise in a Minimally Fatigued State

  • Acute RE can lead to transient fatigue.
  • Beginning each set of RE in a fatigued state can diminish gains in strength.
  • Evidence:
    • Short interset rest intervals diminish strength gains compared with long rest intervals, at least in well-trained individuals.
    • Placing an exercise last in the exercise session diminishes strength gains after long-term training.
    • Performing endurance exercise immediately before RE diminishes long-term strength gains.
  • Initiating each set of RE in a relatively “fresh” (minimally fatigued) state helps maximize long-term strength gains.

Optimizing Recovery Between Training Sessions

  • Recovery from and subsequent adaptation to RE enhances strength.
  • RE causes fatigue and muscle damage, which can impair strength in the short term.
  • Sufficient recovery leads to “supercompensation” (incremental improvements in performance).

Periodizing the Training Stimulus Over Time

  • Advanced practitioners include “periodization” to maximize strength gains, minimize overtraining, and reduce injury risk.
  • Periodized RE training enhances strength gains compared with nonperiodized training.
  • Experts generally believe that periodization remains the best practice.

Future Directions for Maximizing Strength Gains

  • Four primary themes:
    • Supramaximal training intensity (external load and mental effort).
    • Supplemental activities.
    • Optimizing and individualizing recovery strategies.
    • Technological applications.

Supramaximal Training Intensity

  • Using “supramaximal” muscle contractions might further enhance strength gains.
  • Muscles can produce greater force during eccentric actions than during concentric actions.
  • Incorporating supramaximal eccentric-only training or “accentuated eccentric loading” seems to further enhance strength gains.
  • Eccentric-based overload training might require longer recovery.
  • Options to incorporate eccentric-based overload training:
    • Commercially available exercise hardware.
    • External weight releasers.
    • Lifting an object bilaterally and then lowering it unilaterally.
  • “Variable-load” RE (free weights plus elastic bands or heavy chains) might have merits for enhancing strength gains.
  • Supplementing RE with external stimulation (neuromuscular electrical stimulation (NMES), whole-body electromyostimulation (WB-EMS), repetitive transcranial magnetic stimulation (TMS), or transcranial direct current stimulation (tDCS)) might enhance forceful contractions.
  • Manipulation of sensory input (vibration or sensory nerve electrical stimulation) represents another neural-based strategy.
  • “Supramaximal” mental effort during training might further enhance strength gains.
  • Cognitive strategies (methods to enhance arousal) acutely enhance force output.
  • Physiological arousal is likely linked to enhancements in mental effort and human motor neuronal firing patterns.
  • RE training does not necessarily need to proceed to momentary muscular failure.

Supplemental Activities

  • Supplemental activities (used with traditional RE) might also maximize strength gains.
  • Blood flow restriction (BFR) increases long-term gains in strength due to low-load RE.
  • Possible mechanisms include BFR-induced enhancements in metabolic stress or muscle activation.
  • Conflicting findings on whether BFR enhances strength gains when used with heavier RE.
  • Limited evidence indicates that supplementing traditional heavy RE training with occasional low-load BFR training might further enhance strength gains.
  • Passive BFR (without muscle contractions) might help maintain strength during immobilization.
  • Motor imagery could supplement traditional RE to further enhance strength gains without exacerbating fatigue and muscle damage.

Optimizing and Potentially Individualizing Recovery Strategies

  • The scientific realities of supercompensation theory deserve more attention.
  • Determine optimal recovery timelines between bouts of RE.
  • Determine optimal training frequency for maximizing strength gains.
  • Identify methods to expedite the rate of recovery or maximize the magnitude of supercompensation.

Technological Applications

  • Insights regarding the stimuli and mediators of strength gains indicate opportunities for technological applications.
  • Biofeedback (technology to provide real-time physiological insight) could be used to enhance forceful muscle contractions or to direct and maximize mental effort.
  • Real-time biofeedback through electromyography (EMG) enhances forceful muscle contractions during RE and produces greater strength gains.
  • “Gamification” of EMG biofeedback might further facilitate forceful muscle contractions during RE.
  • Electroencephalography (EEG) or virtual reality could enhance the effectiveness of motor imagery.
  • Wireless sensors (near-infrared spectroscopy [NIRS]) can be used to provide real-time biofeedback to ensure that each set of RE achieves a desired magnitude and duration of metabolic stress.
  • Technology can be used to monitor and minimize fatigue between sets of RE.
  • Technology can reinforce and record the ROM during RE.
  • Smart devices can track the dose of RE.
  • Technology can identify when individuals are physiologically ready for the next training session (heart rate variability [HRV]).
  • Wearable sensors, smartphone applications, and rapid point-of-care tests might capture training stress and recovery to maximize long-term strength gains.
  • Developing individually tailored RE and recovery strategies might help ensure maximally effective strengthening interventions for all individuals.

Practical Applications for Special Scenarios

  • Three key applications for increasing or maintaining strength when traditional heavy RE cannot be performed:
    • No-load interventions.
    • Low-load interventions.
    • Supplemental activities.

No-Load Interventions

  • No-load interventions do not require equipment and do not expose the injured limb to undue mechanical stress.
  • Examples: motor imagery, contralateral limb training, and passive BFR.
  • Motor imagery preserves and slightly improves strength.
  • Contralateral limb training preserves strength in the injured limb through the “cross-education” effect.
  • Passive BFR might help maintain strength during immobilization.

Low-Load Interventions

  • Individuals can compensate for the lack of adequate external load by exercising with maximal mental effort.
  • Low-load high-repetition exercise can increase strength.
  • Low-load high-velocity exercise can increase strength.
  • Forceful muscle contractions without an external load can increase strength over time.

Supplemental Activities

  • Biofeedback and external stimulation can help rehabilitate strength.
  • Adding BFR to low-load RE has applications for special scenarios.

Conclusions

  • Traditional heavy RE is well-established for improving strength because it requires maximal mental effort leading to forceful, concentric-eccentric muscle actions through a full range of motion, as well as induces muscular metabolic stress.
  • Optimize strength gains by optimizing the dose of RE, beginning each set in a minimally fatigued state, optimizing recovery, and potentially periodizing training.
  • Future efforts might consider researching, monitoring, and manipulating stimuli and mediators to advance strength gains.
  • Four key themes to further enhance strength:
    • Supramaximal training intensity.
    • Supplemental activities.
    • Scientific realities of supercompensation theory.
    • Technology.
  • Three key themes to increase or maintain strength when heavy RE is not possible:
    • No-load interventions.
    • Low-load interventions.
    • Supplemental activities.

Practical Applications

  • Implement traditional heavy RE when possible.
  • Enhance effectiveness by optimizing dose, minimizing fatigue, optimizing recovery, and potentially periodizing training.
  • Consider eccentric-based overload training, variable-load RE, external stimulation, cognitive strategies, RE with blood flow restriction, biofeedback, technology, and metrics of recovery readiness.
  • When heavy RE is not possible, use no-load interventions (motor imagery, contralateral limb training, passive BFR) or low-load interventions (high-repetition, high-velocity, forceful contractions without external load).
  • Supplement with biofeedback, external stimulation, and BFR.