Editing Muscle memory (section)

==Physiology==

===Motor behavior===
When first learning a motor task, movement is often slow, stiff and easily disrupted without attention. With practice, execution of the motor task becomes smoother, there is a decrease in limb stiffness, and the muscle activity necessary to the task is performed without conscious effort.<ref name = "Shadmehr">{{cite journal | last1 = Shadmehr | first1 = R | last2 = Holcomb | first2 = HH | year = 1997 | title = Neural correlates of motor memory consolidation | journal = Science | volume = 277 | issue = 5327| pages = 821–25 | doi=10.1126/science.277.5327.821| pmid = 9242612 }}</ref>

===Muscle memory encoding===

The [[neuroanatomy of memory]] is widespread throughout the [[brain]]; however, the pathways important to motor memory are separate from the medial [[temporal lobe]] pathways associated with [[declarative memory]].<ref>{{cite journal | last1 = Brashers-Krug | first1 = T | last2 = Shadmehr | first2 = R. | last3 = Bizzi | first3 = E. | year = 1996 | title = Consolidation in human motor memory | journal = Nature | volume = 382 | issue = 6588| pages = 252–255| doi=10.1038/382252a0| pmid = 8717039 | bibcode = 1996Natur.382..252B| citeseerx = 10.1.1.39.3383 | s2cid = 4316225 }}</ref>  As with declarative memory, motor memory is theorized to have two stages: a short-term [[memory encoding]] stage, which is fragile and susceptible to damage, and a long-term [[memory consolidation]] stage, which is more stable.<ref>{{cite journal | last1 = Atwell | first1 = P. | last2 = Cooke | first2 = S. | last3 = Yeo | first3 = C. | year = 2002 | title = Cerebellar function in consolidation of motor memory | journal = Neuron | volume = 34 | issue = 6| pages = 1011–1020 | doi=10.1016/s0896-6273(02)00719-5| pmid = 12086647 | doi-access = free }}</ref> 
  
The memory encoding stage is often referred to as [[motor learning]], and requires an increase in brain activity in motor areas as well as an increase in attention. Brain areas active during motor learning include the motor and somatosensory cortices; however, these areas of activation decrease once the motor skill is learned. The prefrontal and frontal cortices are also active during this stage due to the need for increased attention on the task being learned.<ref name = "Shadmehr" />

The main area involved in motor learning is the [[cerebellum]]. Some models of cerebellar-dependent motor learning, in particular the Marr-Albus model, propose a single plasticity mechanism involving the cerebellar [[long-term depression]] (LTD) of the parallel fiber synapses onto [[Purkinje cells]].  These modifications in synapse activity would mediate motor input with motor outputs critical to inducing motor learning.<ref>{{cite journal | last1 = Boyden | first1 = E. | last2 = Katoh | first2 = A. | last3 = Raymond | first3 = J. | year = 2004 | title = Cerebellum-dependent learning: the role of multiple plasticity mechanisms | journal = Annu. Rev. Neurosci. | volume = 27 | pages = 581–609 | doi=10.1146/annurev.neuro.27.070203.144238 | pmid=15217344}}</ref> However, conflicting evidence suggests that a single plasticity mechanism is not sufficient and a multiple plasticity mechanism are needed to account for the storage of motor memories over time. Regardless of the mechanism, studies of cerebellar-dependent  motor tasks show that cerebral cortical plasticity is crucial for motor learning, even if not necessarily for storage.<ref name = "Ma" />

The [[basal ganglia]] also play an important role in memory and learning, in particular in reference to stimulus-response associations and the formation of habits. The basal ganglia-cerebellar connections are thought to increase with time when learning a motor task.<ref>{{cite journal | last1 = Packard | first1 = M. | last2 = Knowlton | first2 = B. | year = 2002 | title = Learning and memory functions of the basal ganglia.  | journal = Annu. Rev. Neurosci. | volume = 25 | pages = 563–93 | pmid = 12052921 | doi = 10.1146/annurev.neuro.25.112701.142937 }}</ref>

===Muscle memory consolidation===
Muscle memory consolidation involves the continuous evolution of neural processes after practicing a task has stopped. The exact mechanism of motor memory consolidation within the brain is controversial. However, most theories assume that there is a general redistribution of information across the brain from encoding to consolidation.  [[Hebb's rule]] states that "synaptic connectivity changes as a function of repetitive firing." In this case, that would mean that the high amount of stimulation coming from practicing a movement would cause the repetition of firing in certain motor networks, presumably leading to an increase in the efficiency of exciting these motor networks over time.<ref name= "Ma">{{cite journal | last1 = Ma | first1 = L. | display-authors = etal   | year = 2010| title = . (2010). Changes in regional activity are accompanied with changes in inter-regional connectivity during 4 weeks motor learning | journal = Brain Res. | volume =  1318| pages =  64–76| doi = 10.1016/j.brainres.2009.12.073 | pmid=20051230 | pmc=2826520}}</ref>

