Neural Adaptations associated with Strength Training
Resistance training will play an important part in achieving the health or performance aims of many. Those who are newer to exercise will see quicker gains in strength capacity for any given exercise and these first signs of muscle adaptation to strengthening exercises are largely down to neural adaptations. After initial neural adaptations occur, morphological changes (including hypertrophy) contribute to strength gains and from then on these two are the major factors in strength development. Moritani and deVries (1979) were among the first to find that “neural factors” accounted for the significant improvements observed during the first 4 weeks of an 8-week resistance-training program and more experienced exercisers can also benefit from neural adaptations as they progress through different stages of a periodised programme.
This page is designed as a primer for those seeking to understand key strength-based neural adaptations.
Neural adaptations created by resistance training broadly lead to greater efficiency in neural recruitment patterns which includes greater synchronicity of the discharge of motor units, reduced neural inhibitory reflexes, decreased co-contraction of antagonists, increased central motor drive and elevated motor neuron excitability. Neural adaptations are also seen in the non-exercising limb after a period of training in the partner limb with eccentric exercise being shown to be more effective in producing this phenomenon.
The timescale of neural and morphological adaptations has long been studied and it's clear that novice exercisers and experienced, well trained individuals have different potential for gaining strength. It's also clear that neural adaptations can play a role for both experienced and less experienced individuals and depending on the type of training that is undertaken, neural adaptation will allow the individual to better coordinate the activities of the muscle groups, thereby affecting a greater net force even in the absence of morphologic change within the muscles themselves.
1. Motor Unit Recruitment
A motor unit consists of one motor neuron and all of the muscle fibers that it stimulates. Motor unit recruitment refers to the activation of additional motor units to accomplish an increase in contractile strength in a muscle.
Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons, and thus slow to fast twitch) as contraction increases. This is known as Henneman's Size Principle (Henneman et al., 1965)
Read more on Motor Unit Recruitment
Henneman, E., Somjen, G., & Carpenter, D. O. (1965). Excitability and inhibitibility of motoneurons of different sizes. Journal of Neurophysiology, 28(3), 599-620.
Duchateau, J., & Hainaut, K. (2003). Mechanisms of muscle and motor unit adaptation to explosive power training. In P. V. Komi (2003) Strength and power in sport, 315. [full text]
Van Cutsem, M., Duchateau, J., & Hainaut, K. (1998). Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans. The Journal of physiology, 513(1), 295-305.
Folland, J. P., & Williams, A. G. (2007). Morphological and neurological contributions to increased strength. Sports Medicine, 37(2), 145-168. https://doi.org/10.2165/00007256-200737020-00004
2. Firing Frequency (rate coding)
The force produced by a single motor unit is partly determined by the number of muscle fibers in the unit. Another important determinant of force is the frequency with which the muscle fibers are stimulated by their innervating axon. The rate at which the nerve impulses arrive is known as the motor unit firing rate and may vary from frequencies low enough to produce a series of single twitch contractions to frequencies high enough to produce a fused tetanic contraction. Generally, this allows a 2 to 4-fold change in force. In general, the motor unit firing rate of each individual motor unit increases with increasing muscular effort until a maximum rate is reached. This smooths out the incremental force changes which would otherwise occur as each additional unit was recruited
Read more on firing frequency and rate coding
Aagaard, P., Simonsen, E. B., Andersen, J. L., Magnusson, P., & Dyhre-Poulsen, P. (2002). Increased rate of force development and neural drive of human skeletal muscle following resistance training. Journal of Applied Physiology, 93(4), 1318-1326. https://doi.org/10.1152/japplphysiol.00283.2002
Enoka, R. M., & Duchateau, J. (2017). Rate coding and the control of muscle force. Cold Spring Harb Perspect Med, 7, a029702. DOI: 10.1101/cshperspect.a029702
3. Motor Unit Synchronisation
Synchronisation occurs when two or more motor units fire at fixed time intervals. It's possible that simultaneous activation of more than two motor units enhances peak force production by expressing greater Rate of Force Development over short time periods.
