Time off happens. Whether it’s due to injury, illness, a holiday gone on too long, or just life getting in the way, even the most motivated clients (and coaches) sometimes end up skipping training. What comes next? Usually, a slightly panicked look at the mirror or the squat rack, followed by questions like, “How much muscle have I lost?”, “Will I bounce back?” or “Is my body ruined?”
As personal trainers or strength and conditioning coaches, we’ve all had that conversation. And thanks to a recent study by McIntosh and colleagues (2025), we’ve now got some new science to back up our answers. In particular, when it comes to how the vascular system inside muscle responds to disuse and how resistance training can bring it back online.
This research adds an important layer to what we already know about rebuilding muscle mass. It shows that recovery after time off is about what’s happening at the level of capillaries and angiogenic signalling, not just muscle size or strength. And the good news? Your clients can bounce back, whether they’re seasoned lifters or complete novices.
And if you’re the kind of coach who loves to stay on top of new research or if you’re currently digging into on of our PT or personal training courses, this one’s definitely worth your time.
What the Study Looked At
Researchers from Auburn University took a group of young adults, some with a history of regular resistance training and others without, and immobilised one of their legs for two weeks using a locked brace. After this period of forced disuse, participants jumped into an eight-week supervised resistance training programme designed to bring that leg back to full function.
Throughout the process, the researchers took muscle biopsies from the vastus lateralis, part of the quadricep group of muscles, to measure changes in:
- Capillary number: how many small blood vessels feed the muscle
- Angiogenesis markers: VEGF, VEGFR2 and TSP-1
- Muscle fibre size: focusing on type II fibres, aka the powerful, fast-twitch ones
Why all the fuss about capillaries and growth factors? Because angiogenesis, that is the formation of new blood vessels, is a key part of muscle adaptation and it plays a bigger role than we often give it credit for.
Disuse Sends the Wrong Signals (But Only Temporarily)
After just two weeks of immobilisation, things had already started to shift at the molecular level. The researchers found that levels of VEGF (vascular endothelial growth factor), a protein that kickstarts new capillary growth, had dropped significantly. At the same time, TSP-1, which acts like a brake on angiogenesis, went up. It was an anti-growth signal, biologically speaking.
Surprisingly though, actual capillary number didn’t decline during those two weeks. That’s a big deal. It suggests that the vascular system in skeletal muscle is quite resilient, at least over short periods, even when the signalling environment takes a turn for the worse. In other words, the muscle prepares to shrink, but doesn’t fully commit yet.
This mirrors what we know from previous research. For example, studies on bed rest and step-reduction have shown that capillary rarefaction (loss) is a slower process compared to muscle size or metabolic shifts (Hendrickse et al., 2022). But when the molecular markers change, it’s a warning that sets the stage for faster atrophy if inactivity continues.
Resistance Training Restores Muscle and Blood Flow Support
After eight weeks of resistance training however, the negative trends reversed.
- VEGF and VEGFR2 levels increased: these are your green lights for capillary growth.
- TSP-1 dropped back down: removing the brakes.
- And most notably, the number of capillaries feeding type II muscle fibres went up.
These capillaries are vital. They support oxygen delivery, nutrient transport and waste removal in the muscle. These are all things that influence endurance, recovery and ultimately training quality. If you’ve ever worked with a powerlifter who burns out too early in a volume block, or an athlete struggling with repeat sprint fatigue, these microvascular changes could be part of the explanation.
What is even more interesting is that muscle fibre size also bounced back. Type II fibre cross-sectional area increased by 25% from the post-disuse low point, demonstrating a strong hypertrophic response over just eight weeks. But here’s the twist. The study found no strong correlation between capillary growth and muscle size changes in these young adults.
That doesn’t mean capillaries don’t matter. It just means that in young, healthy people, capillary number might not be the limiting factor for hypertrophy, at least not in the short term. In older adults or those with metabolic issues, it’s a different story. Research from Snijders et al. (2017) and Moro et al. (2019) shows that low capillarisation can actually blunt the hypertrophic response to training in those populations.
Why This Matters
So, what can you do with all this information? Quite a bit, actually.
First, if your client is returning from time off due to surgery, illness, burnout or any other reason, you can reassure them with confidence that muscle and its vascular support systems are highly adaptable. Just two months of targeted RT can reverse disuse effects at both the fibre and molecular level.
Second, you don’t have to overthink a client’s training history. Trained and untrained participants in this study responded similarly to the disuse and re-training protocol. So if you’re working with an ex-athlete or a total beginner, a smart, progressive training plan will still do the trick.
Third, think beyond just size and strength. If you’re coaching athletes in team sports, combat sports, or anything that involves high work rates, capillarisation plays a key role in performance. More capillaries mean better oxygen delivery, quicker recovery between efforts and less fatigue. All of this is supported by resistance training.
And finally, it’s a reminder that even short periods of inactivity can kick off changes at the cellular level. You might not see muscle loss immediately, but the body starts shifting gears internally. That’s why a structured return to training and not a YOLO leg day, is the smarter approach.
The Bottom Line
Muscle recovery after disuse is about restoring the infrastructure that keeps muscles functioning at their best. The capillaries, the enzymes, the growth factors. They are all part of the comeback story.
This study by McIntosh et al. (2025) gives strength and conditioning coaches and personal trainers more than just reassurance. It offers a clear roadmap and shows that two weeks of disuse isn’t the end of the world, and eight weeks of smart resistance training is enough to turn things around both inside and out.
It doesn’t matter if you’re guiding a weekend warrior through post-surgery recovery or helping a pro athlete return to form after a layoff, understanding these underlying processes adds depth to your programming and clarity to your coaching.
Reference
- McIntosh, M.C., Michel, J.M., Godwin, J.S., Plotkin, D.L., Anglin, D.A., Mattingly, M.L., Agyin-Birikorang, A., Kontos, N.J., Baweja, H.S., Stock, M.S., Mobley, C.B., & Roberts, M.D. (2025). Disuse and subsequent recovery resistance training affect skeletal muscle angiogenesis-related markers regardless of prior resistance training experience. Journal of Applied Physiology. Click here to review the full research article.
- Hendrickse PW, Wüst RCI, Ganse B, Giakoumaki I, Rittweger J, Bosutti A, Degens H. Capillary rarefaction during bed rest is proportionally less than fibre atrophy and loss of oxidative capacity. J Cachexia Sarcopenia Muscle. 2022 Dec;13(6):2712-2723. Click here to review the full research article.
- Snijders T, Nederveen JP, Joanisse S, Leenders M, Verdijk LB, van Loon LJ, Parise G. Muscle fibre capillarization is a critical factor in muscle fibre hypertrophy during resistance exercise training in older men. J Cachexia Sarcopenia Muscle. 2017 Apr;8(2):267-276. Click here to review the full research article.
- Moro T, Brightwell CR, Phalen DE, McKenna CF, Lane SJ, Porter C, Volpi E, Rasmussen BB, Fry CS. Low skeletal muscle capillarization limits muscle adaptation to resistance exercise training in older adults. Exp Gerontol. 2019 Nov;127:110723. Click here to review the full research article.
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