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Principles of Exercise Physiology and Adaptation

Original Editor - Wanda van Niekerk based on the course by Professor Mike Lambert

Top Contributors - Wanda van Niekerk and Jess Bell  

The Biological Stress of Exercise

  • Exercise and physical training are biological stressors. The body reacts similarly to exercise as it does to other stressors.[1]
  • Stress disrupts homeostasis and this leads to an adaptive response.[2]
  • The body's responses to a single bout of exercise are regulated by the principles of homeostasis. Homeostasis is defined as the ability of the body to maintain a stable internal environment for cells by closely regulating various critical variables such as pH, acid-base balance, oxygen tension, blood glucose concentration and body temperature.[3]
  • Specifically, in exercise – muscle contractions disturb homeostasis resulting in various responses.
  • The type of adaptation is dependent on the overload stimulus.
  • It is important for clinicians using exercise as a modality to understand the biological process of adaptation.[4]

Stress

  • Stressor = anything that causes stress or elicits the stress response
  • Phases of stress[5]:
    • Acute phase – homeostatic adjustment (alarm reaction)
    • Chronic phase – stressors accommodated by adaptations (stage of resistance)
    • Exhaustion phase – maladaptations occur (state of exhaustion)

[6]

  • Examples of stressors to the human body:
    • Exercise
    • Food deprivation
    • Hypo- or hyperthermia
    • Psychological challenges
    • Social challenges

Exercise as a Stressor

  • Acute phase:
    • Acute responses to training - i.e. how the body responds to one bout of exercise
    • Physiological, metabolic and neuromuscular changes that persist for the duration of the exercise bout and are in proportion to the increase in metabolic rate
    • Metabolic rate = "metabolism per unit time especially as estimated by food consumption, energy released as heat, or oxygen used in metabolic processes"[7]
      • Oxygen consumption is a measure of metabolic rate (VO2 Max)[8]
      • When exercise stops, oxygen consumption decreases and returns to baseline measures a few hours after exercise[9]
      • The metabolic rate a few hours after moderate exercise is the same as the metabolic state before the exercise bout
    • Changes are transient
  • Chronic phase:
    • When exercise bouts are repeated regularly, longer-term changes occur
    • Training-induced adaptations occur and these are associated with improvement in exercise performance
    • For example, a trained person[10]:
      • Has an increased capacity to resist fatigue
      • Can generate more muscle power
      • Has more refined motor coordination to perform specific tasks
    • The nature of training adaptations is dependent on the type of exercise (endurance vs strength training)[11]
    • Recovery
      • Recovery is defined as "the set of processes resulting in an athlete's renewed ability to meet or exceed previous performance levels"[12]
      • Recovery period is defined as "the time necessary for the various physiological parameters that were altered by exercise to return to resting values"[12]
      • Recovery should occur after each training session
      • The metabolic rate should revert to the level it was before the start of exercise on the previous occasion
      • The time course of recovery differs, depending on the marker of recovery
      • Markers of recovery:
        • Heart rate (within minutes)[9]
        • Blood lactate concentrations (within minutes)[9]
        • Breathing rate (within minutes)[9]
        • Metabolic rate (VO2Max) (within several hours)[9]
      • If muscle damage is caused by exercise, changes may take weeks to months to revert back to pre-exercise values[9]
  • Exhaustion Phase:
    • Inadequate recovery leads to maladaptation[12]
    • Accompanying symptoms of fatigue and impaired muscle function
    • Increased risk of injury and illness[12]
    • Increased risk of cognitive and mood disturbances[12]
    • Known as overtraining[13]
      • Management of overtraining in competitive athletes is necessary as there is often an imbalance in wanting to continuously improve performance with training, but insufficient recovery periods between training sessions.[14]

Principles of Biology

Principle of Dose and Response

  • Exercise dose = training session stimulus[4]
  • Exercise dose can be quantified through wearable devices measuring the[15]:
    • Intensity
    • Distance
    • Impact forces
  • Exercise response = the athlete’s outcome from a training session

Overload Principle

  • The overload principle states that habitually overloading a system causes it to respond and adapt[16][17]
  • The overload principle can be quantified according to load (intensity and duration), repetition, rest and frequency[18]
    • Load refers to the intensity of the exercise stressor. For example, in strength training it can refer to the amount of resistance or in swimming it can refer to speed. The greater the load, the greater the fatigue and recovery time needed.
    • Repetition refers to the number of times that a load is applied.
    • Rest refers to the time interval between repetitions.
    • Frequency refers to the number of training sessions per week.
  • Practical ways to do this:
    • Increasing the weight
    • Increasing the distance
    • Increasing the intensity
    • Decreasing the rest period between sets and/or sessions.

