A student athelete measured his running performance on a treadmill and found the following data: 15 sec 2 min 6 MPH 5 min 6 MPH O sec 5 sec 30 sec 4 MPH 10 min 20 min O MPH 10 MPH 7 MPH 5 MPH 3 MPH His trainer explained the findings to him in terms of muscle fiber activation and fatigue: At 5 seconds, the [a] fibers in his legs were running at full force, "spending down" their stockpile of [b]. By 15 seconds, these fibers were starting to burn fuel [c), which cut their "full throttle" speed slightly, but not all that much since they could use stored [d] for fuel. Around 30 seconds, however, this first batch of fibers was starting to fatigue due to [e] and his run slowed down a bit. By 2 minutes, however, his respiratory and cardiovascular systems were delivering enough [f] to his leg muscles for his [g] fibers to take over and give him a speed boost. These fibers were burning fuel [h], which gives them more endurance, but since he had not built up much [i] in these muscles through conditioning, they had to wait for oxygen from the blood to arrive before they could really get going. Finally, around 20 minutes into this stress-test, his (j] fibers were beginning to fatigue as well and he began to seriously lose speed.