Laboratory Numbers on a Swimmer: Cardiac Output, Threshold and the Last 50 Metres
Rupam, a 16-year-old state-level swimmer specialising in the 200 m freestyle, is tested in the school laboratory before the inter-district meet. At rest his heart rate is 62 beats/min and his stroke volume is estimated at 70 mL/beat. During a maximal 200 m swim his heart rate reaches 190 beats/min and stroke volume 110 mL/beat. His measured oxygen uptake is 0.30 L/min at rest and 3.60 L/min at maximal effort. Blood lactate is 1.2 mmol/L at rest, 4.0 mmol/L at a submaximal pace of 1.45 m/s, and 11.5 mmol/L immediately after the race. After he stops he continues to breathe heavily for several minutes and complains of a burning sensation in his arms and legs. His coach wants to know why Rupam fades in the last 50 m and whether the weekly programme is aimed at the right intensity. The school has no treadmill or cycle ergometer, so any future testing must be done in the 25 m pool.
1. Using the data provided, calculate cardiac output at rest and at maximal effort, explain the two mechanisms behind the rise, and estimate the arterio-venous oxygen difference in both conditions. What does the comparison tell you about Rupam's adaptation?
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Step 1 - calculate cardiac output (Q = HR × SV).
- Rest: 62 × 0.070 L = 4.34 L/min
- Maximal: 190 × 0.110 L = 20.9 L/min (about 4.8 times rest)
Step 2 - mechanism. Both factors rise, but unequally. Heart rate rises about 3.1-fold (62 to 190) while stroke volume rises only about 1.6-fold (70 to 110 mL). At maximal effort SV approaches its ceiling because filling time shortens and the ventricle cannot empty further, so most of the remaining increase in output comes from heart rate. This matters practically: a swimmer whose output is limited by SV benefits from interval work that fills the ventricle and improves contractility (longer hard efforts with short recovery), not from more steady distance.
Step 3 - a-vO2 difference (VO2 ÷ Q).
- Rest: 0.30 ÷ 4.34 = 0.069 L per L of blood = 69 mL/L (6.9 mL/100 mL)
- Maximal: 3.60 ÷ 20.9 = 0.172 L/L = 172 mL/L (17.2 mL/100 mL)
Interpretation. The roughly 2.5-fold widening means the working muscle extracts far more oxygen per litre of blood - more capillaries, more mitochondria, better redistribution of blood to active muscle. Rupam therefore has good peripheral adaptation; his fade in the last 50 m is more plausibly a central and pacing problem (he is swimming above his sustainable pace early) than an inability to extract oxygen. Caveat: stroke volume here is estimated, so treat the figures as trends, not absolutes.
2. Interpret the lactate values. Identify the approximate lactate threshold, explain the burning sensation and the prolonged heavy breathing after the race, and state what training zone the 1.45 m/s pace represents.
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- Threshold. Lactate climbs from 1.2 to 4.0 mmol/L at 1.45 m/s, then to 11.5 mmol/L after maximal work. The 4 mmol/L figure is the conventional OBLA marker; the true lactate threshold (first sustained rise above baseline) sits slightly below 1.45 m/s. So 1.45 m/s is approximately his threshold pace - the fastest pace at which production and clearance stay roughly balanced, close to maximal lactate steady state.
- Burning. It is not the lactate. Hydrogen ions accumulate when glycolytic ATP production outruns buffering and oxidation, lowering muscle pH and stimulating pain afferents. Lactate is itself a fuel and a gluconeogenic precursor, cleared by heart, liver and less active muscle.
- Heavy breathing after stopping. This is EPOC. He must repay the oxygen deficit: restore phosphocreatine and myoglobin oxygen stores, oxidise lactate, clear CO2 and H+, restore body temperature and circulating hormones. Ventilation stays elevated because blood pH, CO2 and temperature remain above resting values and the respiratory centres retain some drive after the work stops.
- Training use. 1.45 m/s is threshold pace, so repeated 200-400 m at about that speed with short rest trains the ability to hold pace without lactate spiralling. Work in the 11.5 mmol/L territory (all-out speed) should be a small weekly dose, not the base of the programme. A diet of near-maximal 400s would explain both the flat last 50 m and the fatigue he is showing.
3. The school has no laboratory equipment. Design a pool-based test the coach can repeat every six weeks to track threshold pace, justify your choice, and state how you would keep it reliable.
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Test: 30-minute swim time trial (T30). Warm up 600 m easy plus 4 × 50 m build, then swim as far as possible in exactly 30 min in the same 25 m pool, counting lengths; record total distance, average pace (m/s) and heart rate immediately on finishing.
Justification.
- Average pace over 30 min sits very close to maximal lactate steady state, so it estimates threshold pace far better than a single all-out 200 m.
- Distance and time are objective and need no laboratory; the cost is a stopwatch and a lane.
- It tests him in his own environment and body position - specificity.
Reliability rules. Same pool, same water temperature, same time of day, 48 h of easy work beforehand, no hard session the day before, same feeding, a partner counting lengths, and a recorded RPE to check that effort was genuinely maximal. Retest every six weeks, not weekly, and compare pace rather than raw distance.
Weakness and remedy. A single trial depends on motivation; a poor effort reads as a poor result. Add a second data point (e.g. 2 × 200 m with full recovery) or have the coach judge effort against RPE. Use the pace to set zones: easy swimming at 70-80% of T30 pace, threshold sets at 95-100%, race-pace work above it.