The Muscular System

What Muscles Are Made Of

Skeletal muscle — the type that training develops — is composed of bundles of muscle fibres, each one a single elongated cell that can be several centimetres long. Each fibre contains hundreds of myofibrils, and within those myofibrils sit the functional units of contraction: sarcomeres. A sarcomere contains two proteins — actin (thin filaments) and myosin (thick filaments) — that slide past each other when a nerve signal arrives, causing the muscle to shorten. This is the sliding filament theory of muscle contraction, and it is the mechanism behind every rep of every exercise you have ever performed.

At each end of a skeletal muscle is a tendon — dense connective tissue that attaches the muscle to bone. Muscles can only pull, never push. When you press a barbell overhead, the triceps contracts and pulls the forearm into extension. When you lower it, the triceps controls that lowering eccentrically. Understanding that muscles only pull — and that both the shortening and lengthening phases are active — changes how you think about every exercise.

Muscle Fibre Types — Why They Matter

Not all muscle fibres are the same. The body contains three primary types — slow twitch (Type I), intermediate (Type IIa) and fast twitch (Type IIx) — each with distinct characteristics that determine when and how they are recruited. Your genetic distribution of these fibre types influences what you are naturally good at, but training can shift the balance. Click each fibre type below to explore its characteristics in detail.

How Muscles Grow — The Full Process

Hypertrophy — the increase in muscle size — does not happen during the training session. It happens in the hours and days after it, provided the conditions for repair and growth are met. The process is more complex and more interesting than most people realise. Click each step below to understand what is actually happening.

DOMS — What It Actually Is

Delayed onset muscle soreness — felt twenty-four to seventy-two hours after training — is not caused by lactic acid. Lactic acid clears within an hour of exercise. DOMS is caused by microscopic damage to the muscle fibres — particularly from eccentric loading — and the subsequent inflammatory response as the body repairs that damage. This is why the lowering phase of a lift causes more soreness than the lifting phase: the muscle is producing greater force while lengthening, which generates more fibre disruption. Moderate DOMS indicates a productive training stimulus. Severe DOMS that limits normal movement is a sign that the volume or intensity was too high too soon.

The Three Energy Systems

Muscles require energy in the form of ATP (adenosine triphosphate) to contract. The body produces ATP through three distinct systems, each dominant at different intensities and durations. Understanding which system is working during a given activity tells you exactly what kind of training produces the adaptation you are looking for. Click each system in the diagram below.

How Training Improves Muscular Performance

The early strength gains from a new training programme are not primarily from muscle growth — they are from neural adaptations. The nervous system becomes more efficient at recruiting motor units, firing them faster and coordinating them more effectively. This is why untrained individuals can see substantial strength increases within two to four weeks before any significant muscle growth has occurred. Actual hypertrophy — the increase in muscle fibre cross-sectional area — begins meaningfully from around four to eight weeks of consistent training and accelerates as load increases progressively over time.

Progressive overload is the non-negotiable principle. The muscle must be presented with a stimulus that exceeds what it has previously adapted to. This does not mean adding weight every session — it can mean more repetitions, shorter rest periods, a greater range of motion, or better technique under the same load. But if the stimulus never increases, the adaptation stops. The body adapts to exactly the level of demand placed on it and no further.

Recovery — Where Growth Actually Happens

Muscles do not grow during training. They are damaged during training and they grow during recovery — provided adequate protein is available and sleep is sufficient. Protein synthesis remains elevated for twenty-four to forty-eight hours after a session, making the nutritional window immediately following training genuinely important. Seven to nine hours of sleep is when the majority of growth hormone is released. Attempting to build muscle while chronically sleep-deprived is physiologically counterproductive — the hormonal environment for repair simply does not exist. Training is the signal. Recovery is the response. Both are equally important.

A Coaching Observation

The most common training mistake I have observed across thirty years is not insufficient intensity — it is insufficient recovery. People train hard on Monday, hard on Tuesday, feel dreadful on Wednesday and cannot understand why they are not progressing. The muscle needs the stimulus and then it needs to be left alone to adapt. Some of the best training sessions I have had have come after a full day of genuine rest. The body is not lazy — it is efficient. It uses every resource available for the purpose it was most recently demanded for. Give it the demand, then give it the time. Trust the process.