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How Muscles Get Stronger: The Science Behind Lifting Weights

Updated: Jul 20, 2023


Lifting weights is a common exercise to increase muscle strength and size. In this blog, we'll explore the two main ways muscles can become stronger during a lifting muscle action and how understanding this process can help us achieve our fitness goals more effectively.

Increasing the Number of Active Muscle Fibers:

The first way muscles become stronger is through changes in the number of active muscle fibers. This is achieved by a process known as motor unit recruitment. Motor units are groups of muscle fibers controlled by a single nerve cell (motor neuron).

  • Small to Large Recruitment: When performing an exercise like bicep curls, the body initially recruits smaller motor units. As these smaller motor units start to tire, larger motor units are activated to handle the load.

  • Motor Unit Size: Small motor units control only a few muscle fibers, while large motor units can control thousands.

Practical Application:

If the goal is muscle building, it's essential to activate the larger motor units. To do this, aim for a weight that challenges you in the rep range of about 5 to 9 reps to failure or near failure. The first 1 to 3 reps will activate the smaller motor units, and the subsequent reps will recruit the high threshold motor units responsible for muscle hypertrophy.

Increasing Force Production in Muscle Fibers:

The second way muscles become stronger is by increasing the amount of force that each muscle fiber produces. This is determined by the force-velocity relationship, which states that muscle fibers exert higher forces when they shorten slowly.

Practical Application:

During a bicep curl, you may notice that the weight feels relatively easy to lift in the beginning. This is because the smaller motor units are initially activated, and they produce less force. However, as you start to fatigue, you naturally reduce the speed at which you lift the weight, leading to the recruitment of larger motor units that can generate higher forces.


Understanding how muscles become stronger during lifting actions can significantly impact your workout routine. To build muscle effectively, focus on recruiting larger motor units by lifting weights challenging enough to fatigue your muscles in the 5 to 9 rep range. By doing so, you'll activate high threshold motor units, promoting muscle hypertrophy and helping you achieve your fitness goals faster and more efficiently.

Remember to always prioritise proper form.

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