There are two basic types of skeletal muscle in the body, fast twitch and slow twitch. Fast-twitch muscles are more suited to providing rapid, but short, bursts of activity, whereas slow-twitch...

There are two basic types of skeletal muscle in the body, fast twitch and slow twitch. Fast-twitch muscles are more suited to providing rapid, but short, bursts of activity, whereas slow-twitch muscles are more suited for slower, sustained activity. Different people have different mixes of these two types of muscle; for example, sprinters typically have about 75 percent fast-twitch fibers in their thigh muscles, and long-distance runners typically have only about 30 percent fast-twitch fibers. In order to predict the aptitude of athletes for certain types of activity, a small piece of muscle is often analyzed biochemically to determine the proportion of the two fibers. A new method of studying molecules in the body without the need to take a piece of tissue is magnetic resonance (MR) spectroscopy. MR spectroscopy works because a pulse of strong electromagnetic energy causes the nuclei in molecules to line up in the same direction. However, the nuclei quickly “relax” by losing energy to return to their normal state of random alignments. Different molecules have different behavior, and by monitoring characteristics of the relaxation, much can be learned about biochemical processes in the body. Spectroscopists typically measure two different relaxation times, one in the direction of the magnetic field of the MR spectrometer—the longitudinal relaxation time, or T1—and one perpendicular to the direction of the magnetic field—the transverse relaxation time, or T2 . Different types or assemblages of molecules are characterized by their relaxation times. Kuno and coworkers* postulated that muscle fiber type F, expressed as a percentage of fast-twitch fibers, could be predicted using MR spectroscopy. A. How well is F predicted by T1 (the data are in Table D-6, Appendix D)? B. Does the regression equation make physiological sense?

Table D-6