Joints, movements and muscles

Welcome to the Mechanics of Movement!

Ever wondered how a gymnast performs a perfect backflip or how a footballer strikes a ball with such precision? It all comes down to the incredible teamwork between your joints and muscles. In these notes, we are going to break down how the body moves, which muscles are doing the hard work, and the "planes" or directions in which these movements happen. Don’t worry if this seems like a lot of names to remember at first—we’ll use plenty of examples to help it stick!

1. Major Joints: The Hinges and Sockets of the Body

To understand movement, we need to look at specific joints. For each joint, you need to know the movements it can do and the muscles that make it happen.

The Shoulder Joint

The shoulder is a ball-and-socket joint, meaning it is the most mobile joint in your body. Think of it like a joystick on a game controller!

Movements:
• Flexion: Lifting your arm out in front of you (e.g., the start of a bowling action in cricket).
• Extension: Moving your arm back behind your body.
• Abduction: Moving your arm away from the midline of your body (lifting it sideways).
• Adduction: Bringing your arm back toward your body (adding it back to your side).
• Horizontal Flexion/Extension: Moving your arm across your chest and back out while it's raised to shoulder height.
• Medial/Lateral Rotation: Twisting the arm inward or outward.
• Circumduction: Moving the arm in a big circle.

Key Muscles: Deltoid, Latissimus dorsi, Pectoralis major, Trapezius, and Teres minor.

The Elbow and Wrist

These are much simpler than the shoulder. The elbow works just like a door hinge.

Elbow Movements:
• Flexion: Bending the arm (e.g., a bicep curl).
• Extension: Straightening the arm (e.g., a chest press).
• Muscles: Biceps brachii (flexion) and Triceps brachii (extension).

Wrist Movements:
• Flexion: Bending the wrist toward the palm.
• Extension: Moving the back of the hand toward the forearm.
• Muscles: Wrist flexors and Wrist extensors.

The Hip Joint

Similar to the shoulder, this is a ball-and-socket joint but much more stable because it has to carry your weight.

Movements: Flexion, Extension, Abduction, Adduction, Medial rotation, and Lateral rotation.
Key Muscles:
• Iliopsoas: Used for flexion (lifting the knee).
• Gluteus maximus, medius, and minimus: Your "butt" muscles used for extension and stability.
• Adductor longus, brevis, and magnus: Used to pull the legs together.

The Knee Joint

Movements: Flexion (bending) and Extension (straightening).
Key Muscles:
• Hamstring Group: Biceps femoris, Semi-membranosus, and Semi-tendinosus (Flexion).
• Quadriceps Group: Rectus femoris, Vastus lateralis, Vastus intermedius, and Vastus medialis (Extension).

The Ankle Joint

Movement here has special names. Memory Tip: "Plantar" sounds like "Planting" your foot in the ground!

Movements:
• Dorsi flexion: Pulling toes up toward the shin (e.g., preparing to kick a ball).
• Plantar flexion: Pointing toes down (e.g., a ballet dancer on point or a swimmer's kick).
Key Muscles: Tibialis anterior (Dorsi flexion), Soleus, and Gastrocnemius (Plantar flexion).

Quick Review: Every movement has an opposite. If you learn them in pairs (like Abduction/Adduction or Flexion/Extension), it’s much easier!

2. Planes of Movement

Imagine the body is being sliced by large sheets of glass. These "planes" help us describe the direction of movement.

• Sagittal Plane: Divides the body into left and right sides. Movements here are forward and backward (e.g., a somersault or a bicep curl).
• Frontal Plane: Divides the body into front and back. Movements here are side-to-side (e.g., a star jump/jumping jack).
• Transverse Plane: Divides the body into top and bottom. Movements here are rotational (e.g., a discus thrower spinning or a golfer's swing).

Key Takeaway: Most movements in sport happen in the Sagittal plane (running, cycling, kicking).

3. How Muscles Work: Roles and Contractions

Muscles never work alone; they work in teams called antagonistic pairs. While one muscle pulls, the other relaxes to allow the movement.

Functional Roles of Muscles

• Agonist: The "prime mover." This muscle is doing the contracting (shortening) to create movement.
• Antagonist: The "opponent." This muscle relaxes and lengthens to allow the agonist to work.
• Fixator: The "stabilizer." This muscle stays still to keep the joint steady while others move (e.g., muscles around the shoulder during a bicep curl).

Example: In a bicep curl, the Biceps brachii is the Agonist, and the Triceps brachii is the Antagonist.

Types of Muscle Contraction

There are two main ways a muscle can contract:

1. Isotonic Contraction: The muscle changes length and moves the joint.
• Concentric: The muscle shortens under tension (e.g., the "up" phase of a pull-up).
• Eccentric: The muscle lengthens under tension (e.g., lowering yourself down slowly from a pull-up). Careful! This is still a contraction because the muscle is working to control the movement.

2. Isometric Contraction: The muscle stays the same length. There is tension, but no movement (e.g., holding a "plank" position or a rugby scrum).

Common Mistake to Avoid: Many students think "eccentric" means the muscle is relaxing. It isn't! It is working hard to resist gravity, like a brake on a car going downhill.

Summary: Putting it All Together

When you analyze a sporting movement, always follow this step-by-step process:
1. Identify the Joint (e.g., Elbow).
2. Identify the Movement (e.g., Extension).
3. Identify the Agonist muscle (e.g., Triceps).
4. Identify the Antagonist muscle (e.g., Biceps).
5. Identify the Type of Contraction (e.g., Concentric).

Key Takeaway: Understanding the agonist and antagonist relationship is the "golden key" to answering movement analysis questions in your exams!