While the exact location of muscle memory storage is not known, studies have suggested that it is the inter-regional connections that play the most important role in advancing motor memory encoding to consolidation, rather than decreases in overall regional activity.  These studies have shown a weakened connection from the cerebellum to the primary motor area with practice, it is presumed, because of a decreased need for error correction from the cerebellum. However, the connection between the basal ganglia and the primary motor area is strengthened, suggesting the basal ganglia play an important role in the motor memory consolidation process.<ref name = "Ma" />

=== Sleep effects on muscle memory ===
Sleep and quality habits are required for maximizing motor memory and motor skill consolidation. Sleep has been shown to consolidate motor skills acquired via the reactivation and consolidation of neural pathways .<ref name=":0">{{Cite journal |last1=Cheng |first1=Larry Y. |last2=Che |first2=Tiffanie |last3=Tomic |first3=Goran |last4=Slutzky |first4=Marc W. |last5=Paller |first5=Ken A. |date=2021-11-17 |title=Memory Reactivation during Sleep Improves Execution of a Challenging Motor Skill |journal=The Journal of Neuroscience |language=en |volume=41 |issue=46 |pages=9608–9616 |doi=10.1523/JNEUROSCI.0265-21.2021 |pmid=34663626 |pmc=8612481 |issn=0270-6474}}</ref> This is particularly beneficial with complex motor movements, where motor performance is improved following sleep.  

Sleep duration and exercise also influence motor skill learning and memory. It has been proven through experiments that sleep after night training improves skill consolidation compared to morning training without sleep .<ref name=":1">{{Cite journal |last1=Truong |first1=Charlène |last2=Ruffino |first2=Célia |last3=Gaveau |first3=Jérémie |last4=White |first4=Olivier |last5=Hilt |first5=Pauline M. |last6=Papaxanthis |first6=Charalambos |date=2023-09-01 |title=Time of day and sleep effects on motor acquisition and consolidation |journal=npj Science of Learning |language=en |volume=8 |issue=1 |page=30 |doi=10.1038/s41539-023-00176-9 |pmid=37658041 |bibcode=2023npjSL...8...30T |issn=2056-7936|pmc=10474136 }}</ref> This therefore implies that sleep is a time of heightened processing and consolidation of motor learning, allowing athletes and individuals maximizing their motor skills to attain maximum performance.  

Furthermore, formal sleep therapies have also been discovered to enhance the performance of sports through enhanced reaction time, coordination, and overall execution of skills. Maintenance of proper quantities of sleep in addition to strict compliance to consistency in sleeping schedule can maximize the results of motor learning as well as support long-term memory for body skills .<ref name=":2">{{Cite journal |last1=Cunha |first1=Lúcio A. |last2=Costa |first2=Júlio A. |last3=Marques |first3=Elisa A. |last4=Brito |first4=João |last5=Lastella |first5=Michele |last6=Figueiredo |first6=Pedro |date=2023-07-18 |title=The Impact of Sleep Interventions on Athletic Performance: A Systematic Review |journal=Sports Medicine - Open |language=en |volume=9 |issue=1 |page=58 |doi=10.1186/s40798-023-00599-z |doi-access=free |pmid=37462808 |pmc=10354314 |issn=2198-9761}}</ref> The application of sleep-based interventions, including following a constant sleeping pattern and minimizing disruptions to an absolute degree, can therefore be a significant assistant for the person who wants to optimize their motor capacity.  

===Strength training and adaptations===
{{See also|Muscle memory (strength training)}}
When participating in any sport, new motor skills and movement combinations are frequently being used and repeated. All sports require some degree of strength, endurance training, and skilled reaching in order to be successful in the required tasks. Muscle memory related to [[Muscle memory (strength training)|strength training]]  involves elements of both motor learning, described below, and long-lasting changes in the muscle tissue.