It appears that training strategies that include heavy RT and/or ballistic-type movements may improve MU synchronization.
Read more on Motor Unit Synchronisation
Semmler, J. G. (2002). Motor unit synchronization and neuromuscular performance. Exercise and Sport Sciences Reviews, 30(1), 8-14. [www]
4. Neuromuscular Inhibition
Neuromuscular inhibition refers to a reduction in the neural drive of any given muscle group during voluntary muscle actions that may negatively affect force production due to the neural feedback received from muscle and joint receptors. A control mechanism that inhibits muscle force production is sometimes needed to prevent damage to musculoskeletal tissue however it can also affect potential training adaptations.
Read more on Neuromuscular Inhibition
Aagaard, P., Simonsen, E. B., Andersen, J. L., Magnusson, S. P., Halkjaer-Kristensen, J., & Dyhre-Poulsen, P. (2000). Neural inhibition during maximal eccentric and concentric quadriceps contraction: effects of resistance training. Journal of Applied Physiology, 89(6), 2249-2257. https://doi.org/10.1152/jappl.2000.89.6.2249
Gabriel, D. A., Kamen, G., & Frost, G. (2006). Neural adaptations to resistive exercise. Sports Medicine, 36(2), 133-149. https://doi.org/10.2165/00007256-200636020-00004 [www]
Glover, I. S., & Baker, S. N. (2020). Cortical, corticospinal, and reticulospinal contributions to strength training. Journal of Neuroscience, 40(30), 5820-5832. https://doi.org/10.1523/JNEUROSCI.1923-19.2020
Kidgell, D. J., Bonanno, D. R., Frazer, A. K., Howatson, G., & Pearce, A. J. (2017). Corticospinal responses following strength training: a systematic review and meta‐analysis. European Journal of Neuroscience, 46(11), 2648-2661. https://doi.org/10.1111/ejn.13710
Implications for Training
An understanding of the various neuromuscular adaptations that occur with strength training will allow the practitioner to prescribe a range of training activities across an extended period of time. Appropriately periodised programmes should be planned that account for the individual's training history, stage of season and identified individual needs among other factors.
Read this: Walker, O. (2016) Rate of Force Development https://www.scienceforsport.com/rate-of-force-development-rfd-2/
If you only read one article, choose from...
Suchomel, T. J., Nimphius, S., Bellon, C. R., & Stone, M. H. (2018). The importance of muscular strength: Training considerations. Sports Medicine, 1-21. https://doi.org/10.1007/s40279-018-0862-z
Aagaard, P. (2003). Training-induced changes in neural function. Exercise and Sport Sciences Reviews, 31(2), 61-67. https://journals.lww.com/acsm-essr/Fulltext/2003/04000/Training_Induced_Changes_in_Neural_Function.2.aspx
References and Further Reading
Aagaard, P., Simonsen, E. B., Andersen, J. L., Magnusson, P., & Dyhre-Poulsen, P. (2002). Increased rate of force development and neural drive of human skeletal muscle following resistance training. Journal of Applied Physiology, 93(4), 1318-1326. https://doi.org/10.1152/japplphysiol.00283.2002
Bompa, T., & Buzzichelli, C. (2015). Periodization Training for Sports, 3E. Human Kinetics. [extract: Strength Training and Neuromuscular Adaptations]
Carroll, T. J., Riek, S., & Carson, R. G. (2001). Neural adaptations to resistance training. Sports Medicine, 31(12), 829-840. https://doi.org/10.2165/00007256-200131120-00001
de Salles, B. F., Simao, R., Miranda, F., da Silva Novaes, J., Lemos, A., & Willardson, J. M. (2009). Rest interval between sets in strength training. Sports Medicine, 39(9), 765-777. https://doi.org/10.2165/11315230-000000000-00000
Dideriksen, J. L., Del Vecchio, A., & Farina, D. (2019). Neural and muscular determinants of maximal rate of force development. Journal of Neurophysiology. https://doi.org/10.1152/jn.00330.2019
Duchateau, J., Semmler, J. G., & Enoka, R. M. (2006). Training adaptations in the behavior of human motor units. Journal of Applied Physiology, 101(6), 1766-1775. https://doi.org/10.1152/japplphysiol.00543.2006
Eklund, D., Pulverenti, T., Bankers, S., Avela, J., Newton, R., Schumann, M., & Häkkinen, K. (2015). Neuromuscular adaptations to different modes of combined strength and endurance training. International Journal of Sports Medicine, 36(02), 120-129. DOI: 10.1055/s-0034-138588
Farthing, J. P., Borowsky, R., Chilibeck, P. D., Binsted, G., & Sarty, G. E. (2007). Neuro-physiological adaptations associated with cross-education of strength. Brain topography, 20(2), 77-88.
