2.5 Explain the immediate physiological responses to training, heart rate, ventilation rate, stroke volume, cardiac output and lactate levels

1. Heart rate

Heart rate (HR) is the number of heart beats per minute (bpm). It rises quickly when exercise starts so the cardiovascular system can deliver more oxygen and glucose to working muscles.

A typical resting heart rate is about 60 to 72 bpm. In some champion endurance athletes, resting values as low as 27 to 28 bpm have been recorded. A lower resting HR usually shows a more efficient cardiovascular system, often linked to a larger stroke volume at rest.

When exercise begins, HR rises sharply because the sympathetic nervous system becomes more active and hormones such as adrenaline are released. HR then increases in line with intensity. A light jog might sit near 120 bpm, while exhaustive exercise can approach an 18-year-old’s maximum, often close to 200 bpm.

During steady, submaximal work, HR can plateau after several minutes once oxygen delivery meets demand. This is the steady state response. Fitness changes the pattern. A trained person is more likely to reach a steady state and recover quickly. An untrained person may show a gradual upward drift during longer exercise and slower recovery once exercise stops.

After exercise, HR drops quickly at first, then more slowly as the body returns towards resting conditions. How quickly HR recovers is a useful indicator of cardiovascular fitness.

Example: In elite netball, a mid-court player may sustain HR around 160 bpm for extended periods, with peaks closer to 180 bpm during repeated fast breaks, jumps, and defensive bursts. At quarter-time, HR drops quickly at first, then continues to fall more gradually during recovery.

2. Ventilation rate

Ventilation rate is the rate and depth of breathing. It includes inspiration (air moving into the lungs) and expiration (air moving out). Ventilation is commonly described using:

• Breathing rate (breaths per minute)
• Tidal volume (air per breath)
• Minute ventilation (total air moved per minute)

At rest, breathing is often around 12 breaths per minute. Minute ventilation is about 6 L per minute, with an average tidal volume close to 500 mL per breath.

Ventilation can increase slightly before exercise starts because of anticipation and nervous system activity. Once exercise begins, breathing becomes faster and deeper to increase oxygen uptake and remove extra carbon dioxide produced by working muscles.

As intensity increases, ventilation rises in a fairly steady way at first. During high-intensity work, it increases more sharply as the body tries to regulate carbon dioxide levels and blood acidity.

During vigorous exercise, ventilation can exceed 100 L per minute. During maximal work it can be around 150 L per minute (or higher) in healthy individuals, with some elite endurance athletes reaching even higher values. After exercise, ventilation stays high briefly, then gradually returns to resting levels.

Example: In AFL, after a long chase or repeat sprint passage, a midfielder may reach around 40 to 50 breaths per minute, taking deep breaths at the next stoppage. Within a minute or two, ventilation drops as intensity falls.

3. Stroke volume

Stroke volume (SV) is the amount of blood pushed out of the left ventricle with each beat (mL per beat). SV increases early in exercise so more oxygenated blood is delivered to working muscles with each heartbeat.

Typical SV values vary with training status:

• Untrained: about 50 to 70 mL at rest, rising to around 80 to 110 mL at maximum effort
• Trained: about 70 to 90 mL at rest, rising to around 110 to 150 mL at maximum effort
• Highly trained: about 90 to 110 mL at rest, and can rise to 150 mL to above 200 mL at maximum effort

SV rises quickly when exercise begins because more blood returns to the heart (venous return). This increases how much the ventricles fill, which helps the heart contract more strongly. This is the Frank–Starling mechanism, where extra filling stretches the heart muscle and leads to a more forceful contraction.

SV is influenced by:

• how well the ventricles fill (blood volume returning to the heart)
• how well the ventricles empty (contractility and force of contraction)

Most of the increase in SV happens from rest to moderate intensity. As intensity becomes very high, SV usually plateaus because HR is so high that there is less time for the ventricles to fill. At that point, further increases in blood flow depend more on HR than SV.

Example: In an Australian 1500 m track race, a runner’s SV may rise from about 90 mL at rest to around 140 to 160 mL once settled into race pace. Even if SV plateaus later in the race, it stays well above resting levels.

4. Cardiac output

Cardiac output (CO) is the total amount of blood pumped by the heart per minute. It is calculated as:

Cardiac output = Heart rate × Stroke volume

At rest, CO is commonly around 4 to 6 L per minute. As soon as exercise starts, CO increases because both HR and SV increase.

During maximal work:

• Untrained individuals may reach around 15 to 20 L per minute
• Trained individuals may reach around 20 to 25 L per minute
• Highly trained endurance athletes may reach up to around 40 L per minute

A key part of the immediate response is blood flow redistribution. At rest, only a small proportion of CO goes to skeletal muscles. During vigorous exercise, the body redirects more blood towards the active muscles. Working muscles can require around 84% of blood flow during intense exercise.

This immediate rise in CO, along with redistribution, increases the supply of oxygen and glucose to muscles and supports high rates of aerobic ATP production.

Example: In a high-level 400 m freestyle race, a swimmer may have HR close to 190 bpm and SV around 160 mL, producing CO of around 30 L per minute during the effort. When the race ends, HR and CO begin dropping immediately as demand falls.

5. Lactate levels

Lactate is produced when the body breaks down carbohydrate, especially when intensity rises and energy demand increases faster than oxygen delivery. There is always a small amount of lactate in the blood, usually about 1 to 2 mmol/L at rest.

At low to moderate intensity, lactate production and removal are balanced, so lactate levels stay close to resting values. As intensity increases, lactate rises more. At high intensity it starts to accumulate because production becomes greater than clearance.

Common reference points include:

• OBLA (Onset of Blood Lactate Accumulation), often described around 4 mmol/L
• the lactate inflection point, where the intensity cannot be held for long

For trained individuals, lactate can build up quickly above the lactate inflection point, often around 80 to 90% of maximum heart rate. This point is generally lower for untrained individuals.

High-intensity exercise can produce lactate values around 8 to 12 mmol/L. Maximal sprint efforts often produce around 15 mmol/L. In extreme exhaustive efforts in elite athletes, values above 20 mmol/L have been recorded.

Lactate is not simply a waste product. It can be used as a fuel and moved to other tissues to be broken down. However, as lactate levels rise, hydrogen ions (H⁺) also rise. The hydrogen ions are more closely linked to reduced muscle function and fatigue because they affect acidity.

Neutral pH is 7.0, and resting blood pH is about 7.4 (slightly alkaline). As exercise intensity increases, pH drops, which contributes to fatigue and makes muscle contraction harder to maintain.

Example: In a 400 m sprint, an athlete often feels heavy legs and a strong burning sensation in the final 100 m. Lactate levels may rise to around 15 mmol/L by the finish. Lactate then drops over the next 10 to 20 minutes as it is transported and metabolised, with light activity often helping clearance.

6. How the responses work together

These responses work together, not separately. HR and SV rise to increase CO, which is the main way the body quickly increases oxygen delivery. Ventilation rate rises alongside CO to increase oxygen uptake and remove carbon dioxide.

As intensity goes above the lactate inflection point, rising hydrogen ions and carbon dioxide drive a steeper rise in ventilation. At the same time, higher lactate levels show the exercise is relying more on anaerobic glycolysis and is harder to maintain. Redistribution also makes sure more of the increased CO is sent to the muscles doing the work, which supports performance in both continuous and intermittent activity.