Investigate acute physiological responses (cardiovascular, respiratory, muscular) and chronic adaptations to aerobic and resistance training

What this sub-topic is asking

NESA wants you to distinguish acute responses (what happens during one exercise bout) from chronic adaptations (structural and functional change after weeks of training), describe each across the major systems, and explain why specific training types produce the observed adaptations.

The answer

Acute responses (during exercise)

Cardiovascular.

• Heart rate rises immediately at exercise onset (anticipatory and then driven by sympathetic activity and metabolic demand). Continues to rise with intensity until plateau at HRmax.
• Stroke volume rises through low-to-moderate intensity, then plateaus around 40-60 percent VO2max.
• Cardiac output (HR x SV) rises with intensity. Resting cardiac output ~5 L/min; can rise to 20-25 L/min in trained adults at maximal effort.
• Blood pressure: systolic rises (greater cardiac output through similar arterial diameter); diastolic stays similar or rises slightly during dynamic aerobic exercise.
• Blood flow redistributes: away from gut and kidneys, toward working skeletal muscle (up to 80-85 percent of cardiac output) and skin (for thermoregulation).
• Blood viscosity: plasma volume drops slightly through sweat loss, raising haematocrit.

Respiratory.

• Ventilation rate (breaths per minute) and tidal volume both rise. Resting ventilation ~6 L/min; maximal exercise ventilation 100-150+ L/min in trained adults.
• Oxygen uptake (VO2) rises in proportion to demand, up to VO2max.
• Respiratory exchange ratio (RER, CO2/O2) reflects substrate use: ~0.7 fat oxidation, ~1.0 carbohydrate oxidation, >1.0 at very high intensity (CO2 from buffering).

Muscular.

• Motor unit recruitment increases as intensity rises (size principle: small motor units recruited first, large units recruited at high intensity).
• Muscle blood flow increases via local vasodilation.
• Muscle temperature rises (improves enzyme kinetics and contraction speed).
• Glycogen depletes during sustained moderate-to-high intensity work.
• Lactate accumulates at intensities above the lactate threshold.

Chronic adaptations (after weeks-to-months of training)

Aerobic training adaptations.

• Cardiovascular: cardiac hypertrophy (left-ventricular wall and chamber size increase); stroke volume at rest and during exercise rises; resting heart rate falls (bradycardia of training); maximal cardiac output rises; capillary density in trained muscle rises.
• Respiratory: tidal volume rises; ventilatory efficiency improves; VO2max rises (typical 10-25 percent improvement in untrained individuals over 12-16 weeks); lactate threshold shifts to higher percentage of VO2max.
• Muscular: mitochondrial density and size rise; oxidative enzymes (citrate synthase, succinate dehydrogenase) rise; myoglobin content rises; slow-twitch (Type I) fibres become more oxidative; fat oxidation capacity rises.
• Metabolic: glycogen storage capacity rises; insulin sensitivity improves.

Resistance training adaptations.

• Neural (first 4-6 weeks): increased motor unit recruitment, rate coding and synchronisation; the strength gains in the first weeks are predominantly neural, not muscle hypertrophy.
• Muscular: muscle fibre hypertrophy (myofibrillar growth; sarcoplasmic and myofibrillar protein synthesis); cross-sectional area rises; predominantly Type II fibre hypertrophy.
• Connective tissue: tendon and ligament strength rises (slower timeframe than muscle).
• Bone: bone mineral density rises in loaded sites (weight-bearing and high-impact training).
• Hormonal: acute and chronic shifts in testosterone, growth hormone and IGF-1 (acute spikes around heavy sessions).

Why specificity matters at the adaptation level

Aerobic adaptations and resistance adaptations are physiologically distinct. Concurrent training (mixing both) can blunt the adaptation in either direction (the "interference effect") if not programmed carefully. Endurance training does not produce major hypertrophy; heavy resistance training does not produce major VO2max gains. The principle of specificity is grounded in this molecular biology.

Examples in context

Example 1. Cadel Evans' Tour de France training adaptations. Cadel Evans' physiological profile at his Tour de France-winning peak (2011) showed VO2max approximately 80 ml/kg/min, lactate threshold at approximately 90 percent of VO2max, and resting heart rate in the 30s bpm range. These are chronic adaptations to ~20+ years of high-volume aerobic training, mostly cycling. Public profiles of professional cyclists illustrate the upper bound of trainable aerobic adaptation; understanding what these numbers represent (mitochondrial density, capillary density, cardiac chamber size, lactate buffering) is core HMS content.

Example 2. Australian rules football pre-season adaptations. AFL pre-season conditioning programs aim to lift VO2max (more high-intensity match efforts), repeated-sprint ability (which requires both ATP-PC capacity and aerobic recovery), strength and power (for tackling and contested marks). GPS data across pre-season tracks the chronic adaptations: increasing total work, increasing peak speed sustained, improving recovery between bouts. Pre-season testing (Yo-Yo intermittent recovery test, 20m sprint, 3RM trap-bar deadlift) quantifies the adaptations and shapes the next training block.

Try this

Q1. Distinguish between an acute response and a chronic adaptation, with one cardiovascular example of each. [4 marks]

• Cue. Acute: heart rate rises during exercise. Chronic: resting heart rate is lower after weeks of aerobic training (bradycardia of training).

Q2. Analyse the chronic adaptations to aerobic training across cardiovascular, respiratory and muscular systems. [6 marks]

• Cue. Cardiac hypertrophy + stroke volume rise + resting HR fall; tidal volume rise + VO2max rise + lactate threshold shift; mitochondrial density rise + oxidative enzymes rise + Type I fibre adaptation.

Q3. Justify why a strength-and-conditioning coach must plan aerobic and resistance training carefully to avoid the interference effect. [6 marks]

• Cue. Aerobic and resistance adaptations are physiologically distinct; mixing both at high intensity in close proximity can blunt either; programming options include separating sessions by ≥6-8 hours, prioritising the goal adaptation in fresh state, periodising the dominant training type. Reference Super Rugby / NBL / AFL practice.