Human biology
Start here. Every training effect in Parts 2 and 3 is biology doing its job.
Levels of organization
The body is built in layers: atoms form molecules, molecules form organelles, organelles form cells, cells form tissues, tissues form organs, organs form organ systems, and systems form the organism. Damage or change at one level shows up at the next, so a tiny change in muscle cells can change how you lift.
The cell and its parts
| Part | Job | Why athletes care |
|---|---|---|
| Nucleus | Holds DNA, the instructions for making proteins | Muscle cells have many nuclei; more nuclei support more growth |
| Mitochondria | Make most of the cell's ATP using oxygen | Endurance training increases their number and size |
| Ribosomes | Build proteins from amino acids | This is where muscle protein synthesis happens |
| Endoplasmic reticulum, Golgi | Fold, package and ship proteins and lipids | The sarcoplasmic reticulum, a muscle version, stores calcium for contraction |
| Lysosomes | Break down worn-out parts | Part of muscle protein turnover |
| Cell membrane | Controls what enters and leaves | Nerve and muscle signals depend on ions crossing it |
The four tissue types
Epithelial
Covers surfaces and lines cavities: skin, gut lining, airways.
Connective
Supports and connects: bone, cartilage, tendon, ligament, fat, blood.
Muscle
Contracts to create force: skeletal (voluntary), cardiac (heart), smooth (organ walls).
Nervous
Carries electrical signals: neurons plus support cells.
Explore the 11 organ systems
Homeostasis: keeping conditions steady
Homeostasis means holding internal conditions (temperature, blood sugar, pH, fluid balance) within a narrow range. Most control uses negative feedback: a sensor detects a change, a control centre compares it with a set point, and an effector pushes the value back. When you exercise and heat up, sweating and wider skin blood vessels cool you. When blood sugar rises after a meal, the pancreas releases insulin and cells take up glucose. Positive feedback amplifies a change instead, as in childbirth contractions and blood clotting.
How a muscle contracts (sliding filament theory)
- A motor neuron releases acetylcholine at the neuromuscular junction.
- The muscle fibre's membrane fires and the sarcoplasmic reticulum releases calcium.
- Calcium binds troponin, which moves tropomyosin and uncovers binding sites on actin.
- Myosin heads attach to actin and perform a power stroke, pulling thin filaments toward the centre of the sarcomere.
- ATP detaches the myosin head so the cycle can repeat. Without ATP, muscles lock stiff (as in rigor mortis).
- When nerve signals stop, calcium is pumped back and the muscle relaxes.
The sarcomere, the repeating unit of actin and myosin, is the smallest contractile unit. Stacked end to end, sarcomeres make myofibrils; bundles of myofibrils make a muscle fibre (one muscle cell); bundles of fibres make a muscle.
Energy and metabolism
The cell's energy currency is ATP. Food supplies the raw material: carbohydrates become glucose (stored as glycogen in muscle and liver), fats become fatty acids, and protein becomes amino acids that mainly build and repair tissue. Metabolism is all the chemical reactions in the body: catabolism breaks molecules down to release energy, anabolism builds them up and uses energy.
Sport science
Sport science applies physiology, biomechanics, psychology and nutrition to performance and health.
The three energy systems
| System | Fuel | Approx. duration | Example |
|---|---|---|---|
| ATP-PC (phosphagen) | Stored ATP and creatine phosphate | Up to about 10 seconds | Max lift, 100 m start, jump |
| Glycolytic | Glucose or glycogen without oxygen | About 10 seconds to 2 minutes | 400 m run, a hard set of 12 reps |
| Oxidative (aerobic) | Carbohydrate and fat with oxygen | Beyond about 2 minutes | 5 km run, cycling, rest between sets |
All three run at once. Intensity and duration decide which one leads. Times are approximate and overlap.
Muscle fibre types
Type I (slow-twitch)
Low force, very fatigue-resistant, rich in mitochondria. Dominant in postural muscles and marathon runners.
Type IIa (fast oxidative)
Moderate-to-high force, moderately fatigue-resistant. Trains toward either direction.
Type IIx (fast glycolytic)
Highest force and speed, tires fast. Recruited in near-maximal and explosive efforts.
Muscles recruit fibres in order of size: small, slow units first, large, fast units only when force demands rise. Heavy loads, or light loads taken close to failure, recruit them.
Heart, lungs and aerobic fitness
Cardiac output = heart rate x stroke volume. Training raises stroke volume, so a fit heart pumps more blood per beat and resting heart rate often drops. VO2max is the maximum rate your body can use oxygen, a standard measure of aerobic capacity. A common rough estimate of maximum heart rate is 220 minus age, but individual values vary widely, so treat it as a starting point only.
The principles of training
Overload
The body adapts only when stress exceeds what it is used to.
Progression
Increase load, reps, sets or difficulty gradually over time.
Specificity
You get better at what you practise: sprinting trains sprinting.
