Resistance Training: Physical Adaptations & Injury Prevention

The obvious physical benefit of lifting weights is bigger muscles. In this post we will dive deep into the anatomy and physiology of muscles and how the body adapts to training. This week’s post is a little more technical. Next week will get back to the more practical evidence and advice!

Hypertrophy: What is happening in the muscle?

Hypertrophy means making a muscle bigger by increasing the volume/size of the existing muscle fibers. This is the main way your muscles grow.

Hyperplasia is the enlargement of a muscle group by increasing the number of muscle fibers. There is a lack of strong evidence supporting hyperplasia in human skeletal muscle and the main way our muscles get bigger is via hypertrophy.

Essentially, training makes your muscles grow by making your current muscle fibers thicker and not by growing new muscle fibers.

The total number of muscle fibers a person has is largely determined by genetics and established early in life. So training makes the muscle you already have bigger.

We’re going to dive deep into the anatomy and physiology of skeletal muscle to better understand how training makes us stronger.

Muscles fibers are made up of two main filaments: actin and myosin.

When you contract a muscle, these filaments pull on each other to shorten the muscle and move your bones.

(Image by brgfx on Freepik)

Training your muscles frequently against resistance increases the strength and thickness of these filaments, making your muscles bigger and stronger.

The Nervous System

The nervous system is another major site of adaptation, particularly at the neuromuscular junction — the point where the nervous system communicates with muscle. This is shown by the blue arrows in the figure.

(Image by brgfx on Freepik)

Resistance training enhances the speed and control of signals sent from the brain to your muscles, resulting in more powerful and controlled movements.

When you first start training, early strength gains (weeks 1-4) are primarily driven by nervous system adaptations. Muscular hypertrophy begins to contribute more substantially around weeks 8-12 of training.

Connective Tissue Adaptation

Connective tissues such as tendons and ligaments also adapt to resistance training. This plays a significant role in injury prevention.

Muscles are contractile tissues. This means two things:

• They have direct nervous system innervation

  • This lets the brain tell muscles to contract

• They have direct blood flow

  • This provides nutrients to fuel activity and recovery.

Tendons and ligaments are non-contractile tissues, meaning they don’t have either of those two things. Tendons connect muscles to bone and transmit force, while ligaments connect bone to bone and contribute to joint stability.

The lack of blood supply and nervous system connection means that these tissues get less of a stimulus to grow and less fuel to recover. This explains why injuries often happen at these locations and why these tissues take longer to grow and recover.

Stronger tendons and ligaments are longer term benefits of resistance training. Initial changes are seen around 8 weeks with more significant gains after months of training. This highlights the importance of long-term consistency over short term frequency.

Some more complicated nervous system adaptations also contribute to injury resilience. Proprioception is your sense of where your body parts are in relation to one another. Kinesthesia is your sense of speed and direction of movements. These two senses together make up your overall body awareness and coordination.

These senses are made up of complex feedback loops between sensors in your muscles that rapidly communicate with your brain and spinal cord. Resistance training enhances the sensitivity of these receptors and the speed at which they can communicate, resulting in overall increased balance and coordination. This is an often overlooked benefit of resistance training that is important for people of all ages and activity levels.

Next week we will continue more practical tips for how to train for injury prevention and staying healthy!

References:

1.      Simoneau J, Bouchard C. Genetic determinism of fiber type proportion in human skeletal muscle. FASEB J. 1995;9(11):1091-1095. doi:10.1096/fasebj.9.11.7649409

2.      Haff G, Triplett NT, National Strength & Conditioning Association, eds. Essentials of Strength Training and Conditioning. Fourth edition. Human Kinetics; 2016.

3.      Deschenes MR, Judelson DA, Kraemer WJ, et al. Effects of resistance training on neuromuscular junction morphology. Muscle Nerve. 2000;23(10):1576-1581. doi:10.1002/1097-4598(200010)23:10%3C1576::AID-MUS15%3E3.0.CO;2-J