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The dose-response effects of exercise in reduced gravity on musculoskeletal health have not been well documented. It is not known whether or not individualized exercise prescriptions can be effective in preventing the substantial loss in bone mineral density and muscle function that have been observed in space flight and in bed rest. In this study, typical daily loads to the lower extremities were quantified in free-living subjects who were then randomly assigned to control or exercise groups. Subjects were confined to 6-degree head-down bed rest for 84 days. The exercise group performed individually prescribed 1 g loaded locomotor exercise to replace their free-living daily load. Eleven subjects (5 exercise, 6 control) completed the protocol. Volumetric bone mineral density results from quantitative computed tomography demonstrated that control subjects lost significant amounts of bone in the intertrochanteric and total hip regions (p < 0.0125), whereas the exercise group showed no significant change from baseline in any region (p > 0.0125). Pre-and post-bed rest muscle volumes were calculated from analysis of magnetic resonance imaging data. The exercise group retained a larger percentage of their total quadriceps and gastrocnemius muscle volume (− 7.2% ± 5.9, − 13.8% ± 6.1, respectively) than their control counterparts (− 23.3% ± 5.9, − 33.0 ± 8.2, respectively; p < 0.01). Both groups significantly lost strength in several measured activities (p < 0.05). The declines in peak torque during repeated exertions of knee flexion and knee extension were significantly less in the exercise group than in the control group (p < 0.05) but work done was not significantly different between groups (p > 0.05). The decline in VO2max was 17% ± 18 in exercising subjects (p < 0.05) and 31% ± 13 in control subjects (p = 0.003; difference between groups was not significant p = 0.26). Changes in blood and urine measures showed trends but no significant differences between groups (p > 0.05). In summary, the decline in a number of important measures of musculoskeletal and cardiovascular health was attenuated but not eliminated by a subject-specific program of locomotor exercise designed to replace daily load accumulated during free living. We conclude that single daily bouts of exposure to locomotor exercise can play a role in a countermeasures program during bed rest, and perhaps space flight, but are not sufficient in their own right to ensure musculoskeletal or cardiovascular health.

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From 1991 to 1996, Jeffrey pine beetles (Dendroctonus jeffreyi Hopkins) (JPB) caused tree mortality throughout the Lake Tahoe Basin during a severe drought. Census data were collected annually on 10,721 trees to assess patterns of JPB-caused mortality. This represents the most extensive tree-level, spatiotemporal dataset collected to-date documenting bark beetle activity. Our study was an exploratory assessment of characteristics associated with the probability of successful JPB massattack (P JPB ) and group aggregation behavior that occurred throughout various outbreak phases. Numerous characteristics associated with P JPB varied by outbreak phase although population pressure and forest density had positive associations during all phases. During the incipient phase, JPBs caused mortality in individual trees and small groups within toeslope topographic positions and P JPB had a negative relationship with stem diameter. In the epidemic phase, JPB activity occurred in all topographic positions and caused mortality in spatially expanding clusters. P JPB had a curvilinear relationship with tree diameter and a negative relationship with proximity to nearest brood tree. Majority (92-96 %) of mass-attacked trees were within 30 m of a brood tree during the peak epidemic years. During the post-epidemic phase, mortality clusters progressively decreased while dispersal distances between mass-attacked and brood trees increased. Post-epidemic P JPB had a negative relationship with stem diameter and mortality was concentrated in the mid and upper-slope topographic positions. Results indicate mortality predictions are reasonable for the epidemic phase but not for incipient and post-epidemic phases. Ecological factors influencing JPB-caused tree mortality, clustered mortality patterns, and transitions from environmental to dynamic determinism are discussed.

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