When we think of resistance training, we picture bigger muscles, better strength and improved function. Collagen, on the other hand, probably brings to mind skincare ads or supplements in fancy tubs. But inside your muscles, collagen plays a far more interesting role and it’s one that matters just as much to performance as it does to aesthetics.
New research has shown that resistance training sparks collagen remodelling in your muscles, regardless of age. That has big implications for trainers working with clients across the lifespan.
If you’re studying a personal trainer course, you’ll come across plenty about muscle protein synthesis, mechanical tension and hypertrophy. But the scaffolding that holds all of that together, known as the extracellular matrix, is equally important. It’s not just a background player. It’s an active part of how muscles adapt.
A recent study by Schweitzer and colleagues (2025) looked at what happens to collagen after a single session of resistance training, focusing on the breakdown side of the equation. And the results were far from expected.
What Is Collagen Remodelling and Why Should You Care?
Skeletal muscle isn’t just made of contractile fibres. Those fibres sit within a support network called the extracellular matrix (ECM). Think of it like scaffolding. It maintains structure, helps transmit force and even plays a role in muscle regeneration. Collagen is the key structural protein in this matrix.
Inside muscle, the main types of collagen are type I and type III, found around the fibres, and type IV, found in the basal lamina. These aren’t static, they’re turned over through a process known as remodelling. That means the body is constantly balancing collagen synthesis (building it up) and breakdown (clearing out the old stuff).
Breakdown is especially important. If collagen accumulates faster than it’s cleared, it can stiffen the muscle, disrupt force transmission and reduce the quality of movement. Over time, that can lead to fibrosis, a build-up of fibrous tissue that’s hard to reverse. But when collagen turnover is balanced, it supports healthy adaptation and keeps the muscle environment flexible, literally and biologically.
This process is regulated by enzymes called matrix metalloproteinases (MMPs), which break collagen down, and tissue inhibitors of metalloproteinases (TIMPs), which control that breakdown to prevent it from going overboard.
What the Study Looked At
The researchers recruited 25 healthy adults, 12 younger (19–30 years) and 13 older (65–85 years). All participants were physically active and within the 25th to 75th percentile for age, and sex-adjusted aerobic fitness. In other words, these weren’t elite athletes, but they weren’t sedentary either.
Each person completed one bout of resistance training which included four sets of leg press followed by four sets of knee extension, all at 70% of one-rep max and taken to failure. Muscle biopsies were taken before the session, six hours later and again 72 hours after.
The team measured total collagen content in the muscle, as well as the gene expression, protein levels and activity of key regulators involved in collagen breakdown, specifically MMP2, MMP9 and MMP14, along with TIMP1 and TIMP2.
It’s not something that’s covered in any PT courses. These measurements go beyond surface-level hypertrophy theory into the kind of cellular adaptation where things are super interesting. They show how resistance training works at a tissue level, not just a muscular one.
What They Found
At rest, collagen content in the muscle was nearly identical between young and older participants. That alone was surprising. It challenges the assumption that ageing automatically leads to more collagen build-up or fibrosis in muscle tissue.
After resistance exercise, there were no major changes in collagen breakdown markers at six hours. At 72 hours however, it was a different story. Both young and older adults showed significant increases in:
- MMP gene expression (MMP2, MMP9, MMP14)
- TIMP gene expression (TIMP1, TIMP2)
- Protein levels of MMP14 and TIMP1
- Enzyme activity of MMP2 and MMP9
In short, the machinery for collagen breakdown was fired up in response to resistance training. And it kicked in strongly after 72 hours, not immediately. So if you’re working with clients and talking about recovery and adaptation windows, this supports the idea that muscle tissue continues responding to a session well into day three.
One slight age-related difference did show up. Older adults had more MMP14 enzyme 72 hours post-exercise than the younger group. But this didn’t result in any abnormal collagen degradation or imbalance. It was simply a measurable difference and possibly linked to subtle changes in the composition of older muscle tissue, like differences in laminin or immune cell behaviour. More research is needed on that.
