Aging Isn't the Problem, Inactivity Is: How Muscle Protects Your Bones, Blood Sugar and Brain

Built to Move, Episode 1, Part 2 of 3 · Featuring strength and performance coach Kurt Smith

Most people assume that getting weaker, losing bone and needing more help around the house are just part of getting older. In Part 2 of Built to Move, strength coach Kurt Smith (@kurtsmith_fit, @temple_coaching) pushes back on that idea. His point is simple: much of what we blame on aging is really the result of years of not challenging our muscles.

In Part 1, Kurt explained why muscle is the best investment you can make in your long-term health. This time, he goes deeper into what happens when we stop building it, and how muscle affects your bones, blood sugar, brain and hormones.

What Happens to Your Body If You Stop Building Muscle?

Kurt opens with a famous set of MRI scans that cut across the thigh. In a 2011 study of masters athletes, researchers scanned the quadriceps of people aged 40 to 81 who trained four to five times a week. Their thigh muscle didn't shrink with age. The sample images made the point clear: a 70-year-old triathlete's thigh looked almost the same as a 40-year-old triathlete's. A 74-year-old sedentary man's thigh, by contrast, showed much less muscle surrounded by fat.

Kurt compares the change to the marbling in a steak. Without regular training, fat works its way into lean tissue, and the bone in the middle of the scan can look thinner too. The researchers' conclusion matches Kurt's message: much of the decline we call aging may really be the effect of disuse.

Why It Matters: Your Independence

Losing muscle and bone isn't just about how you look. It shows up as lost freedom:

• Relying on a cane or walker to get around
• Needing help getting in and out of the shower or tub
• Remodeling a bathroom for safety
• Being unable to use the second floor of your home without a stair lift

Is It Too Late to Reverse?

No. Kurt points to research showing that bone and muscle still respond to training later in life. The best-known example is the LIFTMOR trial in Australia, which studied postmenopausal women with low bone mass, most in their 60s. Early results showed the women who did brief, high-intensity resistance and impact training gained about 1.6% bone density in the lumbar spine, while the control group lost about 1.7%. The full trial used just two 30-minute sessions a week and improved bone density, structure and function without adverse effects, under close supervision.

Kurt also notes that measurable gains in muscle can show up within roughly 6 to 12 weeks of starting resistance training, even at an older age.

Is Aging the Problem, or Is Inactivity?

Kurt puts it bluntly: "The lack of activity creates aging." We've all seen the 80-year-old who's still muscular and running down the beach. According to Kurt, he isn't an outlier. "He just kept going where everyone else stopped."

Kurt also argues that you control much more of your health than you've been led to believe:

• What you eat fuels your muscles, brain and organs.
• How much effort you put in determines your cardiovascular fitness and how much muscle you build.

He acknowledges that chronic disease, hereditary conditions or limb loss can change the picture. Even then, he points to adaptive bodybuilding athletes who built remarkable upper bodies because they kept using what they had. "You can work around so much," he says. "Sometimes you need a little help to do so."

How Hard Do You Need to Train to Build Muscle?

This is one of the most practical points in the episode. If you pick a weight you can easily lift 12 times, stop at 12 and walk away, your body has no reason to change. You only did what it could already do.

To build muscle, you need to challenge your muscles enough that your body decides it needs to adapt, usually by building more tissue. That means working close to the point where you can't do another good rep.

Expect it to feel uncomfortable at first because it's new. Once it becomes routine, Kurt says the soreness and fatigue start to feel "more of a sense of accomplishment than discomfort."

Is Walking Enough to Stay Strong as You Age?

Walking is great for metabolism, digestion and keeping calories in check, and Kurt encourages everyone to keep doing it. But he explains that the human body was designed to cover long distances while using very little energy. That efficiency is exactly why walking alone doesn't push your muscles and bones to grow.

"Walking, I think, is definitely good," Kurt says, "but it's not going to be the end-all be-all." To improve, you need resistance training of some kind.

How Does Muscle Build Stronger Bones?

Muscle and bone work as a team. Kurt describes two ways that building muscle strengthens bone:

1. Mechanical pull. Your skeletal muscles attach to your bones to move your joints. As your muscles get stronger and pull harder, your bones respond by getting denser and stronger.
2. Chemical signals. When muscles contract, they release proteins and peptides called myokines, which act almost like hormones. These help support bone mineral density and bone remodeling.

Kurt adds that training can even help healing bones regain their strength and proper shape. As always, follow your surgeon's or physical therapist's guidance after a fracture.

Why Is Strength Training Especially Important for Women?

Kurt says resistance training matters even more for women, because women face a higher risk of bone density problems. Two trends stack on top of each other.

1. Muscle declines with age

Research shows muscle mass decreases about 3–8% per decade after age 30, and the decline speeds up after 60. That means someone who has done little strength training could reach their 40s or 50s with noticeably less muscle than they had at 30.

2. Bone loss speeds up after menopause

Falling estrogen around menopause accelerates bone loss. On average, women lose up to 10% of their bone density in the first five years after menopause.

Put the two together, and Kurt says women can "exponentially increase the downfall" of their structural support. Less muscle means less pull on the bone, at the same time hormones are speeding up bone loss.

"Won't Lifting Weights Make Me Bulky?"

Kurt says this fear keeps many women away from strength training, and it's a myth. Women aren't built to put on muscle that easily. It would take a lot of very deliberate effort to get "so big that it looks strange." Lifting will make you stronger and more capable, not bulky.

Ways to Get Started If the Gym Feels Intimidating

• Group strength classes give you structure and coaching.
• A qualified personal trainer can teach you safe form.
• Bodyweight exercises at home are a great place to build confidence.

What Is Resistance Training?

