Skeletal muscles are often thought of as the muscles we use for hard work and exercise, like when people say, “You can’t have somebody else do your pushups for you.” But did you know that these muscles, of which there are 640 in the human body, do much more than just heavy lifting?
Skeletal muscles come in all sorts of shapes and sizes. For example, the longest muscle is the sartorius in your upper thigh, the largest is the gluteus maximus, and the tiniest is the stapedius in your middle ear. These muscles help us do everything from lifting heavy objects to performing delicate tasks like plucking an eyebrow or cuddling a kitten.
A key thing to remember about muscles is that they always pull; they never push. When a muscle contracts, it pulls the bone it is attached to closer to the bone it is anchored on. For instance, when you do a push-up, your pectoralis major muscle pulls your arm bone (humerus) toward your chest bone (sternum). This pulling action is how all skeletal movements happen, whether you’re lifting weights or sipping tea.
Skeletal muscles can be grouped based on their roles in movement:
These are the main muscles responsible for a specific movement. For example, when doing jumping jacks, the pectorals and latissimus dorsi are the prime movers for bringing your arms back to your sides.
These muscles work against the prime movers to help control and stabilize movements. In jumping jacks, the deltoids act as antagonists to slow down your arms.
These muscles assist the prime movers by adding extra force or stabilizing joints. For arm movements, the rotator cuff muscles often help out as synergists.
These muscles help keep your posture steady and stabilize your body during movement.
Muscle contractions can be of two main types: isotonic and isometric.
These happen when the muscle changes length while lifting something, like picking up a mug.
These occur when the muscle generates tension without changing length, like trying to lift something too heavy to move.
Muscle fibers contract when they get signals from motor neurons, which form motor units. A motor unit includes a motor neuron and the muscle fibers it controls. Big muscles, like the quadriceps, have large motor units for powerful movements, while small muscles, like those controlling eye movements, have small motor units for precise control.
When a motor unit is activated, the muscle fibers go through these phases:
Muscles can produce different levels of force through two methods: temporal summation and recruitment.
This happens when motor neurons send signals quickly one after another, causing stronger contractions because the muscle fibers don’t have time to fully relax between signals.
This involves activating more motor units to increase muscle force. Smaller motor units are used first, and larger ones are added as more force is needed.
Skeletal muscles are crucial for a wide range of movements, from delicate tasks to heavy lifting. Understanding how they work, including the pulling action, the roles of different muscle groups, and the mechanics of contraction, helps us appreciate the complexity of human movement. Whether doing isotonic or isometric contractions, the coordination of motor units allows us to be both strong and precise in our physical activities.
Explore the diversity of skeletal muscles by identifying and labeling different muscles on a diagram. Focus on the sartorius, gluteus maximus, and stapedius. Discuss their functions and how they contribute to various movements. Consider how these muscles work together during activities like jumping jacks or lifting objects.
Conduct a simple experiment to understand isotonic and isometric contractions. Use a small weight to perform bicep curls (isotonic) and then hold the weight steady at a 90-degree angle (isometric). Record your observations on how the muscle feels and discuss the differences between these types of contractions.
In groups, role-play the different functional groups of muscles: prime movers, antagonists, synergists, and stabilizers. Create a short skit demonstrating how these groups work together during a common activity, such as playing a sport or doing a household chore. Reflect on the importance of each group in achieving smooth and controlled movements.
Simulate the concept of motor units using a group activity. Assign each student a role as a motor neuron or muscle fiber. Practice sending signals from the motor neuron to the muscle fibers to mimic muscle contraction phases: latent period, contraction phase, and relaxation phase. Discuss how temporal summation and recruitment affect muscle force.
Create a project that illustrates the pulling action of muscles. Use materials like rubber bands and cardboard to construct a model showing how muscles pull on bones to create movement. Present your model to the class, explaining the mechanics of muscle contraction and the roles of different muscle groups in facilitating movement.
Muscles – Tissues in the body that have the ability to contract and produce movement or maintain the position of parts of the body. – The muscles in your arms help you lift objects by contracting and relaxing.
Contraction – The process in which a muscle becomes shorter and tighter, generating force and movement. – During exercise, the contraction of leg muscles helps propel the body forward.
Movement – The act or process of changing position or place, often facilitated by the contraction of muscles. – The movement of your fingers is controlled by the muscles in your hands and forearms.
Fibers – Long, slender cells that make up muscles and are capable of contracting to produce force and movement. – Muscle fibers are classified into different types based on their speed and endurance capabilities.
Force – The strength or energy as an attribute of physical action or movement, often generated by muscle contraction. – The force generated by the contraction of your calf muscles allows you to jump high.
Skeletal – Relating to the bones of the body, which provide structure and support for muscles and other tissues. – The skeletal system works with muscles to enable movement and protect vital organs.
Agonists – Muscles that are primarily responsible for generating a specific movement by contracting. – When you bend your elbow, the biceps act as the agonists to perform the movement.
Antagonists – Muscles that oppose the action of agonists, providing balance and control during movement. – As the biceps contract to bend the elbow, the triceps serve as antagonists to control the motion.
Isotonic – Referring to a type of muscle contraction in which the muscle changes length while the tension remains constant. – Lifting a dumbbell involves isotonic contractions of the arm muscles as they shorten and lengthen.
Recruitment – The process of activating additional muscle fibers to increase the strength of a contraction. – During heavy lifting, recruitment of more muscle fibers helps generate the necessary force.
