Chronic Musculoskeletal Adaptations to Exercise

Original Editor - Wanda van Niekerk based on the course by James Laskin

Top Contributors - Wanda van Niekerk and Jess Bell  

Effects of Deconditioning[edit | edit source]

Deconditioning or detraining refers to changes in the body that occur after a period of inactivity. These changes occur in the heart, lungs and muscles. The effects of detraining on body systems[1] are summarised in Table 1.

Table 1. Effect of detraining on body systems[1]
Neuromuscular Cardiorespiratory Musculoskeletal
↓ EMG activity

↓mean muscle fibre cross-sectional area

↓ flexibility

↓ muscle strength and power

↓ muscle mass

↓ maximal oxygen uptake

↑ mean blood pressure

↑ submaximal heart rate

↓ maximal cardiac output

↓ lactate threshold

↓ endurance performance

↓ blood volume

↓ plasma volume

↓ heart volume

↓ orthostatic tolerance

↓ up to 2.5% in bone mineral density

↓ oxidative enzymes activity

↓ glycogen synthase activity

↓ mitochondrial ATP production

↓ tendon quality

Read more: The effect of detraining on body systems and the evidence for the effects of detraining.


Common Training Principles[edit | edit source]

  • Reversibility principle:
    • benefits of training are transient and reversible[3]
    • after 2 weeks of detraining there is a measurable reduction in work capacity
  • Overload principle:
    • a system must be exercised at a level beyond which it is presently accustomed to for a training effect to occur[4]
  • Specificity principle:
    • any exercise will train a system for the particular task being carried out as the training stimulus[4], thus only the system or body part repeatedly stressed will adapt to chronic overload[5]
  • Individuality:
    • there is variation in response to a training programme as individuals respond differently to the same training programme[4]

Read more: Common Training Principles.

Musculoskeletal Adaptations to Exercise[edit | edit source]

Resistance Training[edit | edit source]

  • Resistance training yields substantial strength gains via neuromuscular changes[6]
  • Important for overall fitness and health[6]
  • Critical for athletic training programmes[7]

Gains in Muscular Fitness[edit | edit source]

  • After 3 to 6 months of resistance training, there is:
    • increased ability to more effectively produce force[8]
    • increased ability to produce true maximal movement[8]
  • Strength gains result from[6]:
    • increased muscle size
    • altered neural control

Mechanisms of Muscle Strength Gains[edit | edit source]

Neural Control[edit | edit source]

  • Strength gains cannot occur without neural adaptations via plasticity[9][10]
  • Strength gains can occur without hypertrophy

Read more: The knowns and unknowns of neural adaptations to resistance training.[10]

Neural adaptations[edit | edit source]
  • Increased central drive (from the higher centres of the brain) after resistance training is partly responsible for the increase in strength[11]
  • Increased motor unit (MU) synchronisation (i.e. several motor units firing at similar times)[12][10]
  • Decrease in the force threshold at which motor units are recruited[13]
  • Increased motor unit firing rate[14]
  • Decrease in the level of co-activation of antagonist muscles after training[15]

Strength Training Model[edit | edit source]

Please refer back to the course video, Chronic Adaptations to Exercise - Musculoskeletal, for a detailed discussion of these concepts. In summary:

  • hypertrophy
    • low-intensity (40% of maximum) resistance training can lead to hypertrophic changes and neuromuscular changes over a long period (this is more pronounced in untrained individuals)[16]
    • high-intensity low repetition training can lead to hypertrophy in a short period of time[17]
  • neurological changes with resistance training occur predominantly in the early phases of a training programme
  • strength
    • strong, rapid gains in strength occur relatively quickly and consistently
    • over time, improvements in strength reach a plateau

The video below provides a good explanation of neural adaptations to resistance exercise, strength and hypertrophy.


Motor Unit Recruitment[edit | edit source]

Motor unit = efferent neuron and all of the muscle fibres that it innervates.

  • Normally motor units are recruited asynchronously
  • Synchronous recruitment leads to strength gains:[12]
    • facilitates contraction
    • may produce more forceful contraction
    • improves rate of force development
    • increased capability to exert steady forces
  • Resistance training leads to synchronous recruitment[19]
  • Strength gains may also result from greater motor unit recruitment[10]
    • increased neural drive during maximal contraction
    • increased frequency of neural discharge (rate coding)[20]
      • rate coding = the rates at which motor units discharge action potentials[21]
    • decreased inhibitory impulses
  • It is likely that some combination of improved motor unit synchronisation and motor unit recruitment results in strength gains[20]

Read more: Motor Units.

