For a signal from the brain to result in skeletal muscle contraction, a sequence of steps needs to take place. Initially, the brain sends an impulse down a motor nerve, which runs along the spinal cord and out towards the targeted muscle. The signal reaches the synapse at the “neuromuscular junction,” where the nerve ending meets the muscle fiber, and neurotransmitters are released by the nerve to bind on the muscle fiber’s receptors. The receiving muscle then transmits the signal along its cell membrane and down into the contractile filaments, through a process called Excitation-Contraction Coupling. It is here where the muscle converts ATP into mechanical energy, shortens the contractile filaments, and generates force.

Muscle Fatigue is the physiological inability to contract. In this lesson, a few reasons contributing to this phenomenon are described. The “imbalance of ions” could occur from the loss of potassium during the excitation-contraction coupling process, but sodium and other ions may also become imbalanced during repeated cycles. The imbalance of other ions such as calcium can also occur. This would include exercise’s potential for damage to the sarcoplasmic reticulum, which can interfere with the calcium ion’s regulation and release. While ATP provides the power for contraction, calcium is the trigger for contraction. Without a consistent and stable method of calcium release and reuptake, the muscle could end up with a physiological inability to contract. In some rare cases disruption to the neuromuscular junction may also prevent the muscle from receiving or transmitting the signal down to the contractile filaments, and this would also result in muscle fatigue.

After exercise, even without fatigue setting in, the muscle still needs to return to its resting state. This involves a few processes, such as replenishing its oxygen stores, and removing accumulated wastes. Glycogen stores are used up during contractions, and so these will need to be replaced, as well as the stores of energy, such as ATP. The increased demand for oxygen to assist with these processes is noticeable through the need for heavy breathing after exercise.

This TED-Ed video focusses on voluntary skeletal muscle contraction. However, there are a range of differences between the function, appearance and force between skeletal and smooth muscle, which are summarized in this video: https://youtu.be/UNyKlwO23w4. The mechanisms of activation are also different in smooth muscle, and do not always require stimulation from a nearby nerve. If you’d like to dig deeper into how the body’s muscular systems work, examining the differences between smooth and skeletal muscle would be a great place to start.