Physiological adaptations in response to training: resting heart rate, stroke volume and cardiac output, oxygen uptake and lung capacity, haemoglobin level, muscle hypertrophy, effect on fast-twitch and slow-twitch muscle fibres

Training works because the body adapts. The HSC syllabus expects you to know the specific physiological adaptations that occur in response to aerobic and anaerobic training, and to be able to explain how those adaptations improve performance. This dot point lists each adaptation, what changes, and what kind of training causes it.

Cardiovascular adaptations

Resting heart rate

The average untrained adult has a resting heart rate around 70-80 beats per minute. Trained endurance athletes can have resting heart rates in the 40s or even 30s. Resting heart rate decreases with training because the heart muscle (specifically the left ventricle) gets stronger and pumps more blood per beat, so it needs fewer beats per minute to circulate the same blood volume at rest.

Training that produces this: aerobic training (continuous, fartlek, aerobic interval). Adaptation is most pronounced in long-duration aerobic training over months to years.

A resting heart rate measurement first thing in the morning, before getting out of bed, is the cleanest indicator. Recreational athletes often track it as an over-training indicator: if RHR jumps 10+ beats above baseline, the athlete is likely under-recovered.

Stroke volume and cardiac output

Stroke volume is the volume of blood ejected from the left ventricle per heartbeat. An untrained adult averages around 70 mL per beat at rest; a trained endurance athlete can hit 100 mL or more.

Cardiac output is the volume of blood pumped per minute. It is the product:

At rest, cardiac output is roughly 5 L/min for everyone (trained or untrained) because the body's demand is the same. The trained athlete simply produces it with a lower heart rate and higher stroke volume.

At maximal effort, cardiac output is the limiting factor. An untrained adult might reach 20 L/min; a trained endurance athlete can reach 30-40 L/min. That difference is what allows the trained athlete to deliver more oxygen to working muscle and sustain higher absolute work rates.

Training that produces this: aerobic training, especially sustained moderate-to-high intensity work that drives the heart to pump higher volumes for long periods (continuous, threshold work, longer interval sessions).

Respiratory adaptations

Oxygen uptake (VO2 max)

VO2 max is the maximum rate at which the body can take up and use oxygen. It is the gold-standard measure of aerobic fitness and is expressed in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min).

Typical values:

• Untrained 20-year-old male: 40-45 mL/kg/min.
• Untrained 20-year-old female: 35-40 mL/kg/min.
• Trained recreational endurance athletes: 55-65 mL/kg/min.
• Elite endurance athletes: 75-90+ mL/kg/min.

VO2 max improves with aerobic training through better cardiac output, more efficient oxygen extraction at the muscle (more mitochondria, more capillaries, higher myoglobin), and improved respiratory efficiency. Improvements of 15-25% are achievable in beginner trainees over 3-6 months; smaller improvements continue with progressive overload over years.

Lung capacity

Lung capacity itself (total lung volume) does not change much with training - it is largely determined by genetics, body size, and age. What does change is the efficiency of gas exchange: stronger respiratory muscles (intercostals, diaphragm), more efficient breathing pattern, and improved oxygen and carbon dioxide diffusion at the alveolar-capillary membrane.

The practical effect is that trained athletes ventilate more efficiently at any given workload, breathing deeper rather than faster, and tolerating higher CO2 levels without the panic-breathing of an unfit person sprinting.

Blood adaptations

Haemoglobin level

Haemoglobin is the iron-containing protein in red blood cells that binds oxygen. Total haemoglobin mass increases with aerobic training, especially when training includes time at altitude (real or simulated). This increases the oxygen-carrying capacity of the blood and is one of the key reasons VO2 max improves with training.

Females typically have lower haemoglobin concentrations than males, but the relative increase with training is similar. Iron deficiency (more common in adolescent female athletes due to menstrual losses combined with high training demands) blunts this adaptation and is a common cause of unexplained fatigue in young female endurance athletes.

Muscular adaptations

Muscle hypertrophy

Hypertrophy is the increase in muscle cross-sectional area, primarily driven by an increase in the size of individual muscle fibres (rather than an increase in fibre number). The driver is mechanical loading sustained over weeks to months.

Strength training produces hypertrophy across both fibre types but is most effective at building fast-twitch fibre size. Adaptations are visible within 4-6 weeks of consistent training, though early strength gains (in the first 2-3 weeks) come primarily from improved neural recruitment, not yet from muscle growth.

Training that produces this: resistance training, especially with moderate-to-heavy loads (around 65-85% of one-rep max) for 6-12 reps per set, with sufficient volume over weeks.

Fast-twitch and slow-twitch fibre adaptations

Slow-twitch (Type I) fibres are aerobic-dominant: high mitochondrial density, lots of capillaries, fatigue-resistant. They adapt to aerobic training by increasing mitochondrial number and size, capillary density, and myoglobin content. They get better at oxidising fat and glucose aerobically.

Fast-twitch (Type II) fibres come in two flavours:

• Type IIa are intermediate - capable of both aerobic and anaerobic work.
• Type IIx (sometimes IIb) are pure anaerobic - high force production, rapid fatigue.

Fast-twitch fibres adapt to anaerobic and strength training by increasing in size, increasing the activity of glycolytic enzymes (faster anaerobic ATP production), and shifting their contractile machinery toward the bias of the training (more aerobic-leaning with endurance work, more glycolytic with sprint and strength work).

The fibre-type ratio is largely genetic. Elite sprinters tend to have 70-80% fast-twitch fibres in their key muscles; elite marathon runners tend to have 70-80% slow-twitch. Training can shift Type IIx toward IIa (more endurance-leaning) and the reverse with sustained sprint training, but the broad ratio is set by birth.

Linking adaptations to performance

The point of memorising these adaptations is to explain performance improvement. A canonical extended response chains them.

"A trained 800m runner has a lower resting heart rate (cardiac adaptation), a higher stroke volume and maximal cardiac output (delivering more oxygen during the race), a higher VO2 max (sustaining higher work rates aerobically), more capillaries per slow-twitch fibre (more oxygen reaching the muscle), and increased glycolytic enzyme activity in their fast-twitch fibres (better lactic acid system performance). Together these adaptations let them hold faster pace through the race than an untrained runner with the same raw effort."

That is what HSC markers are looking for in a 6-8 mark adaptation question: specific adaptations named, linked to specific systems, and tied to a performance outcome.