What Science Reveals About Early Muscle Aging
It all starts with the central nervous system
There’s a debate on social media: what’s the ideal weight training for women’s midlife muscle?
We’ve heard that lifting heavy weights builds midlife muscle for a while now.
Yet another expert calls out this advice, saying there’s no difference between different loads; do what you want.
This is totally confusing for women our age, and that’s why I’m diving into midlife muscle.
As I mentioned in my first post, it’s not just about gaining muscle, but the quality of muscle. And this decrease in quality happens long before we substantially lose muscle mass.
I don’t do the whole “always do this, never do that” thing, use scare tactics, or publicly call anyone out. But I will help you understand your muscle so you can make your own decisions.
In this post, we’re exploring the nervous system’s role in muscle health. My next post will get into what you can do.
The neuromuscular system
The neuromuscular system is one of the largest and most important organ systems in the body.
In this system, the central nervous system instructs muscles to contract via motor units. A single motor unit includes a nerve cell or alpha motor neuron, its long fiber (called an axon), and the muscle fibers it innervates (supply of nerves to muscle).
A very important junction called the neuromuscular junction (NMJ) is where nerve meets [synapse] individual muscle fibers. One nerve can control several muscle fibers at once, and when it sends a signal, all those fibers in the motor unit contract together.
I like the example of a light switch. The switch is like the motor neuron, and the light bulbs its connected to are like the muscle fibers. When you flip the switch (send a signal from the neuron), all the bulbs light up at the same time (all the muscle fibers contract together).
During exercise, the nervous system recruits more muscle fibers, instructing them to contract and create force, power, and eventually grow.
This is often overlooked when discussing aging muscle, but it matters–and here’s why.
Aging Pause effects
A major culprit behind muscle mass loss with Aging Pause is something called denervation. This is removal or disruption of the nerve supply to muscle, which can cause muscle fibers to atrophy.
Luckily, the body has a way to make up for denervation by rescuing muscle fibers and re-innervated them. This is called motor unit remodeling.
These changes don’t first show up as muscle loss per se, but a decrease in muscle’s ability to produce power and generate strength.
Strength is the ability to exert force to overcome resistance, while power is the ability to exert that force quickly, in the shortest possible time.
Changes to strength and power are detectable by age 30 in elite athletes and the general population. At first the changes are slight, less than 1% per year, but this eventually speeds up to 1-2% per year.
Once someone notices these changes, they often have been occurring behind the scenes for years if not decades.
I use the analogy of iron deficiency anemia. Anemia is the final stage of iron deficiency, just like substantial muscle loss is a late stage of functional decline.
Mechanisms behind power and strength decline
Motor unit remodeling increases the size of motor units but decreases the total number of them. This process accelerates at 60. By 71, the average person has 40% fewer motor units.
“Indeed there is increasing consensus that functional muscle denervation is a principal factor leading to sarcopenia, and some even describe sarcopenia primarily as a “disorder of the NMJ”
- lyer SR, Int J of Mol Sci, 2021
It’s not just motor units that decrease, but specific fibers. Motor unit remodeling converts type fibers II (fast) to type I fibers (slow).
Because type II muscle fibers have four times the power output than type I, this causes a drop in power.
Experts believe power is most important for functional movement in older adults. Everyday life is full of quick movements: getting up from a chair, climbing stairs, lifting a little one.
The good news is the neuromuscular system is not fixed, and it shows high plasticity. And this is where exercise comes in.
Exercise stimulates nervous system
Exercise places stress on the body, which causes it to adapt.
Depending on the type of exercise, the neuromuscular system reacts by recruiting motor units, increasing the rate of motor neuron firing, and reducing the number of denervated muscle fibers while promoting new nerve connections.
In short, exercise stimulates the nervous system’s effect on muscle.
Physical activity is helpful, but just how much does it stimulate our nervous system?
A 2024 study with older and younger men set out to answer this question. The older men were an average age of 69 and the young men were 22. Researchers matched them for recreational exercise (2-3 times per week), health status, and body mass.
Compared to the young muscle, the older men had 53% more type I muscle fibers in addition to 32% smaller type II fibers. There was a 2.6-fold higher level of denervation in the older group.
The researchers conclude that “Aged muscle displayed selective deterioration of type II myofibres alongside increased denervation and myofibre grouping.”
You can see from the picture the young have more type II fibers (black) and less type I (green) compared to the older individuals.
But overall, the burden of denervation was low, and the researchers suspect that being active helped and that in sedentary individuals, the burden would be much higher.
I couldn’t find a similar study on women, but it’s clear that the decline in muscle mass with age consists mostly of type II fibers.
There’s no doubt that a physically active lifestyle helped these men, but the deterioration of type II fibers persisted.
The wild west of exercise studies
I started digging into the research on how different exercise affects type II muscle fibers—and wow, is it confusing.
Most studies use young men or much older adults as participants. There’s a glaring gap when it comes to midlife women. To make matters more complicated, researchers don’t even agree on what qualifies as light, moderate, or heavy resistance training, making it difficult to compare studies or draw conclusions.
It’s no wonder midlife fitness experts have to piece together recommendations from what’s available. And it’s easy for critics to dismiss their guidance with the classic “there’s no research.”
Instead of blaming, let’s just agree we need better research, especially for the midlife stage.
A 2024 review on the mechanisms of muscle aging put it best:
To further our knowledge of the mechanisms underlying sarcopenia, we need longitudinal observations that identify the turning point at which the number of MU and the speed and force of MU contraction begin to decline, as well as the events in the nervous system, myofibres, and muscle scaffold (ECM) that precede this moment. Therefore, we should not focus on those who already have manifest or incipient sarcopenia, but on adults who are about to pass the peak of their physical performance.
The three key takeaways
1. The nervous system plays a key role in muscle health. Muscle strength and power decline with age not because of muscle loss, but because of changes in the neuromuscular system—particularly motor unit denervation and remodeling. These changes often begin years or decades before weakness and muscle loss becomes noticeable.
2. Type II (fast-twitch) muscle fibers are most affected by aging. Motor unit remodeling that occurs with aging leads to a loss and conversion of fast type II muscle fibers to slower type I fibers, which reduces physical power and strength. Since type II fibers are critical for everyday movements like standing up quickly or climbing stairs, their decline has real-life functional consequences.
3. Exercise stimulates the nervous system and can slow muscle decline. Regular physical activity can stimulate the nervous system, promote nerve-muscle connections, and help preserve motor units. Yet research specific to midlife women is lacking, and much of the current guidelines are based on data from men or older populations.
Coming up next: I connect the dots between exercise type and maximizing the neuromuscular system—what the research really says (and what it means for you).
What do you think?
Great insights. We'll spread the word!
This article explains why I so much prefer to take longer, slower runs than shorter, quicker runs! Thank you for explaining!