The muscles are one of the parts of our body that we make the most use of. Synonymous with strength and health, it's by doing sport that we develop them. In this article, we focus in more detail on the skeletal striated muscles, that is, those fixed to the bones by the tendons.
The skeletal striated muscles are the most visible and we have more than 600 of them. They give our bodies their shape and their total mass represents more than half of an adult's body weight. These muscles are responsible for our movements as they contract when we tell them to. They allow us to control our posture or to support and move our joints. Our muscles' shapes are also adapted to their functions: the arm and leg muscles are long, the back muscles are flat and those around our lips and eyes are ring-shaped. More detail…
A muscle is made up of many cells called muscle fibres. They are grouped together in bundles called muscle fascicles, which are fed by blood vessels bringing the energy needed by the fibres. The muscle fibres can span from just several millimetres to more than 10 centimetres in length. They contract with the help of very thin filaments, the myofilaments, organised into myofibrils. These filaments slide over each other during exercise as they tighten. This causes the muscle to contract and relax and so on.
The myofilaments are made of two different types of molecule:
- actin (very thin filament)
- myosin (thicker filament)
It is these two sorts of filament that cause muscle contractions.
The muscles are in fact just a tool that allows contractions to occur; it's actually the brain which gives the order and chooses to contract the muscle. When you decide to do an action, you tell your brain to send a signal to your muscle. The motor cortex is the area of the brain that receives this order. It receives the information from several regions of the brain that give the sense of movement, speed, etc. The cortex analyses this information and translates it into a nerve impulse which is sent to a first neuron. The electrical signal generated by the nerve impulse moves through the projection of this neuron.
To reach the muscle, the nerve impulse must pass through several stages. So, in the upper part of the spinal cord, the impulse is passed to a second neuron called the motor neuron, which joins up with the muscle, causing the nerve impulse to stimulate it. This motor neuron is divided into several nerve endings and each of these endings is in contact with the muscle fibre. The combination of the motor neuron and the fibres is called the motor unit.
The meeting point between the motor neuron and the fibres is called the neuromuscular junction. It is here that the electrical signal triggers the release of chemical molecules, the famous neurotransmitters. A series of electrical signals are then produced, allowing the released molecules to move to the filaments so that they contract. This in turn happens in numerous muscle fibres throughout the muscle.
And there you have it, the muscle works!
When overused or poorly cared for, a muscle can hurt. There are different reasons, some more and some less common, that make muscles painful. The three most common are:
- aches: these are caused by micro-tears in the muscle fibres. The body then triggers a small inflammatory response to repair them. They occur when the muscle is not used to the exercise it is doing. They are generally painful for 12 to 48 hours after exercise and last from 5 to 7 days depending on their intensity.
- muscle spasms: these are painful contractions of the muscle that last a while and are linked to excessive use. There are different sorts of spasm: primitive muscle spasms - when the muscle is used intensively for a lengthy period of time without recovery -, muscle spasms with injuries – the muscle is physically damaged in the event of a tear, a pull or a strain – and defensive muscle spasms – a defence mechanism for damaged joints. Muscle spasms particularly affect the calves, thighs, buttocks, neck and back muscles. They result from the contraction of muscle fibres.
- cramps: These are spontaneous and sustained muscle contractions. The muscle contracts involuntarily and does not relax for a short period of time, from several seconds to a quarter of an hour. They often occur during exercise or at night (known as nocturnal cramps).
To avoid this type of pain, you must take care of your muscles before, during and after exercise. To do so, hydration is essential, as well as warming up and stretching at the end of each training session. You should also reduce the amount of exercise and listen to your body when it is in pain. Finally, it's useful to know that heat can relieve muscle pain. Nothing better than a nice warm bath after training!