Everyone knows that exercise is good for building and toning muscle. But, what exactly happens to your muscles during, and after, your workout? How do cardio and strength training effect muscles differently?

Exercise influences the muscles to adapt to the type of work performed. There are four important physiological effects that exercise has on skeletal muscle when performed consistently over a period if time. The four adaptations of exercise are: reinforced muscle density, enhanced muscular force (strength), improved muscular endurance and increased blood flow.

Here's a basic overview of musculoskeletal anatomy. Muscles consist of strands of protein-based fibers designed to contract (shorten) and extend (lengthen). They are attached to tendons - fibrous connective tissue - that adhere to bones across a joint. Therefore, the job of a muscle is to shorten and lengthen the angle of a joint.

During a workout, whether cardio or resistance training, muscles are required to exert a certain amount of force in order to perform the movement. That amount force will dictate how the muscle 'responds' to the work.

Increase in Muscle Density
If the resistance is relatively heavy (but not too heavy!) the muscle fibers will be forced to contract harder than under normal circumstances. This exertion may result in a certain amount of damage to the fibers requiring repair.

When the workout is completed and the body is at rest - and proper nutrition is consumed (a blog for another day) - the muscles will begin repairing and reinforcing the muscle fibers. Over time this consistent routine of 'damage and repair' will result in an increase in muscle density.

Because oxygen is required for muscles to contract, the level of exertion may be more demanding than the lungs and blood can supply. An acidic environment in the working muscle fibers will ensue. Increase in acidity will shut down the neuromuscular (brain-to-muscle) connection and the muscles will begin to fatigue. This is where the infamous 'burn' comes into play.

Strength Improvements
As stated earlier, muscle fibers will become more dense as a result of consistent work. This increase in muscle density promotes the opportunity for the muscle fibers to incrementally lift heavier weight.

A great fundamental principle to follow is Progress Through Progression. Basically, expose the muscle fibers to a certain amount of resistance and let the body adapt. Continue to gradually push their limits. In time the muscle fibers will become stronger and contract with more force.

Extending Stamina
When oxygen demand is relatively lower than normal for muscle exertion the muscles will able to work longer. Basically, the muscles can just keep going - as in walking or swimming - for a extended period of time.

In order, for this to happen the muscle fiber will need to add more mitochondria to increase energy metabolism. Mitochondria are tiny energy processing centers within a muscle fiber. This is where carbohydrates and fat are converted to energy to keep the muscles contracting.

As duration of the work increases, thus will the number of mitochondria to ensure efficient energy processing.

Increased Capillary Density
As workload increases from longer bouts of cardio or tougher resistance training, the demand for oxygen and nutrients will certainly increase. In order to facilitate the passage of these important elements the body will increase the amount of capillaries within the muscles.

Capillaries are microscopic blood vessels that transport blood from arteries into tissue. The body is capable of producing capillaries to adapt to the increased workload of the muscles.

The human body is an incredible organism that is capable of adapting to its environment. Therefore, your muscles are capable of adapting any type of workout you expose them to. Just remember to follow the principle of Progress Through Progression.