Did you know that about half of your body’s weight is in muscle tissue? There are also about 600 skeletal muscles in the body that move bones or other structures! Muscles are classified into three main groups; skeletal, cardiac, and smooth. We are going to dig into skeletal muscle tissue as it is most relevant for physiotherapy and is especially relevant for muscle injuries.
Skeletal muscle are the muscles under our voluntary control and are used for movement. They are comprised of long cells called muscle fibres that have a striped appearance under a microscope.
Skeletal muscle is also broadly classified into two fibre types:
Type 1 Slow Twitch
Slow-twitch or slow-oxidative muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red colour. It can carry more oxygen and sustain aerobic activity.
Type II – Fast Twitch
Fast-twitch muscle has three major kinds that are, in order of increasing contractile speed.
aerobic, rich in mitochondria and capillaries and appears red when deoxygenated.
Type IIx (aka IId)
the fastest muscle type in humans and is less dense in mitochondria and myoglobin. It can contract more quickly and with a greater amount of force than oxidative muscle but can sustain only short, anaerobic bursts of activity
which is anaerobic, glycolytic, “white” muscle that is even less dense in mitochondria and myoglobin.
Anatomy of Skeletal Muscle
Each skeletal muscle is comprised of various integrated tissues. There are three levels of connective tissue that provide structure to the muscle and compartmentalize the muscle fibres within the muscle.
The Epimysium is a dense and irregular connective tissue that wraps around the muscle enabling the muscle to contract and produce force without losing structural integrity. It also separates the muscle tissue from other organs and tissues in the area.
Inside each skeletal muscle, the muscle fibres are organized into bundles called fascicles, which are surrounded by a middle layer of connective tissue called the perimysium. This fascicular organization is common in muscles of the limbs; it allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibres within a fascicle of the muscle. Inside each fascicle, each muscle fibre is encased in a thin connective tissue layer of collagen and reticular fibres called the endomysium. The endomysium surrounds the extracellular matrix of the cells and plays a role in transferring the force produced by the muscle fibres to the tendons. The collagen based connective tissue from all 3 layers merges together to form the tendon, which attaches to the periosteum of the bone.
The area where the muscle tissue joins to the tendon is called the musculotendinous junction. In some muscles like the adductors, this is very small, while in others like the rectus femoris (quad) or the biceps femoris (hamstring) the tendon runs a reasonable length down into the muscle itself – thus making quite a large musculotendinous junction. This is a common location for injury, as we will explore in future blogs.