Polar Fitness Test Development

The Polar Fitness Test resulting in OwnIndex (in Polar M-series heart rate monitors M61 and F92ti) or OwnIndex_S (in Polar S-series heart rate monitors S210, S410, S520, S610i, S625X, S720i, S725 and S810i) predicts maximal aerobic power (maximal oxygen uptake, VO_2max). The test has been developed using artificial neural network calculation, which is a widely used method in signal processing.

In the test development study, 305 laboratory fitness measurements of 15-65- year-old healthy men and women were performed (Väinämö et al. 1996, Väinämö et al. 1998). None of the subjects had any medication. Maximal oxygen uptake was measured with an ergospirometer (Medikro M 909, Kuopio, Finland)) during graded maximal cycle ergometer (Tunturi EL 400, Turku Finland) tests. The fact that each subject reached his or her maximal aerobic power was checked using three criteria: no increase in VO_2max despite load increase, respiratory quotient > 1.1 and blood lactate > 8 mmol/l. Before maximal stress test at least 250 R-R intervals (5 minutes) were recorded from each subject at complete rest in a laying position using Polar R-R Recorder™ (Polar Electro Oy, Kempele, Finland). Measurement errors were removed from R-R intervals using both automatic (Hearts software, Heart Signals Co, Kempele, Finland) and manual methods (visual inspection). Development of the VO_2max prediction in the neural network was done in two phases: firstly 25 subjects were randomly selected as testing (validation) samples. After that the rest of the 305 subjects were used for the teaching of the network. In addition to heart rate and heart rate variability; gender, age and height were used as predictive variables in the neural network analysis. Body weight was used for the calculation of relative VO_2max (ml/min/kg).

Correlation coefficient between the laboratory measured VO_2max and the artificial neural network prediction was 0.97 and the mean error in the VO_2max prediction was 6.5%. The measured maximal aerobic power values in the data varied between 1-6 l/min (25-60 ml/min/kg). In 95% of the cases in the teaching data and in 60 % of the cases in the validation data, the error in the VO_2max prediction was less than 0.5 l/min. The mean error of the prediction is good compared to any other predictive tests of maximal aerobic power. Typically the mean errors vary between 8-15%. In the laboratory measurements of VO_2max, the test-to-test variation within an individual is 3-5% due to physiological day-to-day variation and technical parameters (e.g. calibration of the ergosprirometer).

Further development of the Polar Fitness Test™ was conducted based on the development study, which was described above. At this stage 119 fitness measurements of healthy American men and women, whose maximal aerobic power was measured in a maximal graded treadmill exercise test, were included in the final development of the neural network. Thus the number of subjects used in the final stage was 424, 381 of which were randomly selected for the teaching of the network and 43 for the validation (Kinnunen et al. 2000). The artificial neural network was modified into Polar Fitness Test™.

Polar Fitness Test was further developed to result OwnIndex_S, an advanced modification of OwnIndex. In the test development study, 450 laboratory fitness measurements of 15-65-year-old healthy men and women were performed. Correlation coefficient between the laboratory measured VO_2max and OwnIndex_S prediction in the data was 0.96 and the mean error in the prediction was 8.2% (3.7 ml/kg/min). In 95% of the cases the error in the prediction was less than 9.4 ml/kg/min. Thus, the accuracy of the OwnIndex_S can be considered good. OwnIndex_S was further validated in studies (Peltola et al. 2000, Tschopp et al. 2000) on trained subjects. It was shown that the VO_2max prediction associated reasonably highly with VO_2max measured in the laboratory in both men and women.

Polar Fitness Test

Polar Fitness Test predicts a person's aerobic fitness from the resting heart rate, heart rate variability, gender, age, height, body weight and self-assessment of the level of long-term physical activity. To obtain the measures for heart rate and heart rate variability, 255 heart beats (3 - 5 min) are measured during the test. In M-series heart rate monitors, physical activity is assessed using a three-level scale (low/middle/high) and in S-series heart rate monitors using a four-level scale (low/middle/high/top). The scale has been modified from NASA/JSC physical activity scale (Ross et al. 1990) used also in a non-exercise test for maximal aerobic power prediction (Jackson et al. 1990). The physical activity score should remain the same, if a person's regular exercise habits have not changed during the previous 6 months.

Polar Fitness Test fits best to the follow-up of long-term changes in aerobic fitness. For obtaining a change in aerobic fitness, regular training for a longer period of time is required. A healthy adult can achieve a 10-15% increase in about three months when training 3-4 times weekly for 30-40 min at moderate intensity.

Polar OwnIndex/OwnIndex_S

The effects of the above mentioned variables in Polar Fitness Test cannot be totally separated from each other – the variables always act in concert. In general, however, the activity assessment together with heart rate measurement explain about half of the OwnIndex/OwnIndex_S and the background variables (gender, age, height and body weight) the other half. The index is raised by a reduction in body weight and lowered by an increase in it. Because gender and height are very stable and the changes in age are slow, their effect on the changes of the index is minor.

