METHODS

Subjects

Nine healthy male volunteer footballers (non-professional athletes) were selected randomly to participate in the study. Mean (SD) age, body weight, and height were 18.3 (2.2) years, 67.3 (4.6) kg, and 1.77 (0.57) m respectively. Each participant gave his written informed consent. The ethics committee of the University of Rome “Tor Vergata” approved the study protocol.

Instrumentation

The K4 RQ, as described previously,¹³ consists of three main components: a system to sample the expired air (mask and flowmeter); a portable transmitting unit (which measures 170 × 90 × 50 mm and weighs 550 g), fixed to a chest harness; and a receiving unit. It uses a Cosmed certified oxygen analyser (range 7–24%) and an infrared non-dispersive thermostatically controlled carbon dioxide analyser (range 0.0–8%). Before each test protocol, the instrument was calibrated against inspired air assumed to have a fractional oxygen concentration of 20.9% and a fractional carbon monoxide concentration of 0.03%, as described elsewhere.^10,13 The flowmeter uses a bidirectional digital turbine and opto-electric reader, and, according to the manufacturer, has a linear response in the ventilation range 0.0–300 litres/min. The turbine flowmeter was calibrated using a 3 litre syringe at the beginning of the test protocol. This turbine was found to be accurate as a flowmeter.^13,14 According to the technical description provided by the manufacturer, the 18 mm section turbine was used for measurements at rest, and the 28 mm section turbine was used for measurements during physical exercise. Oxygen consumption (Vo₂) and carbon dioxide production (Vco₂) were sampled every 15 seconds, and their values were measured at rest and at various exercise intensities (8, 10, 12, 14, 16, and 18 km/h).

The Airspec QP9000 (QP9000) is a quadrupole mass spectrometer housed in a robust high vacuum system. It is specifically designed for respiratory gas analysis and consists of a custom built dual microprocessor based controller (digital unit) connected to a personal computer. Levels of inspired and expired O₂ and CO₂ were determined at rest and at each level of the exercise protocol using this instrument.

Test protocol

The test protocol was conducted using the same tapis roulant (Technogym, Pesaro, Italy) for all participants, who performed an incremental exercise running test from 8 km/h to exhaustion, with an increase of 2 km/h for each level of the test protocol. The duration of each running test was set at six minutes. Only the mean values of measurements of the last two minutes were considered for further analysis. Before each exercise level, the participants rested for two minutes. The expired air was collected simultaneously for every subject using the K4 RQ and QP9000, which were disposed in parallel with a special small connector of low resistance respiratory value. Ventilation levels for Vo₂ and Vco₂ calculation were measured with the K4 RQ.

Data analysis

Statistical comparisons between Vo₂ and Vco₂ measured using the K4 RQ and QP9000 were made with an SPSS software package. Planned contrasts to limit comparisons between instruments for each condition were used. Mean values for measurements were analysed using Student’s paired t test. A significance level of p<0.05 was chosen for all comparisons.

RESULTS

Table 1 gives the mean values for Vo₂, Vco₂, and respiratory exchange ratio measured using the K4 RQ and QP9000 at rest and at different exercise levels (8, 10, 12, 14, 16, and 18 km/h). No significant differences were found between measurements using the two instruments either at rest or at any exercise level. Mean (SD) ventilation ranged from 8.8 (1.18) litres/min at rest to 116.50 (11.50) litres/min for the highest exercise level.

DISCUSSION

There are many reports of EE at rest and various workloads.^3,5–17 Each athlete has unique energy requirements, which can be predicted by estimating resting metabolic rate and activity levels.¹⁸ Many studies have also evaluated the accuracy of portable metabolic gas analysis systems, which measure physical activity EE.^6–14 An indirect calorimeter with a telemetric system is a convenient, accessible, and highly accurate instrument for measuring energy requirements of athletes. We have found that the portable metabolic system Cosmed K4 RQ is practical and precise for the determination of EE cost in athletes. Our study shows that data obtained with the K4 RQ portable calorimeter were not significantly different from those obtained with the Airspec QP9000 mass spectrometer. We compared Vo₂ and Vco₂ measurements using simultaneous data collection. McLaughlin et al¹¹ found no significant difference between Vo₂ at rest and at maximal workloads (250 W) measured using a K4 RQ and the Douglas bag method. However, the subjects were tested on consecutive days to avoid interference between the two methods, which may result in differences in Vo₂ caused by day to day variation. One advantage of our study is that participants were tested on the same day. Vo₂ and Vco₂ measured with the K4 RQ were not significantly different from those measured with the QP9000 both at rest and at every exercise level.

The results confirm the validity of the K4 RQ system for gas measurements in the same subject at rest and during both submaximal and maximal exercise, suggesting its possible use for measuring EE over a wide range of exercise intensities. It may prove to be a valuable tool for optimising nutritional assessment of athletes. Measurement of EE during exercise may estimate the total EE compared with calculated predictions for nutritional needs. Application of the K4 RQ portable indirect calorimeter may be of interest to sport physiologists, nutritionists, and doctors.