Skeletal muscle inactivity memory differs in youth and age

Skeletal muscle can retain a molecular trace of previous inactivity, and emerging evidence indicates that this record is written very differently in youth compared with old age.

When stillness is imposed a second time, younger muscle appears to moderate its internal reaction, whereas older muscle shows more pronounced decline under the same repeated challenge.

Repeated rest harms muscle

Multiple bouts of enforced immobility leave a detectable signature within muscle tissue that remains even after the initial reduction in muscle mass.

By following immobilised young adults and, in parallel, very old rats, Adam P. Sharples at the Norwegian School of Sport Sciences (NIH) described how this lingering signature alters the muscle’s response the next time activity is removed.

In younger participants, a repeat period of disuse caused a less dramatic disturbance in energy-related gene activity, despite the fact that the amount of muscle lost was comparable with the first bout.

With ageing, the pattern shifted in the opposite direction: the same repeat stress seemed to translate into greater susceptibility rather than adaptation, setting the stage for the molecular contrasts described below.

Young muscle adapts better

Among the young volunteers, the first and second immobilisation periods led to broadly similar reductions in leg muscle size.

However, at the cellular level, mitochondria-the energy-producing components inside muscle cells-showed a smaller change in key genes when inactivity was repeated.

This more subdued shift resembled a protective form of “memory”, helping the muscle’s energy machinery remain more stable during a second shutdown.

Even so, resilience was not the same as protection from loss: muscle tissue still declined during each enforced rest.

Aging muscle weakens more

In the older rats, the response diverged, with the second inactive period producing a greater reduction in muscle tissue than the first.

Rather than settling into a quieter state, aerobic metabolism-oxygen-driven energy production that supports sustained effort-fell further, alongside a broader switch-off of mitochondria-associated genes.

During the repeat bout, cells also increased signals linked to DNA repair and stress responses, suggesting a stronger internal reaction to the same type of disruption.

Taken together, these changes left older muscle less able to tolerate a new interruption in movement, even when normal activity returned between episodes.

DNA stores inactivity memory

Some of the most persistent signals appeared to sit directly on DNA, where gene activity can be adjusted up or down.

The researchers observed DNA methylation-chemical marks that help regulate gene expression-remaining associated with energy and mitochondrial pathways after disuse.

Greater methylation frequently coincided with lower activity of energy-related genes, consistent with a muscle fuel system operating at a reduced level.

A 2018 study reported long-lasting DNA marks after strength training, and these findings suggest that inactivity can leave enduring marks as well.

Mitochondria reveal the cost

Because mitochondria underpin energy production, repeated disuse placed particular pressure on mitochondrial gene programmes.

In older muscle, many mitochondrial genes stayed suppressed during the second resting period, in line with the larger physical losses.

In younger muscle, gene disruption was less pronounced, although later assessments still detected reduced mitochondrial DNA content after repeated immobilisation.

When mitochondrial capacity drops, routine activities can feel more demanding, as muscle becomes less able to convert oxygen into usable power.

Energy reserves drop

Muscle relies on a small supporting molecule, NAD+, to keep its energy systems functioning, and repeated inactivity appeared to erode this support.

Across both immobilisation periods, one key gene involved in rebuilding this energy helper fell steeply in the muscle of the young volunteers.

In the older rats, the disturbance extended further, and the muscle’s measured energy reserves declined after the second bout of disuse.

With depleted reserves, older muscle found it harder to power cellular processes efficiently, which may make recovery after a further pause in movement more difficult.

Repair cells show changes

At NIH, repair capacity is also tied to muscle stem cells-repair cells that contribute to rebuilding muscle fibres-and the team cultured these cells from biopsy samples.

In laboratory dishes, the cells fused into early muscle fibres, and their size was measured after the disuse period had finished.

When nicotinamide riboside, a nutrient that supports NAD+ production, was added, these lab-grown fibres became larger than those grown without it.

Results from dishes cannot show that a supplement will benefit patients, but they do highlight a potential weak point that muscle may retain.

Illness can speed muscle loss

Illness, surgery and falls can all impose a period of leg rest, and many people experience more than one such episode over time.

As the human arm of the work included ten volunteers in their mid-20s, very old rats were used to represent ageing muscle.

Across both species, repeated disuse altered energy-related genes and left long-lived chemical marks, even after ordinary movement resumed.

If older muscle retains an unfavourable kind of memory, a second illness could call for earlier intervention and closer follow-up than the first.

Training and exercise choices matter

Rehabilitation programmes often approach each setback as though it were starting from scratch, yet muscle appears to keep chemical traces from previous shutdowns.

Muscle retains a record of past strength and weakness, and those molecular traces can build up over time, shaping how it reacts when inactivity strikes again, said Sharples.

“This knowledge will help us determine not only when we should retrain, but also which type and intensity of exercise may be most effective,” said Sharples.

Establishing rehabilitation that accounts for earlier inactivity will require trials in older adults, as this study did not evaluate exercise programmes.

Overall, repeated disuse left younger muscle with a more muted energy-gene response, whereas older muscle carried a pattern linked with worse wasting during later episodes.

Further research can explore which exercise doses might remove harmful chemical marks, and whether raising NAD+ helps older people regain strength more quickly.