Most of the subjects (34 out of 41) completed the 100 repetitions without excessive muscle fatigue according to the post-exercise MVC values. The intermittent PBE increased fatigue in the lumbar and hip extensor muscles in terms of: (1) a decrease in the MVC; (2) an increase in the level of muscle activation; and (3) a decline of the median frequency (MF). There was no gender difference in all EMG measurements. The level of muscle activation in the hip extensors (GM and HA) was associated with task failure (number of repetitions <100) for some subjects and the addition of a weight of 45 N had more impact on HA than ES. In conclusion, the PBE exercise as performed in the present study (including rest intervals), although not very strenuous for our healthy subjects, seems an adequate exercise to measure and train the aerobic capacity of the back muscles. However, to train specifically the back muscles, the exercise must be adjusted to avoid task failure due to possible hip extensor fatigue.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. American Geographical Society is collaborating with JSTOR to digitize, preserve and extend access to Geographical Review.The Rainbow Bridge, a graceful natural arch of sandstone high enough to span the dome.of the Capitol at Washington, surpasses in size and symmetry any other known natural bridge. It is in the former Piute Indian Reservation, in San Juan County, southern Utah, more than I50 miles from a railroad, and can be reached only by traversing in boat the long canyons of the swift Colorado and San Juan Rivers or by traveling with pack train the few and little-used trails. THE SURROUNDING COUNTRYThe arid country adjacent to the bridge is a part of the broad Colorado Plateau, yet it is so dissected by the streams as to be nearly inaccessible, and practically none of its surface is level. The intercanyon areas are characterized by numberless remnants of erosion-large, closely spaced mounds, domes, and "mosques" of bare sandstone-which rise as much as iooo feet above the canyon floors, or 4500 to 5000 feet above sea le-vel.' Above this expanse of rugged canyons and domes, four miles southeast of the bridge, towers the solitary peak of Navajo Mountain to an altitude of IO,4I6 feet; and on the western side of Colorado River the east end of the broad and steep-faced Kaiparowits Plateau reaches an altitude of 7500 feet.In the canyons a variety of shrubs and scanty patches of grass flourish among the yellow sand dunes; but on the higher parts of the country the soil is swept away by wind and rain as fast as it is formed, and only small clumps of grass and sage brush are able to maintain a foothold. The buff, tan, and red colors of the almost unbroken expanse of bare rock contrast sharply with the dark green of the dense coniferous forests on Navajo Mountain. THE BRIDGE CANYONThe Rainbow Bridge may be seen from the lofty summit of Navajo Mountain, from which it appears as a tiny arch; in spite of its large size it is not conspicuous in a landscape view across the country, for it is well hidden in the bottom of a canyon 500 to iooo feet deep. The canyon is so crooked and so narrow-the precipitous walls in places being less than 0oo feet apart that some of its stretches are lighted by sunshine during

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