Recovery
Adaptation happens during rest, sleep and food, not during the session.
Reversibility
Fitness fades when training stops.
Individuality
People respond differently to the same plan.
Biomechanics in brief
Bones act as levers, joints as pivots, and muscles as the force source. A longer moment arm between a joint and the load means more torque is needed. That is why a dumbbell lateral raise feels hardest when the arm is level with the floor. Movement is described by planes (sagittal, frontal, transverse) and by actions such as flexion, extension, abduction and adduction.
What the guidelines say
The World Health Organization's 2020 guidelines recommend that all adults do 150-300 minutes of moderate-intensity aerobic activity per week, or 75-150 minutes of vigorous-intensity activity, or an equivalent mix. They also recommend regular muscle-strengthening activity for all age groups and advise reducing sedentary time.
Recovery, sleep and injury prevention
- Sleep is when most recovery occurs; adults generally do best with 7-9 hours.
- Warm up with easy movement that gradually raises heart rate, then practise the movement with lighter loads.
- Increase weekly training load gradually rather than in sudden jumps.
- Pain that is sharp, joint-based or lasts for days is a reason to stop and seek a clinician, not push through.
Bodybuilding
Bodybuilding is the deliberate shaping of muscle size and symmetry through training, food and recovery. The core process is muscle hypertrophy.
How muscle grows
Muscle gets bigger when muscle protein synthesis, over days and weeks, exceeds muscle protein breakdown. Three mechanisms are commonly discussed: mechanical tension (muscle fibres pulling against a load, the main driver), metabolic stress (the "pump" and burn), and muscle damage (useful in small amounts, but not required for growth). Training signals cells to make more contractile proteins, and satellite cells add nuclei when growth is large.
The training variables
| Variable | Practical starting point |
|---|---|
| Volume | Many lifters do well with roughly 10-20 hard sets per muscle per week; beginners often grow on fewer. |
| Frequency | Train each muscle about 2 times per week; total weekly volume matters more than how it is split. |
| Load and reps | Roughly 5-30 reps can build muscle when sets finish close to failure; 6-12 is the common, practical range. |
| Effort | End most sets with 0-3 reps left in reserve (RIR). |
| Rest between sets | About 2-3 minutes for big compound lifts; 1-2 minutes for isolation work. |
| Progression | Add reps first, then weight, while keeping control and full range of motion. |
Common training splits
| Split | Days / week | Best for |
|---|---|---|
| Full body | 3 | Beginners; busy schedules; high frequency per muscle |
| Upper / lower | 4 | Intermediates; easy to reach 2x per muscle |
| Push / pull / legs | 3 or 6 | Lifters who like more volume per session and can train often |
Exercise selection
Build the base with compound lifts: squat or leg press, hip hinge (Romanian deadlift), horizontal and vertical press, horizontal and vertical pull. Add isolation exercises for muscles that need extra volume, such as lateral raises, curls, triceps extensions, calf raises and leg curls. Choose movements you can load progressively and perform through a full range of motion.
Nutrition for growth
- Energy: A small surplus (often a few hundred kcal above maintenance) supports gain while limiting fat. Larger surpluses mostly add fat.
- Protein: 1.4-2.0 g per kg of body weight per day is enough for most people trying to build or keep muscle, according to the International Society of Sports Nutrition. A meal dose of about 0.25 g/kg, or 20-40 g, is a sensible target.
- Carbohydrate: Fuels hard training and refills glycogen; set it by training volume and appetite.
- Fat: Needed for hormones and cell membranes; avoid going very low.
- Creatine monohydrate: Among the best-studied supplements; typical dose is 3-5 g per day. Check with a doctor if you have kidney disease or take regular medication.
- Water, sleep, and whole foods matter more than any other supplement.
Cutting (losing fat while keeping muscle)
Use a moderate calorie deficit, keep lifting heavy, and raise protein. The ISSN notes that resistance-trained people in calorie deficits may need about 2.3-3.1 g per kg per day to keep lean mass. Lose weight gradually; fast crash dieting costs muscle and performance.
Protein calculator
Common mistakes
- Changing programs every few weeks instead of progressing on one.
- Counting junk volume: sets stopped far from failure or with sloppy range of motion.
- Too little protein, sleep or calories to support growth.
- Skipping legs, back or the muscles you cannot see in the mirror.
- Ego lifting: more weight with worse technique raises injury risk and does not add muscle.
Sources and further reading
- Bull FC et al. World Health Organization 2020 guidelines on physical activity and sedentary behaviour. British Journal of Sports Medicine, 2020; 54(24):1451-1462. doi:10.1136/bjsports-2020-102955
- Jager R et al. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr, 2017. PubMed 28642676
- Textbooks to go deeper: Guyton and Hall Textbook of Medical Physiology; Tortora and Derrickson Principles of Anatomy and Physiology; ACSM Guidelines for Exercise Testing and Prescription; NSCA Essentials of Strength Training and Conditioning.