Evidence has shown that increases in strength occur well before muscle [[hypertrophy]], and decreases in strength due to detraining or ceasing to repeat the exercise over an extended period of time precede muscle [[atrophy]].<ref name = "Adkins">{{cite journal | last1 = Adkins | first1 = DeAnna L. | last2 = Boychuck | first2 = Jeffery | year = 2006 | title = Motor training induces experience specific patterns of plasticity across motor cortex and spinal cord | journal = Journal of Applied Physiology | volume = 101 | issue = 6| pages = 1776–1782 | doi=10.1152/japplphysiol.00515.2006 | pmid=16959909| s2cid = 14285824 }}</ref> To be specific, strength training enhances [[motor neuron]] excitability and induces [[synaptogenesis]], both of which would help in enhancing communication between the nervous system and the muscles themselves.<ref name = "Adkins" /> [[File:US Navy 071017-N-0995C-008 Chief Mineman Kevin Sperling, an officer recruiter at Navy Recruiting Processing Station Honolulu, presses two 105-pound dumbbells.jpg|thumb|right|alt=A navy man performs strength training exercises.]]However, neuromuscular efficacy is not altered within a two-week time period following cessation of the muscle usage; instead, it is merely the [[neuron]]'s ability to excite the muscle that declines in correlation with the muscle's decrease in strength.<ref>{{cite journal | last1 = Deschenes Michael | first1 = R. | last2 = Giles Jennifer | first2 = A. | year = 2002 | title = Neural factors account for strength decrements observed after short-term muscle unloading | journal = American Journal of Physiology. Regulatory, Integrative and Comparative Physiology | volume = 282 | issue = 2| pages = R578–R583 | doi=10.1152/ajpregu.00386.2001| pmid = 11792669 }}</ref> This confirms that muscle strength is first influenced by the inner neural circuitry, rather than by external physiological changes in the muscle size.

Previously untrained muscles will acquire newly formed nuclei through the fusion of satellite cells preceding hypertrophy. Subsequent detraining will result in atrophy and the loss of myo-nuclei. While it was long believed that a muscle memory effect related to myo-nuclei permanence existed, current studies establish that during detraining, myo-nuclei will be lost.<ref>{{cite journal |last1=Snijders |first1=Tim |last2=Aussieker |first2=Thorben |last3=Holwerda |first3=Andy |last4=Parise |first4=Gianni |last5=van Loon |first5=Luc J C |last6=Verdijk |first6=Lex B |title=The concept of skeletal muscle memory: Evidence from animal and human studies |journal=Acta Physiologica|date=2020 |volume=229 |issue=3 |pages=e13465 |doi=10.1111/apha.13465 |pmid=32175681 |pmc=7317456 }}</ref><ref>{{cite journal |last1=Rahmati |first1=Masoud |last2=Mc Carthy |first2=John H. |last3=Malakoutinia |first3=Fatemeh |title=Myonuclear permanence in skeletal muscle memory: a systematic review and meta-analysis of human and animal studies |journal=Journal of Cachexia, Sarcopenia and Muscle|date=2022 |volume=13 |issue=5 |pages=2276–2297 |doi=10.1002/jcsm.13043 |pmid=35961635 |pmc=9530508 }}</ref>

Reorganization of motor maps within the cortex are not altered in either strength or endurance training. However, within the motor cortex, endurance induces [[angiogenesis]] within as little as three weeks to increase blood flow to the involved regions.<ref name = "Adkins" /> In addition, neurotropic factors within the motor cortex are [[upregulated]] in response to endurance training to promote neural survival.<ref name = "Adkins" />

Skilled motor tasks have been divided into two distinct phases: a fast-learning phase, in which an optimal plan for performance is established, and a slow-learning phase, in which longer-term structural modifications are made on specific motor modules.<ref>{{cite journal | last1 = Karni | first1 = Avi | last2 = Meyer | first2 = Gundela | year = 1998 | title = The acquisition of skilled motor performance: Fast and slow experience-driven changes in primary motor cortex | journal = Proceedings of the National Academy of Sciences | volume = 95| issue = 3| pages = 861–868 | doi=10.1073/pnas.95.3.861| pmid = 9448252 | pmc = 33809| bibcode = 1998PNAS...95..861K| doi-access = free }}</ref> Even a small amount of training may be enough to induce neural processes that continue to evolve even after the training has stopped, which provides a potential basis for consolidation of the task. In addition, studying mice while they are learning a new complex reaching task, has found that "motor learning leads to rapid formation of [[dendritic spines]] (spinogenesis) in the [[motor cortex]] contralateral to the reaching forelimb".<ref>{{cite journal | last1 = Xu | first1 = Tonghui | last2 = Perlik | first2 = Andrew J | year = 2009 | title = Rapid formation and selective stabilisation of synapses for enduring motor memories | journal = Nature | volume = 462| issue = 7275| pages = 915–20 | doi=10.1038/nature08389 | pmid=19946267 | pmc=2844762| bibcode = 2009Natur.462..915X}}</ref>  However, motor cortex reorganization itself does not occur at a uniform rate across training periods. It has been suggested that the synaptogenesis and motor map reorganization merely represent the consolidation, and not the acquisition itself, of a specific motor task.<ref>{{cite journal | last1 = Kleim Jerrery | first1 = L. | last2 = Hogg Theresa | first2 = M. | year = 2004 | title = Cortical Synaptogenesis and Motor Map Reorganization Occur during Late, But not Early, Phase of Motor Skill Learning | journal = The Journal of Neuroscience | volume = 24 | issue = 3| pages = 629–633 | doi=10.1523/jneurosci.3440-03.2004 | pmid=14736848| pmc = 6729261 | citeseerx = 10.1.1.320.2189 }}</ref>  Furthermore, the degree of plasticity in various locations (namely motor cortex versus spinal cord) is dependent on the behavioural demands and nature of the task (i.e., skilled reaching versus strength training).<ref name = "Adkins" />