Autogenic Inhibition[edit | edit source]

  • Normal intrinsic inhibitory mechanisms:
    • Golgi tendon organs inhibit muscle contraction if tendon tension too high[22]
    • this prevents damage to bones and tendons[22]
  • Resistance training can gradually decrease or counteract inhibitory impulses:[23]
    • muscle can generate more force[23]
    • this theory may explain superhuman feats of strength[23]
Muscle Hypertrophy[edit | edit source]
  • Hypertrophy = increase in muscle mass and cross-sectional area[24]
  • Transient hypertrophy (after a single exercise bout) - the "pumping-up" of muscle that happens during a resistance training session
    • this is due to oedema formation from plasma fluid[25]
    • it disappears within hours
  • Chronic hypertrophy (long term)[26]
    • reflects actual structural change in muscle
    • fibre hypertrophy, fibre hyperplasia, or both
Fibre Hypertrophy[edit | edit source]
  • More myofibrils
  • More actin, myosin filaments
  • More sarcoplasma[27]
  • More connective tissue
  • Resistance training results in an increase in protein synthesis:[28]
    • muscle protein content is always changing
    • during exercise, there is a decrease in protein synthesis, and increased degradation
    • after exercise, there is an increase in protein synthesis, and decrease in protein degradation[29]
    • read more: muscle protein synthesis
  • Testosterone facilitates fibre hypertrophy:[30]
    • natural anabolic steroid hormone
    • synthetic anabolic steroid use results in a large increase in muscle mass
Fibre Hyperplasia[edit | edit source]

Hyperplasia refers to an increase in the number of cells or fibres.[31] In humans[31]:

  • most hypertrophy is due to fibre hypertrophy
  • fibre hyperplasia may also contribute
  • fibre hypertrophy versus fibre hyperplasia may depend on resistance training intensity / load
  • higher intensity results in type II muscle fibre hypertrophy
  • fibre hyperplasia may only occur in certain individuals under certain conditions

Whether hyperplasia occurs in adult skeletal muscle is debatable. There are concerns about the methods and measurements used in studies investigating hyperplasia. "A firm conclusion with regards to whether physiologically relevant forms of mechanical loading can induce hyperplasia remains elusive."[32] Hyperplasia has been largely dismissed as a significant contributor to skeletal muscle hypertrophy in mammals.[33]

Neural Activation and Hypertrophy[edit | edit source]

Table 2. Summary of neural activation and hypertrophy
Short-term increase in muscle strength Long-term increase in muscle strength
Substantial increase in 1 repetition max (1RM) Associated with significant fibre hypertrophy
Due to an increase in voluntary neural activation Net increase in protein synthesis takes time to occur
Neural factors are critical in the first 8 to 10 weeks Hypertrophy is the major factor after 8 -10 weeks

Immobilisation and Muscle Strength[edit | edit source]

  • Major changes occur after 6 hours of immobilisation:[34]
    • lack of muscle use leads to reduced rate of protein synthesis
    • initiates process of muscle atrophy
  • During the first week: strength loss of 3-4% per day
    • decrease in muscle size / atrophy[35]
    • decrease in neuromuscular activity[35]
  • (Reversible) effects on types I and II fibres
    • cross-sectional area decreases, cell contents degenerate
    • type I fibres are affected more than type II fibres

References[edit | edit source]