But overall, the takeaway was clear. Both young and older muscle responded similarly to training at the cellular level, at least in terms of collagen remodelling.
So Is Ageing the Problem? Or Is It Inactivity?
This study adds to a growing body of evidence suggesting that ageing alone isn’t the villain when it comes to reduced muscle quality. It’s more likely the combination of ageing with inactivity, illness or chronic inflammation that drives fibrosis and stiffness.
Previous studies that showed age-related collagen accumulation didn’t always account for activity levels or fitness. In this study, both groups were matched for fitness, and that may have made the difference.
It’s also worth noting that comorbidities like type 2 diabetes, obesity and osteoarthritis have all been linked to increased muscle fibrosis. These conditions can change the signalling environment inside muscle, tipping the balance towards collagen build-up. But in the absence of these conditions and with regular activity, collagen turnover seems to function well into older age.
Why This Matters for Training
For us, especially for those of us working with older clients, this is encouraging. It supports the idea that resistance training helps maintain not just strength and size, but also muscle structure. And that benefit extends beyond muscle fibres to the surrounding matrix.
Healthy ECM remodelling means better flexibility, less stiffness and potentially better satellite cell activation, which is critical for repair and adaptation.
If you’re building programmes for clients in their 60s, 70s or 80s, this kind of research gives you confidence that their muscles are still responsive. You’re not just slowing decline, you’re supporting real biological change.
It also supports the case for regular resistance training as we age. Skipping sessions for weeks at a time doesn’t just stall progress, it may reduce the beneficial effects on collagen turnover and long-term muscle quality.
Final Thoughts
Resistance training is about more than just hypertrophy. It’s a biological signal that keeps the entire muscle system active, responsive and dynamic, including the collagen-based structures that hold it all together.
This study by Schweitzer et al. (2025) showed that one bout of resistance exercise significantly increased collagen breakdown markers in both young and older adults. The takeaway is simple: fit older muscle still adapts and collagen isn’t the obstacle it’s often made out to be.
And if you’re exploring personal trainer courses or already helping clients apply evidence-based training principles, this kind of insight strengthens the case for lifting at every age.
Reference
- Schweitzer, A.M., Koehle, M.S., Fliss, M.D., & Mitchell, C.J. (2025). Collagen Remodeling Increases After Acute Resistance Exercise in Healthy Skeletal Muscle Irrespective of Age. American Journal of Physiology – Cell Physiology. Click here to review the full research article.
- Rullman, E., Rundqvist, H., et al. (2007). A single bout of exercise activates matrix metalloproteinase in human skeletal muscle. J Appl Physiol, 102(6), 2346–2351. Click here to review the full research article.
- Wessner, B., et al. (2019). Age-specific response of skeletal muscle extracellular matrix to acute resistance exercise: A pilot study. Eur J Sport Sci, 19(3), 354–364. Click here to review the full research article.
- Aussieker, T., et al. (2024). Ingestion of a whey plus collagen protein blend increases myofibrillar and muscle connective protein synthesis rates. Med Sci Sports Exerc, 57(3), 544. Click here to review the full research article.
Trigger Collagen, Build Lasting Strength
A single bout of resistance training can spark a powerful chain reaction in your muscles. Schweitzer et al. (2025) found that within 72 hours, key markers of collagen breakdown surged in both young and older adults, showing how adaptable muscle structure remains at any age. With the TRAINFITNESS Gym Instructor & Personal Trainer Practitioner, Specialist & Master Diplomas™, you’ll gain the skills to guide clients through training that not only builds muscle but also supports the deep structural changes that keep it strong and healthy. Designed for anyone wanting to become a personal trainer, with the option to specialise in areas like nutrition, women’s health or strength & conditioning, these courses give you the knowledge to create programmes that drive results inside and out.
Gym Instructor & Personal Trainer Practitioner Diploma™ – Distance Study, In-Person & Live-Virtual
Gym Instructor & Personal Trainer Specialist Diploma™ – Distance Study, In-Person & Live-Virtual
Gym Instructor & Personal Trainer Master Diploma™ – Distance Study, In-Person & Live-Virtual
