We determined whether a short-term exhaustive dynamic exercise (cycling) for about 18min induces changes in the intramuscular architecture of the quadriceps in trained athletes. Thirty-five male athletes (age 28.8+/-9.8 years; height: 175.4.+/-5.5cm; weight: 74+/-11.5kg; average years spent training: 11.1+/-8.4 years; mean weekly duration of training: 10.4+/-3.20h) underwent an incremental cycloergometer test to exhaustion. Muscle thickness of the right quadriceps femoris (rectus femoris + vastus intermedius), and the angle of pennation of the right vastus lateralis on the quadriceps tendon were determined by high-resolution real time ultrasonography. Quadriceps thickness increased from 32.1+/-3.3mm at rest to 34.9+/-3.0mm after the test (p=0.001). The pennation angles were significantly greater after exercise (12.8+/-2.1 degrees at rest; 14.4+/-2.5 degrees after the test (p=0.001)). There are marked changes in intramuscular architecture of the quadriceps following a bout of cycloergometer exercise to exhaustion, with significant increase of quadriceps thickness and pennation angle. Ultrasonography allows to evaluate the changes of muscle architecture following exercise.