  1. 1.0 1.1 Aspetar Clinical Guidelines: Safe return to sport during the Covid-19 pandemic version 2.0. 2021
  2. Soccer Physiologist. DETRAINING - What Happens When We Stop Exercising? Available from: [last accessed 4/12/2022]
  3. Kasper K. Sports training principles. Current Sports Medicine Reports. 2019 Apr 1;18(4):95-6.
  4. 4.0 4.1 4.2 Physiopedia. Principles of Exercise
  5. Powers SK and Howley ET. Exercise Physiology: Theory and Application to Fitness and Performance.10th edition. North Ride, NSW, Australia. McGraw Hill. 2014
  6. 6.0 6.1 6.2 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.
  7. Suchomel TJ, Nimphius S, Bellon CR, Stone MH. The importance of muscular strength: training considerations. Sports medicine. 2018 Apr;48(4):765-85.
  8. 8.0 8.1 Dorrell HF, Smith MF, Gee TI. Comparison of velocity-based and traditional percentage-based loading methods on maximal strength and power adaptations. The Journal of Strength & Conditioning Research. 2020 Jan 1;34(1):46-53.
  9. Siddique U, Rahman S, Frazer AK, Pearce AJ, Howatson G, Kidgell DJ. Determining the sites of neural adaptations to resistance training: a systematic review and meta-analysis. Sports Medicine. 2020 Jun;50(6):1107-28.
  10. 10.0 10.1 10.2 10.3 Škarabot J, Brownstein CG, Casolo A, Del Vecchio A, Ansdell P. The knowns and unknowns of neural adaptations to resistance training. European Journal of Applied Physiology. 2021 Mar;121(3):675-85.
  11. Aagaard P, Simonsen E, Andersen J et al. Neural adaptation to resistance training: changes in evoked V-wave and H-reflex responses. J Appl Physiol. 2002;92: 2309-18.
  12. 12.0 12.1 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.
  13. Casolo A, Del Vecchio A, Balshaw TG, Maeo S, Lanza MB, Felici F, Folland JP, Farina D. Behavior of motor units during submaximal isometric contractions in chronically strength-trained individuals. Journal of Applied Physiology. 2021 Nov 1;131(5):1584-98.
  14. Walker S. Evidence of resistance training-induced neural adaptation in older adults. Experimental Gerontology. 2021 Aug 1;151:111408.
  15. Hakkinen K, Alen M, Kraemer WJ et al. Neuromuscular adaptations during concurrent strength and endurance training versus strength training. Eur J Appl Physio. 2003;89:42-52.
  16. Schoenfeld BJ. Is there a minimum intensity threshold for resistance training-induced hypertrophic adaptations?. Sports Medicine. 2013 Dec;43(12):1279-88.
  17. Morton RW, Colenso-Semple L, Phillips SM. Training for strength and hypertrophy: an evidence-based approach. Current Opinion in Physiology. 2019 Aug 1;10:90-5.
  18. Michael Wiggs. Hypertrophy vs Neural Adaptations. Available from: [last accessed 5/12/2022]
  19. Carroll TJ, Riek S, Carson RG. Neural adaptations to resistance training. Sports medicine. 2001 Oct;31(12):829-40.
  20. 20.0 20.1 Maestroni L, Read P, Bishop C, Turner A. Strength and power training in rehabilitation: underpinning principles and practical strategies to return athletes to high performance. Sports Medicine. 2020 Feb;50(2):239-52.
  21. Enoka RM, Duchateau J. Rate coding and the control of muscle force. Cold Spring Harbor perspectives in medicine. 2017 Oct 1;7(10):a029702.
  22. 22.0 22.1 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.
  23. 23.0 23.1 23.2 Kumar P, Singh D. Chapter-6 Strength Gains: Neuromuscular Adaptation to Resistance Training. Chief Editor Dr. Deba Prasad Sahu. 2019;95:95.
  24. Hryvniak D, Wilder RP, Jenkins J, Statuta SM. Therapeutic Exercise. In Braddom's Physical Medicine and Rehabilitation 2021 Jan 1 (pp. 291-315). Elsevier.
  25. Wilmore J, Costill D, Larry Kenney W. Physiology of Sport and Exercise: 3" Edition. Champaign, lL: Human Kinetics. 2005.
  26. Haun CT, Vann CG, Roberts BM, Vigotsky AD, Schoenfeld BJ, Roberts MD. A critical evaluation of the biological construct skeletal muscle hypertrophy: size matters but so does the measurement. Frontiers in physiology. 2019:247.
  27. Haun CT, Vann CG, Osburn SC, Mumford PW, Roberson PA, Romero MA, Fox CD, Johnson CA, Parry HA, Kavazis AN, Moon JR. Muscle fiber hypertrophy in response to 6 weeks of high-volume resistance training in trained young men is largely attributed to sarcoplasmic hypertrophy. PLoS One. 2019 Jun 5;14(6):e0215267.
  28. Abou Sawan S, Hodson N, Malowany JM, West DW, Tinline-Goodfellow C, Brook MS, Smith K, Atherton PJ, Kumbhare D, Moore DR. Trained Integrated Postexercise Myofibrillar Protein Synthesis Rates Correlate with Hypertrophy in Young Males and Females. Medicine & Science in Sports & Exercise. 2022 Jun 1;54(6):953-64.
  29. Damas F, Libardi CA, Ugrinowitsch C. The development of skeletal muscle hypertrophy through resistance training: the role of muscle damage and muscle protein synthesis. European journal of applied physiology. 2018 Mar;118(3):485-500.
  30. Horwath O, Apró W, Moberg M, Godhe M, Helge T, Ekblom M, Hirschberg AL, Ekblom B. Fiber type-specific hypertrophy and increased capillarization in skeletal muscle following testosterone administration in young women. Journal of applied physiology. 2020 May 1;128(5):1240-50.
  31. 31.0 31.1 Macdougall JD. Hypertrophy and hyperplasia. Strength and power in sport. 2003 Jan 1:252.
  32. Jorgenson KW, Phillips SM, Hornberger TA. Identifying the structural adaptations that drive the mechanical load-induced growth of skeletal muscle: a scoping review. Cells. 2020 Jul 9;9(7):1658.
  33. Roberts MD, McCarthy JJ, Hornberger TA, Phillips SM, Mackey AL, Nader GA, Boppart MD, Kavazis AN, Reidy PT, Ogasawara R, Libardi CA. Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions. Physiological Reviews. 2023 Jun 29.
  34. Appell HJ. Muscular atrophy following immobilisation. Sports medicine. 1990 Jul;10(1):42-58.
  35. 35.0 35.1 Campbell M, Varley-Campbell J, Fulford J, Taylor B, Mileva KN, Bowtell JL. Effect of immobilisation on neuromuscular function in vivo in humans: a systematic review. Sports Medicine. 2019 Jun;49(6):931-50.