Using Critical Swim Speed (CSS) for calculating the aerobic capacity of swimmers

A variety of performance assessments are currently used in swimming for two reasons. One is to check on the changes that are made by swimmers as a result of their training and the second is to set specific training intensities which are likely to improve a swimmers level of fitness.

These methods of assessment are both aerobic and anaerobic. One anaerobic test is the assessment of lactate threshold. This test is used to assess at what point blood lactate begins to accumulate above resting levels during exercise.

Moderate exercise intensity

During light to moderate exercise blood lactate levels remain just above resting levels but when exercise intensity increases lactate accumulates more rapidly. There are a number or procedures to represent the lactate threshold with controversy lying about where the lactate threshold actually occurs.

This is because lactic acid is already present in the muscles before the threshold is reached and lactic acid only accumulates in the blood because it cannot be cleared as fast as it accumulates during intense exercise. This makes it difficult to point out a clear lactate threshold.

Because of this, set points are used to represent lactate threshold. OBLA is the onset of blood lactate accumulation and is the point at which 4mmol/litre of blood lactate is accumulated. This point can be a percentage of VO2 Max or a running velocity but is the point where 4mmol/litre of lactate is accumulated in the blood.
Another lactate threshold test is the lactate minimum test (LM) which has shown promise for prescribing optimal race pace for endurance training. This test requires a high level of blood lactate to be accumulated.

It works by firstly performing two maximum 50m sprints which are then followed by five or six 300m swims at gradually increasing speeds. The idea is that blood lactate concentrations will decrease at test speeds lower than lactate minimum but these levels will increase at speeds greater than lactate minimum. Lactate minimum is the speed at which lactic acid accumulation exceeds the rate of removal.

Lactate testing is not ideal as it involves blood sampling, which requires experienced people and can be expensive and time consuming even though it provides accurate information on athletes. Therefore it is sometimes better to use non invasive methods requiring inexpensive equipment that are easy to perform such as the Critical Swim Speed (CSS) test.

The Critical Swim Speed Test

The CSS is an aerobic method, which is valid and reliable. The only equipment needed is a stop watch and it is defined as 'the swimming speed that can theoretically be maintained continuously without exhaustion'.

It is therefore the highest sustainable work rate which enables lactate to remain in steady state. It was first used in 1991 by Wakayoshi who swam subjects at six various speeds until exhaustion.

Time in seconds (T) and distance in metres (D) were recorded and a regression line was plotted between D and T, with the equation D = a + bT. The b represents the CSS (speed in m/s) and the a represents the anaerobic swim capacity (ASC).

In 1992, Wakayoshi performed another test to determine CSS. Subjects swam 50m, 100m, 200m and 400m at maximum pace with the times recorded in seconds. A regression graph was plotted again with CSS and ASC determined.

From the regression lines correlations were found and it was concluded that CSS could be determined by a non-invasive method and should be utilised as a standard value for establishing the optimum training intensity in each swimmer.

Ginn (1993a) used two maximum swims at 50m and 400m to determine CSS. They used a push start and said training times should be used and not competition times. CSS was calculated using the following formula.

CSS = d2 - d1
             t2 - t1

d2 = 400m, d1 = 50m, t2 = time for 400m, and t1 = time for 50m (in seconds).

An example of that formula is, if a swimmer peforms a 50m in 30.2s and a 400m in 290.5s, CSS is calculated as below:

CSS = 400 - 50 = 350
290.5 - 30.2 260.3

CSS = 1.34 m/s
Ginn (1993a) then put forward that the obtained value for CSS could be used to determine training times for sets of different distances. For example, for 6 sets of 400m, the time per repetition would be calculated as follows:

400 / 1.34 = 297.49s, Which = 4 mins and 57.5 secs.

Ginn (1993b) related a lot of his CSS work to actual training programmes and found that it is about 80-85% of maximum 100m swim speed, or 90-95% of 400m swim speed. A system of training intensities was devised and is illustrated in Table 1.

Table 1. A simple system of training intensity levels based on calculations of CSS

Training Level % of Speed % of Max 400m*
Level 1 75-80 > 75
Level 2 80-90 75-85
Level 3 90-100 85-95
Level 4 100 100
Level 5 100-110 105

* These are approximate only and will vary for individual swimmers

Cooper (1996) studied eight competitive swimmers who were efficient in both the front crawl and breaststroke, and determined the CSS for both strokes. Lactate thresholds were also determined as well as velocity for OBLA. The three swimming speeds (m.s-1) were then compared and the results are shown in Table 2.

Table 2. Mean parameter values obtained during breaststroke and frontcrawl testing

CSS (m.s-1)
Tlac (m.s-1)
Vobla (m.s-1)

Tlac = Lactate threshold. Vobla = Velocity at obla (4mM)

More recently, Coulson (1997) studied the effects of training on the CSS to find out if aerobic/anaerobic training increases or decreases CSS (comparing sprinters and middle-distance swimmers) and what variations of maximum swims could be used to determine CSS.

Twelve subjects (seven male and five female) were tested at three periods of the swimming season. These were pre-season (September), post-aerobic training (November), and post-anaerobic training (December). Four maximum swims (50m, 100m, 200m and 400m) were used for a regression line to be plotted and CSS calculated.

The hypothesis put forward was that aerobic training increases CSS and anaerobic training increases ASC. This was due to the expected 'shifts' in the regression line (ie, making it more or less steep and so altering the slope and the intercept).

The results showed a significant increase in CSS as a result of aerobic training (1.38 m.s-1 to 1.42 m.s-1 was the average for the group as a whole) which was maintained as a result of anaerobic training.

This was specifically noticeable in the sprint swimmers compared to the middle-distance swimmers, as they would be more prone to changes in their aerobic capacity due to their high anaerobic composition. Table 3 illustrates the mean CSS for the two groups of swimmers.

Table 3. Mean Critical Swim Speeds for sprinters and middle distance swimmers

Middle Distance
CSS (m.s-1)
CSS (m.s-1)
Pre season
Pre season
Post Aerobic
Post Aerobic
Post Anaerobic
Post Anaerobic

It was concluded that CSS cold be used by swimming coaches as a sensitive measure of training.

If coaches have only a limited time to assess the CSS, how many maximum swims need to be performed? Couldson (1997) examined two, three and four maximum swims and found that the two-trial test of 200m and 400m proved to be the most suitable method for CSS determination compared to the four-trial set (which was classed as the 'gold standard'.)


The use of CSS is a valuable and reliable test of aerobic capacity and is sensitive to changes in training. It is a concept which is practical for all coaches, inexpensive, non-invasive, does not required qualified personnel, and the only piece of equipment needed is a stop-watch.

A Guide to Determining CSS
Standardised Warm Up - 1000m

Method 1
1 Subjects swim either four (50m, 100m, 200m, and 400m) or two (200m and 400m) distances as fast as possible. Good recovery is important so sufficent rest must be given between swims.
2 Swims must be from a push and not a dive start
3 Record the swimmer's time for each swim (in seconds).
4 Plot a graph of distance (in metres) against time (in seconds).
5 Join the two or four points by means of a straight line
6 Calculate the slope (or gradient) of this line. The figure produced is the CSS and is given in metres per second.

Method 2
1 Swim two maximal swims (400m and 50m only) from a push and not a dive start.
2 Record the time for each swim in seconds.
3 Calculate CSS using the formula
4 To determine training times for sets, use the formula

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