There are over 600 muscles in the human body; each is made of small fibers that come together to form a person's shape and definition. Typically, the muscular system itself represents about 40 percent of an adult's weight
. That figure can be 5-10 percent less for children or higher in Olympic and professional athletes. Muscles are attached to the human skeleton by thick chord-like tendons. Muscles contract to perform movements. All movements involve a contraction of one or several muscles. When muscle tissue contracts it shortens, allowing a variety of motion
. Running, jumping, brushing your teeth, and even smiling involve the contraction of muscle tissue. Thus, muscles contractions throughout the body are responsible for walking, running, and lifting objects. Muscles in the face are responsible for facial expressions and eye movements. Increasing muscular fitness helps with joint stability, posture, and bone density. Strong, flexible muscles allow people to perform more complex movements and physical tasks without increasing their risk of injury. Students might also find it interesting that muscular contraction uses energy to create heat.
Types of Muscle
There are three types of muscles in the human body: skeletal, smooth, and cardiac. Each type of muscle is formed by protein synthesis. The fibers that make up muscles are developed through forced contraction. Protein helps restore muscle mass and strength. Skeletal muscle attaches to the bone and exist throughout the entire body. Smooth muscle is found in the walls of the digestive tract and blood vessels. It is not under voluntary control. Cardiac muscles can only be found in the walls of the heart.
Muscle Terminology & Size
While most muscular names and terminology originate from Latin or Greek roots, many muscle names are based in mathematical roots that describe their structure, function, or size
. The names
refer to a large or long muscle respectively. Conversely, a muscle with either
in its name describes a muscle that is small or short. The deltoid, or outside shoulder muscle, is shaped like a triangle. The trapezius, upper shoulder muscle, is cleverly shaped like a trapezoid.
Location & Origin
With teacher assistance, students should be able to dissect long names in the muscular system to determine their location. Names such as intercostal should be broken into their prefix and root. For example,
means across and
. Therefore, the intercostals muscles run between the bones of the rib cage. The word
means head. A prefix added on to
can also inform a person of the number of origins or heads that muscle has. For instance, the biceps and triceps of the upper arm have two and three origins respectively
. Meanwhile, the quadriceps femoris has four origins. The word femoris tells us that the quadricep muscles are related to the femur of the skeletal system. Thus, the ability to memorize skeletal structures and their given names will greatly help students to explore and breakdown complex terminology in the muscular system.
Direction of Fibers
Muscle fibers stretch in different directions to help us navigate our world. Different muscles are responsible for bending, twisting, lifting, pushing, or pulling. Each muscle in the body has one primary function and one or two secondary functions. The study of muscle direction is relative to the midline of the body
. Students may have a good grasp of what it means to be parallel or they may need to relate this concept to a horizontal plane. Having students move around and perform small exercises to test directional movements is an appropriate teaching method.
muscle fibers in the body are perpendicular to the midline. The transverses abdominis in the abdomen is an example of a muscle that contracts at a right angle to the midline of the body. The obliques, side muscles in the stomach, do not move in a straight line perpendicular to the midline. Instead, the obliques contract diagonally in comparison to the body's midline. The rectus abdominus, known to many students as the "six pack" muscle, runs parallel to the midline.
Skeletal muscle is the longest type of muscle in the human body. Students will be familiar with these as the primary muscles in their body. Skeletal muscle is responsible for voluntary motion. We consciously decide to move our arm to lift an object. We knowingly move in a specific direction. Students need to learn that skeletal muscle fibers are strong, but have very little endurance. Skeletal muscle can be trained to become more powerful and gain greater endurance. Running, cross training, and weight lifting are the most common ways of creating stronger skeletal muscle. A cross-section of skeletal muscle will show students that muscle fibers do not simply rest side by side. Skeletal muscle is an intricate bundle of fascicles and connective tissue called perimysium
. A sheath, the epimysium, wraps and protects the bundle of fibers and connective tissue to complete the muscle composition. Tears or strains in skeletal muscle can be painful and may require treatment with ice and rest to heal.
Skeletal muscle fibers that contract quickly tend to fatigue quickly. Higher contraction speed is associated with less endurance. More endurance is associated with slower twitch. While the average adult has 50-60% fast twitch muscle fibers, the body of professional sprinters may contain higher than 80% fast contracting fibers
. Although people are born with specific distributions of fast and slow twitch muscles, exercise can increase their resistance to fatigue.