The overload principle forms the base of all training programmes and assists athletes in reaching peak performance.[4]

Other Principles

Other principles in exercise physiology are:

  • Specificity principle
  • Reversibility principle
  • Individuality principle

Read more about these principles here: The Basic Principles in Exercise Physiology

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[19]

Biological Signal During Exercise

  • At rest, the physiological, metabolic and endocrine systems of the body are in equilibrium (homeostasis)
  • A muscle contraction causes disruption of homeostasis
  • Responses to the disruption of homeostasis are designed to meet the demands of increased metabolic rate or the need to produce muscle power and maintain the body’s internal environment during exercise.
  • Examples of transient homeostatic changes include[9]:
    • Altered blood flow to active muscles
    • Increased heart rate
    • Increased oxygen consumption
    • Increased rate of perspiration
    • Increased body temperature
    • Secretion of stress hormones such as ACTH (Adrenocorticotropic Hormone), cortisol and catecholamines
    • Increased glycolytic flux
    • Altered recruitment of muscles
  • The magnitude of these changes/responses is dependent on the interaction of factors such as[9]:
    • Type of muscle action
    • Duration of activity
    • Whether the individual has been exposed to the activity before
  • Ambient conditions also impact homeostatic response. Examples of ambient conditions[20]:
    • Temperature
    • Altitude
    • Wind speed
  • Inter-individual differences in response to the same exercise stimulus explain why some individuals adapt faster than others when exposed to the same training stimulus.[21] Reasons for this include[22][23]:
    • Pre-training phenotype
    • Pre-training autonomic function
  • The timing and composition of nutritional intake may also modulate the training response[23]

Exercise-Associated Signal

  • The exercise-associated signal that introduces training-induced adaptation is dependent on[23]:
    • Type of muscle contraction
    • Duration of the exercise bout
    • Intensity of the exercise bout
    • Frequency of the exercise bout
  • The mechanical signal is converted to primary and secondary messages for adaptation
  • These messages activate pathways involved in protein synthesis or degradation
  • This results in adaptations associated with performance changes
  • Signal can also be affected by the recovery period between exercise sessions
    • For example, training-induced expression of messenger RNA of several oxidative enzymes are upregulated 24 hours after an exercise bout[24]
  • Endurance training leads to muscles becoming more resistant to fatigue
  • Strength training leads to muscles becoming stronger, more powerful and sometimes bigger

Exercise-Induced Adaptations at Cellular Level

The primary stimulus for exercise-induced adaptations is a combination of[9][25]:

Comparison of primary stimuli for exercise-induced adaptations
Type of Exercise Load Metabolic Stress Calcium Flux Result
Endurance Low High Moderate Increase in mitochondrial mass and oxidative enzyme activity
Strength High Moderate High Muscle fibre hypertrophy and neural changes

For more information on the changes in various systems of the body, read this: What happens during exercise?

Training for Improved Performance

  • Insufficient and too much training results in underperformance
    • Insufficient training – not inducing appropriate training adaptations needed for peak performance
    • Too much training – maladaptation with failure to adapt and leading to symptoms of fatigue and poor performance
    • Training-Induced Fatigue Continuum
      The training-induced fatigue continuum
      • Functional overreaching is overcome with a few days of rest[13]
      • Non-functional overreaching needs a longer rest period, negative phase, coaches try and avoid this phase[13]
      • Overtraining phase when training load persists with inadequate recovery[13]
        • Main symptom
          • Impaired performance
        • Other symptoms:
          • Dysfunction in neuromuscular, endocrine, metabolic and immune systems
          • Inability to accommodate the same training load that was possible before the development of symptoms
        • Ways to avoid overtraining[26]
          • Adopt a more systematic approach to training
          • Ensure correct balance between training load and rest and recovery
          • Important to quantify training load and fatigue arising from each training session to manipulate and manage the dose/response relationship

Read more here: Overtraining Syndrome

Specific Types of Training

Endurance Training

Endurance training increases resistance to fatigue or following training; adaptations occur and more work can be done in the same time compared to before training or at the same submaximal intensity. Thus, it takes longer to fatigue when/after training.

