1. Increased number of muscle fibers: When you do strength training, your muscles will be slightly damaged. This slight injury will stimulate your body to produce more muscle fibers to repair these injuries. Over time, the number of your muscle fibers will increase, which will make your muscles stronger.

2. Muscle tissue growth: Strength training will lead to muscle tissue growth, because your body needs more energy and nutrition to support muscle growth. With the increase of muscle tissue, your muscles will be stronger.

3. Improvement of muscle contractility: Strength training will make your muscles produce stronger contractility, because your muscle fibers will become stronger and stronger during training. This enhanced contractility enables your muscles to bear more loads, thus making your muscles stronger.

4. Increased metabolic rate: Strength training can improve your basal metabolic rate (BMR), which means that your body consumes more energy at rest. This will help you burn more calories after exercise, which will help you build stronger muscles.

5. Changes in hormone levels: Strength training will stimulate the secretion of hormones (such as growth hormone and testosterone) in the body, which are essential for muscle growth and repair. With the increase of hormone level, your muscles will grow faster, thus making your muscles stronger and stronger.

In short, through strength training, your muscle fiber quantity, tissue, contractility and metabolic rate will be improved, and your hormone level will also rise. These factors will work together to make your muscles stronger and stronger.