Folland, J. P., & Williams, A. G. (2007). Morphological and neurological contributions to increased strength. Sports Medicine, 37(2), 145-168. https://doi.org/10.2165/00007256-200737020-00004
Gabriel, D. A., Kamen, G., & Frost, G. (2006). Neural adaptations to resistive exercise: Mechanisms and Recommendations for Training Practices. Sports Medicine, 36(2), 133-149. https://doi.org/10.2165/00007256-200636020-00004 [www]
Glover, I.S. and Baker, S.N. (2022) Both Corticospinal and Reticulospinal Tracts Control Force of Contraction. Journal of Neuroscience https://doi.org/10.1523/JNEUROSCI.0627-21.2022
Gorassini, M., Yang, J. F., Siu, M., & Bennett, D. J. (2002). Intrinsic activation of human motoneurons: reduction of motor unit recruitment thresholds by repeated contractions. Journal of neurophysiology, 87(4), 1859-1866. https://doi.org/10.1152/jn.00025.2001
Hedayatpour, N., & Falla, D. (2015). Physiological and neural adaptations to eccentric exercise: mechanisms and considerations for training. BioMed research international, http://dx.doi.org/10.1155/2015/193741
Jenkins, N. D., Miramonti, A. A., Hill, E. C., Smith, C. M., Cochrane-Snyman, K. C., Housh, T. J., & Cramer, J. T. (2017). Greater neural adaptations following high-vs. low-load resistance training. Frontiers in Physiology, 8, 331. https://doi.org/10.3389/fphys.2017.00331
Moritani, T. and DeVries, H.A. (1979). Neural factors versus hypertrophy in the time course of muscle strength gain. American journal of physical medicine, 58(3), 115-130.
Sale, D. G. (2003). Neural adaptation to strength training. In P. V. Komi (Ed.), The Encyclopaedia of Sports Medicine: Strength and Power in Sport (pp. 281-314). Oxford: Blackwell Scientific https://doi.org/10.1002/9780470757215.ch15
Selvanayagam, V. S., Riek, S., & Carroll, T. J. (2011). Early neural responses to strength training. Journal of Applied Physiology, 111(2), 367-375. https://doi.org/10.1152/japplphysiol.00064.2011
Semmler, J. G. (2002). Motor unit synchronization and neuromuscular performance. Exercise and sport sciences reviews, 30(1), 8-14. [www]
Siddique, U., Rahman, S., Frazer, A. K., Pearce, A. J., Howatson, G., & Kidgell, D. J. (2020). Determining the sites of neural adaptations to resistance training: a systematic review and meta-analysis. Sports Medicine, 1-22. https://doi.org/10.1007/s40279-020-01258-z
Suchomel, T. J., Nimphius, S., Bellon, C. R., & Stone, M. H. (2018). The importance of muscular strength: Training considerations. Sports Medicine, 1-21. https://doi.org/10.1007/s40279-018-0862-z
Zhou, S. (2000). Chronic neural adaptations to unilateral exercise: mechanisms of cross education. Exercise and sport sciences reviews, 28(4), 177-184.