Adaptations After Endurance Training

  • Adaptations after endurance training include[27]:
    • VO2 max increases
    • Increased plasma volume
    • Increase in cardiac output
    • Increase in stroke volume
    • Decreased heart rate
    • Proliferation of capillaries and active muscles
    • Capillary to muscle fibre ratio increases
    • Increase in mitochondrial content after 4 weeks – results in an increase in oxidative capacity of the muscle
    • These changes are lost when regular training stops
    • Time course of endurance training adaptations:
      • Changes in VO2 max, cardiac output and stroke volume start within 3 weeks, but continue linearly for at least 12 weeks[28]
      • An improvement in VO2 max can be seen for at least 12 months[29], but if there is insufficient training stress/load the the increase in VO2 max will reach a plateau after 3 weeks[30]
      • Resting heart rate (HR) drops after 3 months [29]
      • Heart rate (HR) at a given submaximal intensity drops within 3 months[29]
  • At a fixed submaximal workload, the perception of effort decreases as the training status improves
    • This can be related to:
      • Reduced disturbance in homeostasis which occurs at a fixed submaximal workload after training compared to before
      • The athlete is more efficient and less oxygen is used at a fixed submaximal load
    • This is demonstrated by:
      • Slower breathing rate
      • Lower heart rate
      • Lower blood lactate concentration
      • More fat used as fuel during submaximal exercise
        • Demonstrated in a laboratory when the volume of carbon dioxide produced and the volume of oxygen consumed are measured - this is known as the Respiratory Exchange Ratio (RER)
          • RER – close to 1 when glucose and glycogen are the main fuel sources and decreases to 0.7 when free fatty acids are the main fuel source during exercise[31]
          • In an untrained person, exercising at submaximal intensity, RER = around 1, indicating that mainly glucose is used as fuel
          • RER of an endurance athlete = 0.8 -0.9 indicating that fatty acids and glucose are oxidised for fuel
  • The rate of glycogen utilisation decreases during submaximal exercise after endurance training. As a result, it takes longer for glycogen stores to become depleted. Fatigue during endurance training is associated with glycogen depletion. Thus, if glycogen depletion can be delayed, the onset of fatigue will be delayed and will occur at a later stage.[32]
  • Read more: Adaptations to Endurance Training; Endurance Exercise

Strength Training or Resistance Training

Health Benefits of Resistance Training

Treat and manage conditions characterised by muscle weakness such as:

  • Sarcopenia[33]
  • Neuromusculoskeletal disorders[17]
  • Muscle atrophy[34]

Strength vs Power

The musculoskeletal system is fundamental in exercise physiology. The strength of a muscle is mostly determined by its cross sectional area. Therefore size is key.

  • Mechanical Work performed by a muscle is the amount of force applied by the muscle multiplied by the distance over which the force is applied.[35]
  • Muscle Strength is the maximal amount of tension or force that a muscle or a muscle group can voluntarily exert on a maximal effort when the type of muscle contraction, segment velocity and joint angle is specified.[36]
    • Muscle Strength = ability of a muscle to produce force
  • The power of a muscle contraction is different from muscle strength because power is a measure of the total amount of work that the muscle performs in a unit period of time. It is generally measured in kilogram meters (kg-m) per minute.[35]
    • Muscle power = the ability of a muscle to do work over time. Thus, it is an interaction between the force of contraction and speed of contraction
  • Muscle endurance is defined as the ability to perform repeated contractions against a resistance or maintain a contraction for a period of time.
  • Normal daily activities require minimal strength while certain sports depend on high levels of muscle strength, such as weightlifting. Sports like gymnastics require more muscle power than muscle strength.
  • Read more: Strength Training versus Power Training

Types of Resistance Training

  • Free weights
  • Machine weights
  • Resistance bands
  • Bodyweight exercises

Adaptation of Overload Stimulus for Strength Training

Ways to alter training stimuli to impose training overload include:

  • Manipulate the number and intensity of repetitions
  • Manipulate the recovery time between repetitions, sets and sessions
  • Manipulate the frequency of training sessions
  • Change the weight used
  • Manipulate the number of sets per exercise
  • Type of resistance exercise
  • Order of exercise

The outcome of resistance training is dependent on the overload stimulus and the type of resistance training. The interplay between these variables can influence[17]:

  • Muscle strength
  • Muscle endurance
  • Muscle hypertrophy
  • Muscle power

Several factors influence the muscle’s capacity to generate force. These are[17]:

  • Muscle fibre type
  • Muscle cross-sectional area
  • Muscle architecture
  • Neural drive[37]
    • Initially, increases in strength are a result of changes in the neural system. For example, an untrained individual will notice an almost immediate increase in strength when starting a resistance training programme. Initially, there are no changes in muscle size.
  • Signs of an increase in muscle size or hypertrophy occur after about 4 weeks of consistent training
  • There is a significant change in size after 8 to 12 weeks of resistance training

Hypertrophy[38]

  • Muscle sizes are determined mainly by genetic and anabolic hormone secretion. Training can add another 30 to 60 percent of muscle hypertrophy, mostly from increased muscle fibre diameter, but in a small part also from an increased number of fibres (hyperplasia).
  • Hypertrophied muscle is characterised by[39]:
    • An increased number of myofibrils
    • Increased number of mitochondrial enzymes
    • Increase in ATP and phosphocreatine amounts available
    • Increased stored glycogen and triglyceride
    • Thus enhancing both aerobic and anaerobic systems
  • Additional manufacturing and incorporation of contractile proteins such as actin and myosin into existing myofibrils will increase the cross-sectional area of the muscle
  • The increase in cross-sectional area is directly proportional to the force that the muscle can produce
  • Factors that influence the rate and size of the muscle increases after training include:
    • Age
    • Biological sex
    • Genotype

Maximum voluntary contraction

  • Voluntary strength depends on the maximal activation of the agonist muscle, but also minimal activation of the antagonist muscle, and support from the synergistic and stabilising muscles.
  • An untrained muscle cannot be fully activated. This may be because the Golgi tendon organ is activated, which inhibits agonist muscle recruitment.[40]
  • After strength training, this inhibition appears to be overridden
  • During the first 3 to 4 weeks after resistance training there is an increase in maximal voluntary contraction – an increase in surface EMG activity of the agonist muscle is also seen during this period. This confirms the change in neural drive to the muscle and indicates that more muscle fibres can be recruited after training compared to a maximal contraction before training.[41]
  • Once hypertrophy occurs, fewer muscle fibres are recruited during a submaximal contraction, which is seen by reduced EMG activity.
  • Other evidence of neural adaptation after resistance training is visible in the cross-over effect, where strengthening one limb increases strength in the contralateral untrained limb.[4]

Read more: Adaptations to High-Resistance Strength Training; Strength Training

Concurrent Strength and Endurance Training

  • Adaptations are compromised if endurance and strength training occurs at the same time.[11]
  • Gains in strength are reduced if additional endurance training occurs simultaneously. This is due to a suppressed hypertrophic response in the muscle that may be related to an elevated catabolic state induced by endurance training.[11]
  • There is evidence showing that aspects of vascularisation and oxidative enzyme activity may be increased with concurrent training. Various molecular signalling processes induced in skeletal muscle by endurance exercise can inhibit pathways regulating protein synthesis and stimulate protein breakdown. Regulating pathways in this manner may reduce the capacity for muscle hypertrophy.[42]
  • Read more: An updated systematic review and meta-analysis on the compatibility of concurrent aerobic and strength training for skeletal muscle size and function.[43]

Conclusion

  • Exercise-induced adaptations vary depending on the primary stimulus of training
  • Muscles can be remodelled to be either fatigue-resistant, stronger and more powerful, bigger or better coordinated
  • These adaptations have implications for sporting performance, rehabilitation after injury and treatment of disease
  • Training adaptations persist if the training stimulus is provided consistently and systematically
  • Training adaptations slowly regress to the pre-training state when the training stimulus is removed

