Muscle Contraction: Energy Usage Mechanisms

How is a muscle movement accomplished in our body involving the CNS and the musculo-skeletal system?


The movements of our muscles are coordinated and controlled by the brain and nervous system. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum.

muscle contraction stretching

When we want to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.

The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and in what position it is. This feedback results in smooth, coordinated motion.

For example if we want to bend the elbow, our brain sends a message to the muscles in our arm (biceps and triceps) and we move it.

Muscles move body parts by contracting and then relaxing. Our muscles can pull bones, but they can't push them back to their original position. So they work in pairs of flexors (muscles causing flexion) and extensors (muscles causing extension). A flexor muscle contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When we bend our elbow, the biceps contracts and the triceps relaxes. Then the biceps relaxes and the triceps contracts to straighten the elbow.

How does a muscle get its fuel/energy for producing action?


three energy systems muscle contraction

Muscles work by contracting, and in order to contract they require fuel. They get that fuel from ATP (adenosine triphosphate). When ATP breaks down, ADP (adenosine diphosphate) is formed. ADP can be readily recycled back to ATP, but it must use energy from other fuel sources. ATP constantly needs to be replenished. The body has other energy systems in place to continually refill the ATP stores.

1. The Creatine Phosphate Energy System:

When there is a requirement of a short-burst, all-out effort like a 100-meter sprint creatine phosphate is used provide the required ATP from ADP. There is very small amount of creatine phosphate in the body to fuel for 5 to 8 seconds. So the creatine phosphate energy system is our first backup.

2. The Glycogen-Lactic acid (Anaerobic Glycolysis) System:

If there is need of ATP for a longer duration, up to 3 minutes, then glycogen in the muscle is utilized in the absence of oxygen. Due to the absence of oxygen, an undesired byproduct called lactate is produced in this energy system. Build up of lactate in the blood causes fatigue and one has to stop or slow down the pace of the exercise.