Many people think "resistance training" means only light exercise bands. Kurt defines it much more broadly: anything that makes your muscles contract against resistance. That includes:

• Free weights like dumbbells, kettlebells and barbells, where gravity provides the resistance
• Cable machines, which keep constant tension on your muscles
• Resistance bands, which add tension as they stretch
• Weight machines and lifts like deadlifts and squats
• Bodyweight movements like squats, push-ups and lunges

Can Building Muscle Help Prevent Disease?

Yes, and Kurt says this is one of the most overlooked reasons to lift.

Type 2 Diabetes and Blood Sugar

Many people think of type 2 diabetes as only a weight problem. Kurt sees it as just as much a muscle problem. Skeletal muscle is the body's number one place to clear sugar from the blood. Research backs this up: under insulin-stimulated conditions, about 80% of glucose uptake happens in skeletal muscle, and researchers consider muscle insulin resistance an early, primary defect in type 2 diabetes.

Kurt explains that muscle helps in two ways:

• More storage space. More muscle gives your body more places to send the carbohydrates you eat.
• Better insulin sensitivity. Training helps your body respond to insulin better, so the whole blood sugar system runs more efficiently.

He gives the example of NFL linemen. They often weigh 300 pounds or more, but they carry a lot of muscle, and you don't see them all struggling with diabetes.

Heart Disease, Osteoporosis and Inflammation

Kurt says heart disease and diabetes are the two conditions he has most often watched people "grow out of" through weight training and healthier habits. Building muscle also supports bone health (helping protect against osteoporosis) and may help with conditions linked to chronic inflammation.

How Does Muscle Affect Brain Health?

If you feel sharper after a workout, there's a reason. Kurt explains two connections:

• Feel-good chemicals. Physical effort prompts your brain to release dopamine and serotonin.
• Myokines reach the brain. Some myokines released by working muscles are thought to cross the blood-brain barrier. A 2024 review in Frontiers in Physiology describes how myokines such as BDNF, irisin and cathepsin B may support the growth of new neurons and new connections between them.

Research in this area is still developing, but it supports what many people feel: training is good for memory, focus and protection against cognitive decline.

Does Muscle Affect Your Hormones?

Challenging training, especially heavy leg work, triggers a release of hormones and endorphins. Over time, Kurt says, it helps keep your hormones more balanced.

It also works the other way: if your hormones are off, building muscle gets harder. After 30, both men and women produce testosterone at a lower rate, which can affect energy, brain function, recovery and red blood cell production. Kurt notes that low testosterone isn't only a men's issue. Women can be affected too.

Can Building Muscle Reduce Your Reliance on Medication?

Kurt describes how the benefits of building muscle add up. Training improves your cardiovascular health, blood pressure, blood sugar and metabolism at the same time. When your blood sugar is better controlled, organs like your liver and kidneys are under less strain, and the whole system runs more smoothly.

His concern is that we often treat one symptom at a time with one medication after another, instead of asking what we can change at the source. He suggests asking your physician whether building muscle and exercising should be a bigger part of your plan alongside any treatment.

Important: Never stop or change a medication on your own. Talk to your doctor first.

This Week's Built to Move Challenge

Kurt's advice for anyone starting from zero is refreshingly simple:

"If you're doing nothing, [it] makes a big difference," Kurt says. "You'll even probably be sore from it." Stop if you feel sharp pain, and check with your doctor or physical therapist if you have a joint condition or recent surgery.

The One Thing to Remember: Don't Stop

Age doesn't decide how strong you are at 70 or 80. What you keep doing does. Walking is a great start, but your muscles and bones need a real challenge to keep growing. Start small, push a little harder each week, and keep going when others stop.

Watch the Full Episode and Keep Moving

Watch Built to Move, Episode 1, Part 2 on YouTube, and subscribe to Movement Health so you don't miss Part 3. Missed the start? Read Part 1: Why Muscle Is the Best Investment You Can Make in Your Long-Term Health.

Like, subscribe, and comment down below with your questions and the health, fitness, nutrition or longevity topics you'd like us to cover next. Your question could become a future episode. Follow Kurt on Instagram at @kurtsmith_fit and @temple_coaching.

Download the Free FAQ PDF

Want the key points in one place? Download the free Built to Move Part 2 FAQ guide (PDF), with 12 common questions about muscle, bone health, menopause, blood sugar and getting started.

Sources and Further Reading

1. Wroblewski AP, et al. Chronic exercise preserves lean muscle mass in masters athletes. The Physician and Sportsmedicine. 2011;39(3):172–178.
2. Watson SL, et al. Heavy resistance training is safe and improves bone, function, and stature in postmenopausal women with low to very low bone mass: novel early findings from the LIFTMOR trial. Osteoporosis International. 2015.
3. Wiley / Journal of Bone and Mineral Research. Intense strength training benefits postmenopausal women with low bone mass (LIFTMOR randomized controlled trial summary). ScienceDaily, 2017.
4. Volpi E, Nazemi R, Fujita S. Muscle tissue changes with aging. Current Opinion in Clinical Nutrition and Metabolic Care. 2004;7(4):405–410.
5. Better Health Channel (Victoria State Government). Menopause and osteoporosis.
6. DeFronzo RA, Tripathy D. Skeletal muscle insulin resistance is the primary defect in type 2 diabetes. Diabetes Care. 2009;32(Suppl 2):S157–S163.
7. Kostka M, et al. Muscle–brain crosstalk mediated by exercise-induced myokines: insights from experimental studies. Frontiers in Physiology. 2024;15:1488375.

This article is for educational purposes only and isn't individual medical advice. Talk with your doctor or physical therapist before starting a new exercise program, especially if you have osteoporosis, a chronic condition or a recent injury or surgery, and never change a medication without your doctor's guidance.