Muscles in Relation to the Skeletal Structure
There are different variety of muscles in the human body with different functions. Skeletal muscles or voluntary muscles are responsible for movement of the bones of the human skeleton and maintaining posture/proper alignment of the skeletal structure. These muscles are joined to the bones with tendons (fibrous connective tissue) that are found at the end of muscles. Skeletal muscles (along with other muscles in the body) are also responsible of generating body heat. Besides moving the skeleton, these muscles are also supporting the proper movement of lymphatic fluids, as our lymphatic system does not have it’s on engine for circulation (unlike the cardiovascular system) so by engaging our muscles we are helping the toxic fluids to drain from our bodies.
Muscles are also supporting our joints. Although in our bodies there are other fibrous connective tissues called ligaments that stabilise the joints, by maintaining a healthy amount of muscle mass, we are supporting the correct alignment of the skeleton/joints. As muscles contract, they pull the bones not only to generate movement, but they also stabilise their relationships to one another in the joints.
Significance of joint structure
The shape a joint affects the type of movement permitted by the joint.
There are 3 groups we divide joints into based on the movement they allow.
Synarthrosis, allows little to know movement (e.g. cranial elements or teeth)
Amphriarthrosis, permits slight mobility (e.g. articulations between the bodies of the vertebrae or the inferior articulations of the two hip bones)
Synovial joints, also known as freely movable joints. There are 6 different kinds of synovial joints as the picture above shows.
The common similarity in each of these joints that they are connections made between bones. They are constructed for different type of movement as the picture above describes. There is the joint cavity between the bones, filled up with synovial fluid to smoothen the movement between the two bones that are covered with cartilage. This fluid is contained by the synovial membrane and articular capsule. There are the previously mentioned ligaments that support these joints running along the capsule and being attached to both bones.
Leverage system in the human body
The leverage system in the human body builds up of 4 parts. In the human body, the rigid bar (lever) is a bone. The point of support (fulcrum) is a joint and the muscles provide the effort and force to move loads. Based on the arrangement of bones and muscles, different classes of levers are identified in the body. Different levers are classed firs, second and third depending on the relationship between the fulcrum, effort and resistance.
First class levers have the fulcrum in the middle of the bone. A great example of this is the nodding expression of the head, where the neck muscles provide the effort, the neck (cervical spine) is the fulcrum and the weight of the head is the load.
Second class levers have the load in the middle of the fulcrum and the effort. A great example of this is in the ankle area. When standing on tiptoe the ball of the foot acts as the fulcrum, the weight of the body is the load and the calves (gastrocnemius muscle) contracting creates the effort.
Third class levers have the effort in-between the fulcrum and the load. An example of this is found in the arm when doing a biceps curl. The elbow is the fulcrum, the biceps brachii contracts creating the effort, and the load is any resistance created by the triceps brachii plus the weight that might be held in the hand.
Isometric and isotonic movement
Muscles move by contraction. There are contractions called isotonic and isometric contractions.
Isotonic: isotonic contraction maintains constant tension in the muscle while it’s changing length. The length can get longer (eccentric contraction) or shorter (concentric contraction). Isotonic contractions are commonly used to move limbs, such as when walking, running or even lifting an object.
Isometric: isometric contractions generate force without changing the length of the muscle. For example when we grab an object there is muscle contraction in the hand and forearm, and although there is no joint movement, there is tension created in the muscles so the object would not be dropped.
Agonist and antagonist muscles
For almost every major movement there is an agonist and antagonist muscle involved. The agonist muscle is the primary mover, usually being shortened by contraction. The antagonist muscle has multiple purposes during movement. Firstly it relaxes to allow the agonist muscle to function to its fullest. Secondly it slows down the movement to prevent any over-use or tearing of the agonist muscle. Flexors are the muscles that decrease the angle between the bones that converge at the joint. Extensors are the opposite of flexors and help increase the angle between the bones. Rotators are muscles used to rotate a part of the body.