References

  1. ↑ Kiely J. A new understanding of stress and the implications of our cultural training paradigm. New Stud Athl. 2016;30(3):27-35.
  2. ↑ Chu B, Marwaha K, Sanvictores T, Ayers D. Physiology, stress reaction. InStatPearls [Internet] 2021 Jun 8. StatPearls Publishing.
  3. ↑ Powers SK and Howley ET. Exercise Physiology: Theory and Application to Fitness and Performance.10th edition. North Ride, NSW, Australia. McGraw Hill. 2014
  4. ↑ 4.0 4.1 4.2 4.3 Lambert, M. Principles of Exercise Physiology and Adaptation Course. Plus, 2022.
  5. ↑ Cunanan AJ, DeWeese BH, Wagle JP, Carroll KM, Sausaman R, Hornsby WG, Haff GG, Triplett NT, Pierce KC, Stone MH. The general adaptation syndrome: a foundation for the concept of periodization. Sports Medicine. 2018 Apr;48(4):787-97.
  6. ↑ Personal Trainier Collective. General Adaptation Syndrome with Dr Mike Zourdos. Available from: https://www.youtube.com/watch?v=jnsmwlRAIwM[last accessed 6/10/2022]
  7. ↑ “Metabolic rate.” Merriam-Webster.com Medical Dictionary, Merriam-Webster, https://www.merriam-webster.com/medical/metabolic%20rate. Accessed 6 Oct. 2022.
  8. ↑ Crowley E, Powell C, Carson BP, W Davies R. The Effect of Exercise Training Intensity on VO2max in Healthy Adults: An Overview of Systematic Reviews and Meta-Analyses. Translational Sports Medicine. 2022 Feb 24;2022.
  9. ↑ 9.00 9.01 9.02 9.03 9.04 9.05 9.06 9.07 9.08 9.09 9.10 9.11 Lambert MI, Mujika I. Physiology of exercise training. Recovery for Performance in Sport. Champaign, IL: Human Kinetics. 2013:3-8.
  10. ↑ Theofilidis G, Bogdanis GC, Koutedakis Y, Karatzaferi C. Monitoring exercise-induced muscle fatigue and adaptations: making sense of popular or emerging indices and biomarkers. Sports. 2018 Nov 26;6(4):153.
  11. ↑ 11.0 11.1 11.2 Hughes DC, Ellefsen S, Baar K. Adaptations to endurance and strength training. Cold Spring Harbor perspectives in medicine. 2018 Jun 1;8(6):a029769.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 Skorski S, Mujika I, Bosquet L, Meeusen R, Coutts AJ, Meyer T. The temporal relationship between exercise, recovery processes, and changes in performance. International Journal of Sports Physiology and Performance. 2019 Sep 1;14(8):1015-21.
  13. ↑ 13.0 13.1 13.2 13.3 Carrard J, Rigort AC, Appenzeller-Herzog C, Colledge F, Königstein K, Hinrichs T, Schmidt-Trucksäss A. Diagnosing overtraining syndrome: a scoping review. Sports health. 2022 Sep;14(5):665-73.
  14. ↑ Meeusen R, De Pauw K. Overtraining–what do we know?. InRecovery and Well-being in Sport and Exercise 2021 Dec 28 (pp. 51-62). Routledge.
  15. ↑ Seshadri DR, Li RT, Voos JE, Rowbottom JR, Alfes CM, Zorman CA, Drummond CK. Wearable sensors for monitoring the internal and external workload of the athlete. NPJ digital medicine. 2019 Jul 29;2(1):1-8.
  16. ↑ Burton I, McCormack A. The implementation of resistance training principles in exercise interventions for lower limb tendinopathy: A systematic review. Physical Therapy in Sport. 2021 Jul 1;50:97-113.
  17. ↑ 17.0 17.1 17.2 17.3 Maestroni L, Read P, Bishop C, Papadopoulos K, Suchomel TJ, Comfort P, Turner A. The benefits of strength training on musculoskeletal system health: practical applications for interdisciplinary care. Sports Medicine. 2020 Aug;50(8):1431-50.
  18. ↑ Brooks G, Fahey TD, Baldwin KM.Exercise Physiology: Human Bioenergetics and its Applications. 4th Edition. Boston. McGraw Hill. 2005.
  19. ↑ Osiris Salazar.Published on Feb 7, 2018 The Energetics of Exercise. Available from https://www.youtube.com/watch?v=q-NWIDlUCuY&t=341s (last accessed 11 October 2022)
  20. ↑ Travers G, Kippelen P, Trangmar SJ, González-Alonso J. Physiological Function during Exercise and Environmental Stress in Humans—An Integrative View of Body Systems and Homeostasis. Cells. 2022 Jan 24;11(3):383.
  21. ↑ Ross R, Goodpaster BH, Koch LG, Sarzynski MA, Kohrt WM, Johannsen NM, Skinner JS, Castro A, Irving BA, Noland RC, Sparks LM. Precision exercise medicine: understanding exercise response variability. British journal of sports medicine. 2019 Sep 1;53(18):1141-53.
