CSCS Exercise Science Revision Notes: Bioenergetics, Biomechanics & Physiology

The Three Bioenergetic Systems

Every training question that touches on energy systems traces back to three pathways. The phosphagen system (ATP-PC) supplies energy anaerobically and alactically for roughly the first 10-15 seconds of maximal effort, using stored ATP and phosphocreatine; it's the dominant system for a single heavy lift, a sprint start or a jump, and it regenerates over several minutes of rest, which is why true max-effort work needs long rest periods to repeat cleanly. The glycolytic system takes over as the phosphagen system depletes, breaking down glucose or glycogen anaerobically to resynthesise ATP, dominant from roughly 15 seconds out to 2 minutes of continuous effort; it's fast but produces hydrogen ions and metabolic by-products that contribute to the burning sensation and eventual force decline in sets like a 400m sprint or a 60-second maximal set. The oxidative system uses oxygen to break down carbohydrate and fat for ATP over longer durations, dominant beyond roughly 2 minutes, and it's the slowest but most sustainable pathway. Almost no real activity uses only one system in isolation; the exam tests whether you understand which system is dominant, not which system is exclusively active, at a given duration and intensity.

Muscle Fibre Types and the Size Principle

Type I fibres (slow-twitch, oxidative) are fatigue-resistant, produce force slowly, and rely primarily on the oxidative system; they dominate postural and endurance-type activity. Type IIx fibres (fast-twitch, glycolytic) produce force rapidly and powerfully but fatigue quickly, relying on the phosphagen and glycolytic systems; Type IIa fibres sit between the two, fast-twitch but with meaningfully more oxidative capacity than IIx, and they're the fibre type most responsive to training-induced shifts. The size principle (Henneman) governs recruitment order: motor units are recruited from smallest to largest as force demand increases, meaning Type I fibres are recruited first for low-force tasks and the larger, more powerful Type II motor units are only recruited as effort approaches maximal. This is exactly why light-load, high-rep training under-recruits the fibres most responsible for maximal strength and power, and why true strength and power development requires training at intensities high enough to force full motor unit recruitment.

Neuromuscular Adaptations to Resistance Training

In the first several weeks of a new resistance training programme, most of the strength gain you see comes from neural adaptation rather than muscle growth: increased motor unit recruitment, improved rate coding (the frequency at which motor units fire), better intra-muscular coordination within a single muscle, and improved inter-muscular coordination across synergist muscles working together. Hypertrophy becomes the dominant driver of continued strength gains only after this early neural adaptation phase, typically several weeks to a couple of months into consistent training. This distinction explains a genuinely common exam scenario: a beginner getting dramatically stronger in weeks 1-4 without visible muscle growth. That's neural adaptation, not hypertrophy, and it's the expected, normal pattern.

Biomechanics: Levers, Torque and Joint Actions

The human body operates almost entirely on Class 1, 2 and 3 lever systems, and most limb movement, including the biceps curl and knee extension, is a Class 3 lever, where the effort is applied between the fulcrum and the resistance; this arrangement favours speed and range of motion over mechanical advantage, which is why so much of human movement trades raw force output for the ability to move quickly through a large range. Torque, the rotational force around a joint, equals force multiplied by the perpendicular distance from the joint axis to the line of force (the moment arm); this is the mechanical reason why moving a weight further from a joint, holding a dumbbell at arm's length versus close to the body, increases the torque demand on that joint even though the load itself hasn't changed. Understanding torque and moment arms is the basis for reasoning through exercise selection questions: why a front-loaded carry challenges the trunk differently to a side-loaded carry, or why a wide-grip pull-down changes the torque demand on the shoulder compared to a narrow grip.

Acute Physiological Responses vs Chronic Adaptations

The exam consistently distinguishes between what happens during and immediately after a single bout of exercise (acute response) and what happens after weeks or months of consistent training (chronic adaptation), and mixing these up is one of the most common errors candidates make. Acute cardiovascular responses to resistance exercise include increased heart rate, increased blood pressure (particularly during the concentric phase and with Valsalva manoeuvre use), and increased cardiac output. Chronic cardiovascular adaptations to consistent aerobic training include resting bradycardia (a lower resting heart rate), increased stroke volume, and increased maximal oxygen uptake (VO2 max). Acute hormonal responses to resistance training include transient elevations in testosterone, growth hormone and cortisol immediately post-exercise. Chronic endocrine adaptations are more nuanced and include improved insulin sensitivity and, in some populations, altered resting hormone profiles. If a question describes something happening ‘during’ or ‘immediately after’ a session, it's testing acute response; if it describes something after a training programme, it's testing chronic adaptation.

Detraining and the Reversibility Principle

Training adaptations are reversible, and the exam tests specific timelines here. Cardiovascular fitness (VO2 max) declines meaningfully within as little as 2 weeks of complete training cessation, with more substantial losses by 4 weeks. Strength is comparatively well-preserved in the short term, particularly in trained individuals, with more significant losses generally not appearing until beyond 2-4 weeks of inactivity, though this varies with training age and the specific quality being measured. The core principle worth remembering is that adaptations decay roughly in the reverse order of how specific and recently-trained they were; qualities that took longest to build and are most neurally-dependent (fine motor skill, sport-specific power expression) tend to be the most fragile, while general strength in trained lifters tends to be the most robust over short breaks.

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