Whether strength or endurance related, it is plausible that the majority of motor movements would require a skilled moving task of some form, whether it be maintaining proper form when paddling a canoe, sitting with a neutral posture, or bench pressing a heavier weight.{{citation needed|date=July 2022}} Endurance training assists the formation of these new neural representations within the motor cortex by up regulating neurotropic factors that could enhance the survival of the newer neural maps formed due to the skilled movement training.<ref name = "Adkins" /> Strength training results are seen in the spinal cord well before any physiological muscular adaptation is established through muscle hypertrophy or atrophy.<ref name = "Adkins" /> The results of endurance and strength training, and skilled reaching, therefore, combine to help each other maximize performance output.

More recently, research has suggested that epigenetics may play a distinct role in orchestrating a muscle memory phenomenon <ref>{{cite journal|last1=Sharples|first1=Adam P.|last2=Stewart|first2=Claire E.|last3=Seaborne|first3=Robert A.|title=Does skeletal muscle have an 'epi'-memory? The role of epigenetics in nutritional programming, metabolic disease, aging and exercise|journal=Aging Cell|date=1 August 2016|volume=15|issue=4|pages=603–616|doi=10.1111/acel.12486|pmid=27102569|pmc=4933662|language=en|issn=1474-9726}}</ref> Indeed, previously untrained human participants experienced a chronic period of resistance exercise training (7 weeks) that evoked significant increases in skeletal muscle mass of the vastus lateralis muscle, in the quadriceps muscle group. Following a similar period of physical in-activity (7 weeks), where strength and muscle mass returned to baseline, participants performed a secondary period of resistance exercise.<ref name = "seaborne">{{cite journal|last1=Seaborne|first1=Robert A.|last2=Strauss|first2=Juliette|last3=Cocks|first3=Matthew|last4=Shepherd|first4=Sam|last5=O’Brien|first5=Thomas D.|last6=Someren|first6=Ken A. van|last7=Bell|first7=Phillip G.|last8=Murgatroyd|first8=Christopher|last9=Morton|first9=James P.|last10=Stewart|first10=Claire E.|last11=Sharples|first11=Adam P.|title=Human Skeletal Muscle Possesses an Epigenetic Memory of Hypertrophy|journal=Scientific Reports|date=30 January 2018|volume=8|issue=1|pages=1898|doi=10.1038/s41598-018-20287-3|pmid=29382913|pmc=5789890|language=En|issn=2045-2322|bibcode=2018NatSR...8.1898S}}</ref> Importantly, these participants adapted in an enhanced manner, whereby the amount of skeletal muscle mass gained was greater in the second period of muscle growth than the first, suggesting a muscle memory concept. The researchers went on to examine the human epigenome in order to understand how DNA methylation may aid in creating this effect. During the first period of resistance exercise, the authors identify significant adaptations in the human methylome, whereby over 9,000 CpG sites were reported as being significantly hypomethylated, with these adaptations being sustained during the subsequent period of physical in-activity. However, upon secondary exposure to resistance exercise, a greater frequency of hypomethylated CpG sites was observed, where over 18,000 sites reported as being significantly hypomethylated. The authors went on to identify how these changes altered the expression of relevant transcripts, and subsequently correlated these changes with adaptations in skeletal muscle mass. Collectively, the authors conclude that skeletal muscle mass and muscle memory phenomenon is, at least in part, modulated due to changes in DNA methylation.<ref name = "seaborne" /> Further work is now needed to confirm and explore these findings.