  22. ↑ Bouchard C, Rankinen T. Individual differences in response to regular physical activity. Medicine and science in sports and exercise. 2001 Jun 1;33(6; SUPP):S446-51.
  23. ↑ 23.0 23.1 23.2 Mann TN, Lamberts RP, Lambert MI. High responders and low responders: factors associated with individual variation in response to standardized training. Sports Medicine. 2014 Aug;44(8):1113-24.
  24. ↑ Kuang J, McGinley C, Lee MJ, Saner NJ, Garnham A, Bishop DJ. Interpretation of exercise-induced changes in human skeletal muscle mRNA expression depends on the timing of the post-exercise biopsies. PeerJ. 2022 Feb 4;10:e12856.
  25. ↑ 25.0 25.1 25.2 25.3 Baar K. The signaling underlying FITness. Applied Physiology, Nutrition, and Metabolism. 2009 Jun;34(3):411-9.
  26. ↑ Kreher JB. Diagnosis and prevention of overtraining syndrome: an opinion on education strategies. Open access journal of sports medicine. 2016;7:115.
  27. ↑ Hackney AC. Molecular and physiological adaptations to endurance training. InConcurrent aerobic and strength training 2019 (pp. 19-34). Springer, Cham.
  28. ↑ Murias JM, Kowalchuk JM, Paterson DH. Time course and mechanisms of adaptations in cardiorespiratory fitness with endurance training in older and young men. Journal of applied physiology. 2010 Mar;108(3):621-7.
  29. ↑ 29.0 29.1 29.2 Scharhag-Rosenberger F, Meyer T, Walitzek S, Kindermann W. Time course of changes in endurance capacity: a 1-yr training study. Medicine & Science in Sports & Exercise. 2009 May 1;41(5):1130-7.
  30. ↑ RC. Hickson, JM. Hagberg, AA. Ehsani, JO. Holloszy, Time course of the adaptive responses of aerobic power and heart rate to training., Med Sci Sports Exerc, volume 13, issue 1, pages 17-20, 1981,
  31. ↑ Rothschild JA, Kilding AE, Stewart T, Plews DJ. Factors influencing substrate oxidation during submaximal cycling: a modelling analysis. Sports Medicine. 2022 Jul 12:1-21.
  32. ↑ Place N, Westerblad H. Metabolic Factors in Skeletal Muscle Fatigue. InExercise Metabolism 2022 (pp. 377-399). Springer, Cham.
  33. ↑ Hurst C, Robinson SM, Witham MD, Dodds RM, Granic A, Buckland C, De Biase S, Finnegan S, Rochester L, Skelton DA, Sayer AA. Resistance exercise as a treatment for sarcopenia: prescription and delivery. Age and ageing. 2022 Feb;51(2):afac003.
  34. ↑ Yin L, Li N, Jia W, Wang N, Liang M, Yang X, Du G. Skeletal muscle atrophy: From mechanisms to treatments. Pharmacological Research. 2021 Oct 1;172:105807.
  35. ↑ 35.0 35.1 Hall JE. Guyton and Hall Textbook of Medical Physiology, 12e. 12th edition. Philadelphia, Pa: Saunders; 2010. p1120
  36. ↑ Foss ML, Keteyian SJ, Fox EL. Fox’s Physiological Basis for Exercise and Sport. 6th edition. Boston, Mass: William C Brown Pub; 1998. p 620
  37. ↑ Gabriel DA, Kamen G, Frost G. Neural adaptations to resistive exercise. Sports medicine. 2006 Feb;36(2):133-49.
  38. ↑ Suchomel TJ, Nimphius S, Bellon CR, Stone MH. The importance of muscular strength: training considerations. Sports medicine. 2018 Apr;48(4):765-85.
  39. ↑ Hornsby WG, Gentles JA, Haff GG, Stone MH, Buckner SL, Dankel SJ, Bell ZW, Abe T, Loenneke JP. What is the impact of muscle hypertrophy on strength and sport performance?. Strength & Conditioning Journal. 2018 Dec 1;40(6):99-111.
  40. ↑ Chalmers G. Strength training: Do Golgi tendon organs really inhibit muscle activity at high force levels to save muscles from injury, and adapt with strength training?. Sports Biomechanics. 2002 Jul 1;1(2):239-49.
  41. ↑ Pucci AR, Griffin L, Cafarelli E. Maximal motor unit firing rates during isometric resistance training in men. Experimental physiology. 2006 Jan;91(1):171-8.
  42. ↑ Lambert MI. General adaptations to exercise: Acute versus chronic and strength versus endurance training. InExercise and human reproduction 2016 (pp. 93-100). Springer, New York, NY.
  43. ↑ Schumann M, Feuerbacher JF, Sünkeler M, Freitag N, Rønnestad BR, Doma K, Lundberg T. An updated systematic review and meta-analysis on the compatibility of concurrent aerobic and strength training for skeletal muscle size and function.
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