The purpose of this study was to compare the external mechanical efficiency (ME) between power-trained athletes (n = 5) and endurance-trained athletes (n = 5). The relationships between biomechanical variables and metabolic cost were also investigated. The subjects ran at 3 different speeds (2.50 m.s-1, 3.25 m.s-1 and 4.00 m.s-1) both on the treadmill and on the track. The external work of the subjects was determined by a kinematic arm, and energy expenditure was determined by measuring oxygen consumption and respiratory exchange ratio. Biomechanical parameters included ground reaction forces, angular displacements of the knee and ankle joints and electromyography (EMG) of the selected muscles. The mean ME (+/- SD) values during running on treadmill were as follows: 49.6 +/- 8.9%, 60.1 +/- 9.6% and 61.2 +/- 10.4% for the endurance group, and 47.1 +/- 3.7%, 52.0 +/- 4.3% and 57.4 +/- 5.5% for the power group. In running on the track the respective values were 57.5 +/- 11.9%, 51.5 +/- 6.1% and 62.2 +/- 9.2% for the endurance group, and 47.0 +/- 8.3%, 45.3 +/- 10.2% and 60.0 +/- 5.9% for the power group. The subject groups did not differ significantly in ME due to high interindividual variance among both subject groups. The metabolic responses such as heart rate, pulmonary ventilation and oxygen uptake differed clearly between the athletic groups but this was not the case for the most of the biomechanical variables (such as EMG, step length and vertical displacement of the centre of the gravity).(ABSTRACT TRUNCATED AT 250 WORDS)