Why Exactly Do We Lose Muscle Strength With Age?

Maintaining adequate muscle mass and strength is one of the most effective ways to increase longevity. In particular, muscle strength correlates with reduced mortality from all causes in humans. Unfortunately, muscle mass and strength tend to decline with age. We’ve discussed this decline in the past, as well as how to prevent or at least limit it, but why exactly does muscle strength deteriorate?

First, let’s take a look at some of the data on muscle loss with age. This table from Peter Attia’s article on the subject shows the general percentage of muscle loss per decade after our 30s.
As people age, they lose muscle mass and muscle strength at different rates. Muscle strength typically declines faster than muscle mass.

You would be correct in suspecting that muscle mass isn’t the only factor determining muscle strength. To understand what other factors are at play, we first need a brief overview of how muscle contraction works.

• The process of muscle contraction starts in the brain. Motor neurons carry signals to the muscle telling it to contract.
• Each signal arrives at a connection point called a neuromuscular junction.
• Here, neurotransmitters are released from the end of the neuron, which opens channels in the membrane of the muscle cell.
• This allows ions to flood into the cell, which causes the cell to release its stores of calcium ions.
• These calcium ions bind to proteins that are wrapped around the filaments responsible for contraction.
• This allows these filaments to start breaking down the cellular fuel ATP in order to contract the muscle fibre.

How hard the muscle contracts is controlled by how rapidly the motor neurons fire, and by how many neuromuscular junctions are active at a time. From this, we can see that the quantity and quality of the muscle isn’t the only thing that matters for muscle strength: the health of the nervous system is also important.

How muscle quality declines with age:

• As people age, contraction becomes less efficient because there are fewer ion channels in their muscles.
• Age is associated with changes to the contractile proteins that reduce their ability to generate force.
• Over time, we develop a lot of oxidative stress-related DNA damage. This results in a decline in the number and function of mitochondria, the ‘cellular organs’ responsible for providing the muscle with ATP.
• As we age, the proportion of type II muscle fibres (responsible for rapid, high-force contractions) gradually diminishes, eventually reducing muscle strength.

How nervous system quality declines with age:

• The nervous system adapts to demand: a muscle’s ability to contract is increased as it gains more connections with the nervous system (innervation) in response to increased usage.
• The reverse is also true: the less a muscle gets used, the fewer connections it will have (denervation).
• Younger people typically have a balance between the strengthening and weakening of these connections in response to exercise.
• For older adults, denervation seems to outpace reinnervation.
• The reduced density of connections means less force, less speed, and compromised performance.

Unfortunately, there’s currently not a lot we can do about reduced muscle quality in ageing. However, we do have more control over nervous innervation, the less well recognised player in muscle strength. This is because contracting muscles release signals which promote the growth of nerve fibres and improve transmission at the neuromuscular junction. This increases muscle strength and helps to offset the negative changes that occur during ageing. Endurance and power training appear to be particularly effective for achieving this effect.