Muscles & Movement

Every single movement you make — blinking, breathing, walking, laughing — is powered by muscles. Muscles are remarkable tissues that do one thing: contract (shorten). By pulling on bones, they create all the movement your body can produce. The human body has over 600 muscles, making up about 40% of your body weight. They are not just for sport — your heart is a muscle, your digestive system is lined with muscle, and tiny muscles in your eye control your vision constantly. The three types of muscle — a quick recap: | Type | Where | Control | Appearance | |------|--------|---------|------------| | Skeletal muscle | Attached to bones | Voluntary (you control it) | Striped (striated) | | Cardiac muscle | Heart only | Involuntary (automatic) | Striped, but never tires | | Smooth muscle | Gut, blood vessels, bladder | Involuntary | Not striped | This lesson focuses on skeletal muscle — the type you consciously control and the type relevant to movement and exercise.

A muscle is made of bundles of muscle fibres — each fibre is a single very long cell. Inside each fibre are thousands of tiny protein filaments arranged in repeating units called sarcomeres. The sarcomere is where contraction actually happens. The sliding filament theory — in plain English: Inside each sarcomere are two types of protein filaments: - Actin — thin filaments (like thin rope) - Myosin — thick filaments with tiny "heads" that can grab onto actin (like hands) When a nerve signal tells the muscle to contract: 1. Calcium is released inside the muscle fibre 2. Calcium triggers the myosin heads to grab onto actin 3. The myosin heads pull the actin filaments inward — like a person pulling a rope hand-over-hand 4. The actin and myosin do NOT get shorter — they slide past each other. The sarcomere gets shorter. 5. Millions of sarcomeres all getting shorter at the same time = the whole muscle shortens = contraction What ATP has to do with it: Every time a myosin head grabs and pulls (one "power stroke"), it uses one molecule of ATP for energy. This is why exercise burns so much energy — thousands of myosin heads are cycling through millions of power strokes every second. When ATP runs out (in extreme exercise), muscles cramp and stop working. The nerve-muscle connection: A muscle fibre only contracts when it gets a signal from a nerve. The connection point between a motor nerve and a muscle fibre is called the neuromuscular junction (NMJ). The nerve releases a chemical called acetylcholine, which triggers the electrical signal that releases calcium and starts contraction. Curare (used historically as a poison on arrows) and botulinum toxin (Botox) both block the NMJ in different ways — causing muscle paralysis.

Muscles cannot attach directly to bones — they need a tough connector. That connector is a tendon — a cord of dense connective tissue (mostly collagen) that attaches muscle to bone. When a muscle contracts, it pulls on its tendon, which in turn pulls on the bone. This is how movement happens. Think of the tendon as the string in a puppet — the muscle is the puppeteer pulling the string. Famous tendons: - Achilles tendon — connects the calf muscles (gastrocnemius and soleus) to the heel bone (calcaneus). The thickest and strongest tendon in the body. Essential for walking, running, and jumping. Named after the Greek hero Achilles, whose only vulnerable point was his heel. - Patellar tendon — connects the quadriceps muscle to the kneecap (patella) and then to the shin bone. Doctors tap this with a reflex hammer to test knee reflexes. - Rotator cuff tendons — four tendons around the shoulder joint that hold the shoulder ball in its socket. Easily torn — a very common sports and occupational injury. Tendons vs ligaments — a common confusion: - Tendons: connect MUSCLE to BONE (remember T = Tendon = muscle To bone) - Ligaments: connect BONE to BONE (hold joints together) Tendon injuries: Tendons heal slowly because they have a poor blood supply — nutrients reach them mainly by diffusion through synovial fluid. A completely torn Achilles tendon can take 6–12 months to heal with rehabilitation.

A muscle can only pull — it cannot push. So to move a limb in two directions, you need (at least) two muscles working in opposite directions. These are called antagonistic pairs. The classic example — the arm: - Biceps brachii (front of upper arm) — contracts to flex (bend) the elbow - Triceps brachii (back of upper arm) — contracts to extend (straighten) the elbow When the biceps contracts, the triceps must relax (lengthen). When the triceps contracts, the biceps must relax. If both contracted at the same time with equal force, the arm would not move — it would just become very stiff (this is what happens in some types of muscle spasm). Key movement terms: - Flexion — decreasing the angle at a joint (bending). Bending your elbow, crouching down. - Extension — increasing the angle (straightening). Straightening your elbow, standing up. - Abduction — moving a limb away from the midline of the body. Raising your arm to the side. - Adduction — moving toward the midline. Bringing your arm back to your side. - Rotation — turning around the long axis. Turning your head left and right. Muscles and their roles: In any movement, different muscles play different roles: - Prime mover (agonist) — the main muscle doing the work (biceps in elbow flexion) - Antagonist — the opposing muscle that relaxes to allow movement (triceps) - Synergists — helper muscles that support the movement - Stabilisers — muscles that fix nearby joints to give the prime mover a stable base

Muscles are tough — but they can be injured by sudden force, overuse, or insufficient recovery. Common muscle injuries: Muscle strain (a "pulled muscle"): A strain is a tear in muscle fibres — not a complete rupture, but enough to cause pain and swelling. Graded by severity: - Grade 1 — a few fibres torn. Sore, but the muscle still works. Heals in 1–2 weeks. - Grade 2 — a significant portion torn. Pain, swelling, reduced strength. Takes weeks. - Grade 3 — complete rupture. Often requires surgery. Common sites: hamstrings (back of thigh — very common in sprinters), calf, lower back. Delayed Onset Muscle Soreness (DOMS): That deep aching feeling 24–48 hours after unfamiliar or intense exercise. It is NOT caused by lactic acid (a common myth — lactic acid clears within an hour of stopping exercise). DOMS is caused by tiny microscopic tears in the muscle fibres from eccentric contractions (muscle lengthening under load, like walking downhill). The inflammation and repair process causes the soreness. This is actually HOW muscles get stronger — the repair makes them slightly bigger and more resistant to future damage. Cramp: Sudden, involuntary, painful muscle contraction. Causes include dehydration, low electrolytes (especially sodium, potassium, magnesium), fatigue, and poor blood flow. Relief: gently stretching the muscle and rehydrating. Preventing muscle injuries: - Warm up — increasing blood flow and temperature to muscles makes them more pliable and less likely to tear - Progressive training — gradually increasing load, not jumping to extremes - Recovery — muscles rebuild during rest, not during exercise. Sleep is essential for muscle repair (most growth hormone is released during deep sleep). - Nutrition — protein provides the amino acids needed to repair and build muscle fibres