For this article, we interviewed personal trainer and physical therapist Clyde Staley, PT, DPT, CSCS. Clyde is a Doctorate of Physical Therapy and a Certified Strength and Conditioning Specialist, he has over 6 years of experience training athletes and clients to increase hypertrophy and perform their best in their sport.

Weight training is the best way to increase hypertrophy (increase in the size of your muscles). Most gym-goers know this, but many don’t understand how hypertrophy actually occurs.

What exactly triggers hypertrophy?

Hypertrophy occurs as a result of the breakdown and regeneration of muscle fibers during strength training. High-volume weight training causes muscle breakdown, leading to inflammatory, muscular, and hormonal responses that cause our muscle fibers to increase in size.

High-intensity weight training causes damage to our muscle fibers. It is the body’s job to repair the damaged cells and prepare them for the load to be placed on them in the future.

With sufficient loading and recovery time, our muscle fibers grow in size.

Key Takeaways

• High-volume weight training causes damage and trauma to muscle fibers.
• Inflammatory and hormonal responses lead to muscle fiber repair and increased muscle fiber size.
• Weight training leads to an increase in type II-b muscle fiber size.
• Choosing the right training parameters is key to triggering hypertrophy.

Hypertrophy: The Overload Principle

The load from weight training puts our skeletal muscles under stress. Hypertrophy occurs as a result of the overload principle, which states that for hypertrophy to occur, the body must be pushed past its limits. 

Hypertrophy occurs as a result of forcing our muscles to overcome a load greater than what they’re accustomed to.

High-intensity weight training is the best way to provide our muscles with the overload necessary for hypertrophy to occur. Other forms of exercise are beneficial, but weight training is the most researched and shows consistent results for triggering hypertrophy.

During The Training Session

To trigger the physiological changes necessary for hypertrophy to occur, the load placed on the muscles must be sufficient to cause muscle breakdown. High-intensity weight training is the best way to ensure this happens.

High-intensity weight training forces the muscles to overcome the resistance that is beyond their physiological capability. This damages the muscles as a result.

In this case, muscle damage is a good thing. Damage and trauma to existing skeletal muscles trigger a cascade of events in the body that lead to the repair and subsequent growth of our muscles.

Inflammatory Response

Hypertrophy cannot occur without damage or trauma to existing skeletal muscle. The trauma that occurs during weight training causes an inflammatory response in our body necessary for muscle growth and repair.

Normally, inflammation is perceived as a bad thing. In this case, inflammation is necessary.

Specialized inflammatory cells called macrophages are activated and brought to the area of damaged muscle. Their job is to absorb damaged cells and signal other inflammatory cells to come to the area of damaged muscle.

The other inflammatory cells- mostly different types of white blood cells- arrive and continue the process of removing damaged cells. This is necessary to allow other cells to arrive for the repair and growth of our muscle fibers.

This inflammatory process is the trigger for muscle hypertrophy; without it, muscle fibers would remain damaged after weight training.

Muscular Response

When our muscles are damaged during weight training, special cells called satellite cells are activated. Satellite cells are responsible for muscle protein synthesis- the repair and growth of damaged muscle fibers.

Satellite cells proliferate to the damaged muscle fibers and fuse with them, causing the muscle fibers to regenerate and grow in size.

With high-intensity weight training, hypertrophy also occurs as a result of the increase in the size of type II-b muscle fibers. 

Type II-b muscle fibers are larger and produce greater amounts of force than type I or type II-a fibers. They are recruited during high-intensity weight training.

When type II-b muscle fibers break down, they repair to a much greater size than do type I or type II-a fibers. The result is muscle hypertrophy.

Hormones That Cause Hypertrophy

The release of specific hormones is a key trigger for hypertrophy. Hormones such as insulin-like growth factors (IGFs), growth hormone, and testosterone play key roles in facilitating hypertrophy after weight training.

High-intensity weight training causes the release of IGF hormones, which are responsible for the activation and proliferation of satellite cells- the cells responsible for muscle protein synthesis.

Growth hormone is another hormone that is released with high-intensity weight training and contributes to hypertrophy. Growth hormone is responsible for the activation of satellite cells and subsequent muscle protein synthesis. 

The other major hormone that triggers hypertrophy is testosterone. Testosterone levels are typically higher in men but are also present in females.

Testosterone is an anabolic hormone, meaning it is responsible for building muscle. Testosterone is key in facilitating protein synthesis.

Parameters for Triggering Hypertrophy

All of the processes described above are the result of high-intensity weight training. Hypertrophy occurs as a result of choosing the right exercises, resistance level, volume, and rest intervals.

A high-intensity weight training program is the key to activating all of the inflammatory, muscular, and hormonal responses needed for hypertrophy.

The greatest inflammatory and hormonal responses occur by implementing compound exercises, such as squats and deadlifts.

Hypertrophy occurs primarily by performing 3-6 sets of an exercise. The rep range for these exercises should be 6-12 per set.

Intensity should be about 65-85% of the one-rep max (1RM). Rest intervals should be between 60-90 seconds between sets.

The recommended number of total sets per muscle group for hypertrophy varies widely depending on the individual, their training experience, and their goals. 

Typically, 3-6 sets of exercise per muscle group per week are sufficient for hypertrophy. More advanced athletes with greater training experience may benefit from 6-12 sets per week, or 3-5 sets per day twice per week.

Weight training below or above these parameters will likely lead to hypertrophy. However, the parameters listed have shown the most optimal effects for triggering hypertrophy.