More on performance measurements, part 3. Stroke Index.


Stroke Index, shown as SI, equals Stroke Length x Velocity


Original by Damien Gogoll, PLC Aquatic Swim Club, Head Coach

The previous articles covered measurements that coaches use in swimming:

  • Times (splits)
  • Stroke Rate
  • Stroke Count
  • Distance Per Stroke
  • Velocity
  • SWOLF (swimming golf)

All of these measures are extremely useful in assessing improvement in a swimmer’s performance and are widely used among high performance coaches around the world.

The ultimate aim of a swim coach: "To train the swimmer to swim the given distance of the race in the shortest time possible."

This posting's focus is upon a lesser known efficiency measurement, STROKE INDEX. Stroke index is abbreviated as SI. 

Stroke index is another swim coaching measurement tool. To understand stroke index (SI), let’s first take a step back and look at one of the overall aims of swim coaching:

Coaches measure swimmers' performances with simple and complex markers.

Quite simply, coaches worry about the outcomes, the swimmers' times for each of the races and the place they came in the races.

However, as coaches look to improve the race times of the swimmers, coaches can’t just say, "Swim faster." Smarter coaches need to be more specific in what can be improved to reduce their swimmers' times. Many variables can influence a swimmer’s time including technique, fitness, strength, power, psychological state, training phase. The list of variables that impact swim times goes on, seemingly, infinitely. 

Measuring these variables and comparing them is the best way to identify what and how to improve the swimmers time.

From a biomechanical and physiological point of view, how can we improve a swimmers time?

Working on a swimmers efficiency in the water (both technique and energy production) jumps out as the most logical answer. Looking at one basic principle of forces, it is important to ensure that the propulsive forces are significantly larger than the resisting forces that are applied to the swimmer, for the duration of the race. The relationship between these two forces can be complex and many facets can influence changes
to this relationship. Remember the environment we are dealing with is fluid and constantly changing, a movement from the swimmer will influence the forces applied to the swimmer.

Two major facets swim coaches look at to improve the net force created by a swimmer are:

  • Swimming Technique
  • Physiology of the Swimmers 

Net Force = Propulsive Forces minus Resisting Forces

Both technique and physiology influence the net force created.


Finding the optimal positions for the swimmer to be in to allow them to efficiently apply
the greatest propulsive force while minimizing the resisting forces for the entire race is

As each swimmer is different, it is important to teach each swimmer and each stroke
individually, but apply the key principles of reducing resistance and then increasing

Finding the right movement pattern for each individual to achieve effective
and efficient propulsive forces while reducing the resisting forces is the aim. This can be
limited by the swimmers' physique, age, cognition and movement-pattern-retention

Great coaches can visually observe efficient and inefficient technique. However it is important to be able to actually measure techniques. There are many ways to measure efficient technique in training. In racing; Stroke Count, Stroke Length and Stroke Rate are simple measurements that are often used.

Efficient technique sets the swimmer up to apply the propulsive forces in the most effective way.


The definition of physiology in swimming means, the ability to apply the propulsive forces on the water efficiently for the duration of the race while maintaining a body shape and position that reduces the resisting forces applied to the body.

This includes, but is not limited to, building up a swimmers strength, power, production of energy within the muscles, and the removal of by-products from the
energy production.

Once a swimmer has efficient technique, it is much easier to develop the physiological systems to increase the propulsive forces.

Stroke Index

Stroke index measures a swimmer’s efficiency for an exact point in their race or training. The higher the stroke index value, the more efficient the swimmer is.

Stroke index = Stroke Length X Velocity of swimmer during stroke length
Stroke Length

Stroke length is the distance traveled by the swimmer in one complete arm cycle. A complete arm cycle (One stroke in Breaststroke or Butterfly, a left and right arm stroke combined in both backstroke and freestyle). This is otherwise known as Distance per Stroke (DPS).

How is stroke length useful to a swimmer and a coach?

If we look back at the overall aim of the swimmer in a race, it is to get to the finish as fast as possible (ultimately to be the first to finish). Increasing the distance travelled for each arm cycle will reduce the number of strokes taken. Reducing the number of strokes taken results in less disruption of the water, less changes in body position and most importantly reducing the resistant forces applied to the swimmers body.

With less resisting forces applied to the body, overall net force is increased. Furthermore, the body is able to use less energy in counteracting the resisting forces which frees up more energy to create propulsive forces. However as coaches, we know that having the longest stroke alone doesn’t result in swimming as fast as you can. It is a combination of stroke length and the speed of the stroke that determines fast swimming. As mentioned above, there are other factors that influence fast swimming, however we are just focusing on these two factors.

Velocity during measured Stroke Length

The velocity of a swimmer is simply the distance traveled divided by the time it took to travel that distance. In relation to Stroke Index, the velocity is as follows:

VELOCITY = Stroke Length (SL) ÷ Time taken to travel SL

The velocity of a swimmer’s stroke is one of the most under used measurements within swimming. Ultimately, velocity is under used because velocity requires calculations. Go figure, calculations need to be done. Let’s face it, the majority of coaches don’t have
time nor desire to a lot of calculations.

The higher the velocity (also known as speed) of the swimmer, the faster the swimmers are going, However doing the velocity calculations with a lone velocity point doesn’t give an accurate indication of how a swimmer is performing in a race. For instance, a swimmer may have a velocity of (1.8meters per second) in the first 50-m of a 200-meter race. But then on the second length, the velocity might drop to (1.0-meters per second) for the last three segments. By only taking and looking at one velocity point, the coach is sure to miss the fact that there is a significant drop in velocity after the first 50-meters.

By taking velocities during each lap of a race, and comparing them to each other, can quickly show a coach and swimmer the laps in a race that need addressing in order to improve.