The effect of long-term physical activity on the Index is essential: the greater the activity, the better the fitness. In general, OwnIndex/OwnIndex_S is raised by a decrease in the resting heart rate and an increase in heart rate variability, and lowered by an increase in the resting heart rate and a decrease in heart rate variability. To achieve long-term changes in the resting heart rate and heart rate variability, regular physical activity of at least 6 weeks is needed.

Resting heart rate and heart rate variability are sensitive measures and reflect the status of the body. Short-term changes in these variables explain the changes in consecutive Polar Fitness Test™ measurements. There is normal daily variation in the resting heart rate and heart rate variability, and breathing and momentary changes in blood pressure cause normal momentary variation. There can also be undesired, transient changes in the measured heart rate values due to coughing, speaking, body movement, excitement or other disturbances. It is therefore important to minimize the disturbing effects and standardize the testing conditions in order to achieve accurate and reliable results (always perform the test in a similar way at the same time of the day). However, there is no need to control the effects of breathing to the normal variation of heart rate and heart rate variability during the test.

Validity and reliability of the test

Polar Fitness Test has been validated in a study, where 52 healthy 20-60-year-old men were measured before and after an 8-week exercise training. Fifteen men were in the control group. Before the training period the mean error in VO_2max prediction by Polar Fitness Test was 2.2% and after the training -0.7% compared to the laboratory measurement of maximal aerobic power. The mean deviation in the prediction before and after the training was 4-5 ml/min/kg in all groups. Because this is less than standard deviation of the mean VO_2max values within an age group (5-7 ml/min/kg), the validity of Polar Fitness Test can be considered good. In this study Polar Fitness Test was also validated as a measure of fitness change. The effect of the exercise training was on the average 4.1 ml/min/kg (10 %) when measured in the laboratory. Polar Fitness Test predicted this change to be 2.4 ml/min/kg (6 %) on an average. The test thus detected the direction of the change correctly but slightly underestimated the change. The mean deviation in the estimated change of aerobic power was 4.5 ml/min/kg.

The reliability of Polar Fitness Test in consecutive tests for the same individual is good. When 11 subjects repeated the test in the morning, in the middle of the day and in the evening during 8 days, in both sitting and laying positions, the average individual standard deviation of the consecutive test results was less than 8 % from the individual mean value. The standard deviations calculated separately for each time of the day were smaller than the standard deviation of all results. This indicates that the test can be conducted at any time of the day but it should always be repeated at about the same time.

Practical Conduction of Polar Fitness Test

The background variables, gender, age, height and body weight as well as physical activity level, are given to the heart rate monitor. Height must be given to the nearest centimetre (or feet and inches) and weight within the nearest kilogram (or pounds).

Activity assessment is done by selecting the alternative that best describes your general long-term activity level.

In M-series heart rate monitors the activity levels are:
1.Low: You do not participate regularly in programmed recreation sport or heavy physical activity. E.g. you walk only for pleasure or occasionally exercise sufficiently to cause heavy breathing or perspiration.
2.Middle: You participate regularly in recreation sports. E.g. you run 5 miles a week or spend 30-60 minutes a week in comparable physical activity or, your work requires modest physical activity
3. High: You participate regularly, at least 3 times a week, in heavy physical exercise. E.g.you run regularly more than 5 miles a week or spend more than 1.5-2 hours in comparable physical activity.

In S-series heart rate monitors the activity levels are:
1. Low: You do not participate regularly in programmed recreation sport or heavy physical activity. E.g. you walk only for pleasure or occasionally exercise sufficiently to cause heavy breathing or perspiration.
2. Middle: You participate regularly in recreation sports. E.g. you run 3-6 miles per week or spend 0.5-2 hours per week in comparable physical activity or, your work requires modest physical activity
3. High: You participate regularly, at least 3 times a week, in heavy physical exercise. E.g.you run 6-12 miles per week or spend 2-3 hours per week in comparable physical activity.
4. Top: You participate regularly in heavy physical exercise at least 5 times a week. E.g. you exercise to improve performance for competitive purposes.

For reproducible heart rate and heart rate variability measurement the test should be conducted in well-standardised conditions. It is recommended to be performed in a peaceful environment, since talking (even a cough), noisy music and telephone ringing disturb the testing. Eating a heavy meal or smoking 2-3 hours prior to the testing should be avoided. Unusually heavy physical effort as well as alcoholic beverages or pharmacological stimulants should be avoided on the test day and the day before.

How to interpret Polar OwnIndex/OwnIndex_S

OwnIndex/OwnIndex_S is equivalent to the maximal aerobic power, VO_2max, in ml/min/kg. This indicates how many milliliters of oxygen your body is able to transport and use per each kilogram of your body weight in one minute. The maximal aerobic power, as any other fitness test result, is most meaningful when used in comparing individual values and changes. Norms, rather national, can be used to compare the fitness results to the average values of those with the same age and gender. Below an example of normal values presented as a mean(standard deviation) according to the age group (Fletcher et al. 1995).

Individual OwnIndex/OwnIndex_S can be compared to the population norms as follows: One standard deviation around the mean (half SD up and half down) represents "average fitness". E.g. for a 33-year-old woman any index between 31-37 (34-3 and 34+3) represents "average fitness" compared to other women of the same age. Values less than 31 are below the average and those higher than 37 are above the average.

