Patients ages 18 to 60 who are in good health and who do not have a history of major conditions affecting the bones, joints or nervous system may be eligible for this study. Patients will undergo a medical examination. There will be 1 or 2 testing sessions, 20 training sessions, and 1 or 2 final testing sessions, with patients being asked to come to the laboratory for as few as 4 times (about 10 hours total) or for up to 25 visits (about 20 hours total). The investigator will indicate which of six different groups that a patient is selected for.