Understanding Joints and Their Role in Human Movement

The joints are structures that connect the skeletal bones and allow
mobility of most of their parts.

Types of Joints

• Motionless: They impede the movement of the bones that group. For example, the joints connecting the skull bones.
• Semi-mobile: Only allow small movements between the bones. This is the case of the joints of the vertebrae.
• Mobile: They allow greater mobility between the bones, such as the elbow, hip, and knee.

The Knee Joint

The knee has the following elements:

• Ligaments: Very strong fibers that connect bones.
• Cartilage: Pieces of cartilage that cover and protect the surfaces of the bones in contact.
• Synovial Capsule: A bag located between the bones, filled with a lubricant called synovial fluid.

The knee joint also has fibrocartilaginous structures between the bones, known as the meniscus, which aims to adapt the interface to deliver load better.

Lesions in the Locomotor System

The most common injuries in the musculoskeletal system can affect muscles, bones, or joints. For example:

• Muscle contractures: Persistent, involuntary contraction of a muscle.
• Bone fractures: Total or partial rupture of a bone.
• Dislocation: Bones coming out of the joints.
• Sprains: Ligament injury.
• Torn ligaments and cartilage: Damage to connective tissues.

Each muscle is attached to two different bones through some very tough cords of connective tissue called tendons.

Muscle Contraction and Movement

The muscle contracts, i.e., shortens, when it receives a motor nerve impulse. Thanks to the joints, muscles move the bones, causing them to approach, separate, or rotate relative to each other. The combination of simple movements produces more complex ones, such as walking or athletic exercise. To perform some moves, the muscles work in coordination for couples that work antagonistically or in contrary ways. For example, the contraction of the biceps brachii causes flexion of the arm, while the contraction of the triceps leads to extension.

Levers of the Human Body

The bones and joints form a system of levers on which muscles act. Thus, muscle forces are applied in the most appropriate way to produce large movements or overcome high resistance. In any lever, there are three elements:

• Power Point: Where the force is applied.
• Point of Support: Which supports the lever.
• Point of Resistance: Where the strength to be overcome lies.

Types of Leverage

• First Class: Allows versatility, mobility, and power.
• Second Class: No major shifts, but manages to overcome considerable resistance.
• Third Class: Results in extensive travel but cannot overcome strong resistance.