Measured individually, Velocity and Stroke Length are great indicators of efficiency. However, both have downfalls.

A swimmer can have great stroke length. However there is a point at which this comes at
a sacrifice of speed. All swim coaches will have had a swimmer who is able to get a really long stroke, but it isn’t fast.

A swimmer can have an extremely high velocity for part of the race, but at what point is it detrimental to the rest of the race. If you just look at the velocity you will not know the length of each stroke.

How the Stroke Index helps coaches

Stroke Index allows for easy comparison:

  • a) within a swimmers race (each 25-meter or 50-meters)
  • b) Between a swimmer's races (heats and semi-finals, or between swim meets)
  • c) Among swimmers (International swimmers compared to National or State level swimmers)
Care with Comparisons

Comparisons of Stroke Index (SI) values among strokes or among swimmers can be useful. However, care must be taken. Comparisons are most accurate when comparing to a swimmer of similar ability and similar physical size (e.g. stroke length) for the same stroke. However the ability to compare the efficiency of groups of swimmers (e.g. international level to State level) can provide useful data.

As Stroke Index (SI) multiplies Stroke Length and Velocity, it takes the two values to produce one score which is an efficiency factor. In a similar way to SWOLF (combining Time & Stroke Count) giving one score, Stroke Index also gives one score. This score can tell us how efficiently a swimmer is moving through the water, A higher value indicates greater efficiency. Therefore, it is easy to compare sections of a race to establish when a swimmer is more or less efficient.

A coach and swimmer can easily identify and analyze the sections of the race that are less efficient in an effort to establish why. It would be assumed that the Net Force produced by the swimmer has decreased either by a reduction in Propulsive forces or by an increase in resistive forces. Further analysis to identify the cause would be carried out, with many outcomes possible.

The swimmers kick (and how much contribution it has to the overall stroke) can influence their performance. Using a comparison between a swimmer’s Pull and Swim SI’s can identify the contribution their kick has. Simply measure the SI over a 50-meter Pull and a 50-meter Swim. E.g. 50-M free index pull = 3.6, 50-M free index with kick index = 4.1, therefore kick contribution is 0.5. Testing the kick contribution in this manner is can be more beneficial than timing swimmers with kick only, as some swimmers may not efficiently integrate their kick into the stroke body movement cycle.

Calculating Stroke Index

There are two ways to calculate the Stroke index (this requires some simple middle school math, a spreadsheet or calculator or simply just use an AutoCoach stop watch.

In order to calculate the Stroke Index you will need to take:

  • 1) A Stroke Rating
  • 2) A time for the distance traveled
  • 3) The distance traveled

It is recommended that measurements are taken after the first 15-meters of a length to allow the swimmer to reach a more consistent velocity and ensures the measuring the actual stroke and not effects of the start, turn or push-offs.


An elite freestyle swimmer recorded a rating of 36 with a time of 20.0 for the last 35-meters of a 50-meter lap.

Option 1

Step 1: Calculate the number of stroke cycles for the section being measured:

Number of stroke cycles = stroke rating (per minute) * time (in minutes)

Number of stroke cycles = 36 * (20/60)
Number of stroke cycles = 12 stroke cycles

Step 2: Calculate the Distance per stroke for the section being measured:

Distance per stroke = distance / number of stroke cycles.

Distance per stroke = 35 / 12
Distance per stroke = 2.91-meters

Step 3: Calculate the Average Velocity for the section being measured:

Average velocity = Distance (M) / Time(S) in meters per second

Average velocity = 35 / 20
Average velocity = 1.75-meters per second

Step 4: Calculate the stroke index

Stroke Index = Velocity * Distance per stroke

Stroke Index = 1.75 * 2.91
Stroke Index = 5.09

Option 2

Step 1: Calculate the number of stroke cycles for the section being measured:

Number of stroke cycles = stroke rating (per minute) * time (in minutes)

Number of stroke cycles = 36 * (20/60)
Number of stroke cycles = 12 stroke cycles

Step 2: Calculate the Distance per stroke for the section being measured:

Distance per stroke = distance / number of stroke cycles.

Distance per stroke = 35 / 12
Distance per stroke = 2.91-meters

Step 3: Calculate the Stroke Frequency (number of stroke cycles per second)

Stroke Frequency = stroke rating / 60

Stroke Frequency = 36 / 60
Stroke Frequency = 0.6 /second

Step 4: Calculate the Stroke Index

Stroke Index = Distance per stroke * Distance per stroke * stroke frequency

Stroke Index = DPS * DPS * SF
Stroke Index = 2.91 * 2.91 * 0.6
Stroke Index = 5.08

Putting meaning to figures: Based on the example above (Freestyle)

  • A Velocity of 1.75meters / second would result in a 50-m time of 28.57 seconds
  • A DPS of 2.91-meters per arm cycle equates to 1.46-meters per arm stroke.

If you consider the arm span of a swimmer is approximately their height (1.8-meters) then the hand will travel 1.8-meters backwards. However, the body will only travel 1.46-meters forwards. This equates to a slip of 0.34-meters. Using the analogy of a swimmer grabbing onto a fixed solid object (e.g. a lane rope) and pulling themselves forwards, the body would move forward 1.8-meters. Due to the fluidity of water, it is a challenge for a swimmer to travel forwards without any slip (unless they have an extremely solid pull and propelling kick.

Hate math or don’t have time to do the above calculations?

The Autocoach stop watch can automatically calculate the following measures by simply taking a rating and split time:

  • Velocity
  • Stroke Length (Distance Per Stroke)
  • Stroke Index.

About the author 

Mark Rauterkus

Swim, water polo and SKWIM coach in Pittsburgh, PA, USA. Also the webmaster for the International Swim Coaches Association.

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