For international use the fitness classification by Shvartz & Reibold (1990) presented in Table 1 is recommended.

Table 1. Classification of maximal oxygen uptake (Shvartz & Reinbold 1990). Data from adults in USA, Canada and 7 European countries.

MEN / MAXIMAL OXYGEN UPTAKE (VO_2max, [ml x kg^-1 x min^-1])

WOMEN / MAXIMAL OXYGEN UPTAKE (VO_2max, [ml x kg^-1 x min^-1])

In this classification, class 1 corresponds to "very poor", class 2 "poor", class 3 "fair", class 4 "average", class 5 "good", class 6 "very good" and class 7 "excellent" cardiovascular fitness compared to individuals of the same gender and age. In a population, 11 % of the people belong to classes 1-2 and 6-7, 22% in classes 3 and 5 and 34% in class 4. This corresponds to "gaussian distribution", because the classification has been developed in representative samples of individuals from different countries.

Fitness class is a useful reference when interpreting the individual test results. Because cardiovascular health is related to aerobic fitness, the people in classes 1-3 would most probably obtain lots of heath benefits and improve their fitness by starting regular exercise. Those in class 4 should at least maintain their exercise habits to ensure better health. However, increase in exercise is recommended for fitness improvement. The people in classes 5-7 most probably already have good health, and their exercise increase targets to improve their performance. Top athletes in endurance sports typically score VO_2max values (ml/kg/min) above 70 (men) and 60 (women). The values differ to some extent according to the sport but there are no reliable reference values in the literature for the different sports. Regular exercisers participating occasionally in competition events score 60-70 (men) and 50-60 (women). Individuals exercising regularly, but not in competitive level, have values between 40-60 (men) and 30-50 (women) and sedentary adults most probably below 40 (men) and 30 (women). General guidelines to help the interpretations are illustrated in Picture 1.

Picture 1. How to interpret the result of Polar Fitness Test?

Maximum heart rate prediction in Polar S-series

Maximum heart rate prediction(HR_max-p) is carried out simultaneously with Polar Fitness Test in S-series heart rate monitors. HR_max-p is based on resting heart rate, heart rate variability at rest, age, gender, height, body weight and maximal oxygen uptake, VO_2max (measured or predicted). HR_max-p has been developed on 431 15-65-year old men and women (Hannula et al. 2000). Of the subjects 175 were used in the development of the HR_max prediction formula and the rest 256 in a validation study. The study showed that HR_max-p predicted the individual HR_max more accurately than the age-based formula (220-age). The mean absolute error in HR_max-p was 6.5 bpm (3.5%) compared to 7.6 bpm (4.1%) in the age-based formula. The standard deviation (SD) of the prediction error was 7.9 bpm with the HR_max-p method and 9.4 bpm with the age-based formula.

References

Fletcher, Balady, Froelicher, Hartley, Haskell, Pollock. Exercise Standards. A statement for healthcare professionals from the American Heart Association. Circulation 91,2,580-615,1995.

Jackson, Blair, Mahar, Ross and Stuteville. Prediction of functional aerobic capacity without exercise testing. Med Sci Sports Exerc 22,6,863-870,1990.

Hannula, Nissilä, Kinnunen, Virtanen. Development of a new HR_max prediction model. Proc 5th Annual Congress of the ECSS, Jyväskylä, Finland, 19-23 July 2000, p 306.

Kinnunen, Hautala, Mäkikallio, Tulppo, Nissilä. Artificial neural network in predicting maximal aerobic power. Med Sci Sports Exerc 32,5,1535,2000.

Peltola, Hannula, Held, Kinnunen, Nissilä, Laukkanen, Marti. Validity of Polar Fitness Test based on heart rate variability in assessing VO_2max in trained individuals. Proceedings of 5th Annual Congress of ECSS, Jyväskylä, Finland, 19-23 July 2000, p 565.

Ross, Jackson. Exercise concepts, calculations, and computer applications. Carmel, IN, Benchmark Press, 1990, pp 95-103,109.

Shvartz, Reibold. Aerobic fitness norms for males and females aged 6 to 75 years: a review. Aviat Space Environ Med 61,3-11,1990.

Tshopp, Peltola, Held, Kinnunen, Hannula, Laukkanen, Marti. Traditionelle und neue Ansätze zür Schätzung der maximalen Sauerstoffaufnahme in Ruhe. Schweizerische Zeitschrift für Sportmedizin und Sporttraumatologie 48(2),58-63,2000.

Väinämö, Tulppo, Mäkikallio, Röning. An atrificial neural network for human aerobic fitness approximation. Proceedings of International Conference on Neural Networks (ICNN), Washington DC, June 3-6, 1996, pp 1939-1949.

Väinämö, Tulppo, Mäkikallio, Röning. A neuro-fuzzy approach to aerobic fitness classification: a multistucture solution to the context-sentive feature selection problem. Proceedings of ICNN, Anchorage, 1998.