Our bodies are systems of adaption. In other words, they respond to stress. Specifically, our muscles respond to an increased demand or work load placed upon them.
Work is a force divided by a unit of time. A force is a mass multiplied by acceleration. Therefore in order to increase the work load placed upon your muscles and cause them to grow you need to either
- Increase the amount of weight lifted
- Increase the acceleration of the weight lifted (i.e. lift more explosively)
- Decrease the amount of time you perform the same amount of work (i.e. decrease rest periods)
There is a reason you often hear people state that muscle size and muscle strength are synonymous. This is only partly true. Weight is the only infinite variable you can keep adding endlessly. However, you can also manipulate acceleration and time to increase the work load and increase muscle size.