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Finite element models of the proximal femur at birth, 2 years of age, and at 8 years of age were constructed to investigate stress patterns under different loading conditions. These loading conditions represent typical activities of a normal developing child and abnormal activity associated with muscle spasticity. The hypothesis is that the shear stresses in the growth plate correlate with the neckshaft angle as associated with valgus and normal development. Loads for the finite element models were derived from a separate muscle model used to calculate the forces across the hip joint for an arbitrary subject and activity. Results show there is an inverse relationship between the relative magnitude of the shear stress in the growth plate and the developing neck-shaft angle. The relatively high shear stresses generated by normal activity in the 2-year-old’s growth plate correlate with the decrease in neck-shaft angle that accompanies normal development. Alternatively, lower shear stresses are generated in the growth plate by loading conditions representing spasticity. These lower magnitude shear stresses correlate with a valgus deformity, which is often observed clinically.

Purpose: Although the effect of caffeine in thermoneutral or cool environmental conditions has generally shown performance benefits, its efficacy in hot, humid conditions is not as well known. The purpose of this study was to further examine the effect of caffeine ingestion on endurance running performance in the heat. Methods: Ten trained endurance runners (6 males; mean [SD] age = 26 [9] y, height = 176.7 [5.1] cm, and mass = 72.1 [8.7] kg) came to the lab for 4 visits. The first was a VO₂max test to determine cardiorespiratory fitness; the final 3 visits were 10-km runs in an environmental chamber at 30.6°C and 50% relative humidity under different conditions: 3 mg·kg⁻¹ body mass (low caffeine dosage), 6 mg·kg⁻¹ (moderate caffeine dosage), and a placebo. Repeated-measures analyses of variance were used to determine the effect of condition on the 10-km time, heart rate, core temperature, rating of perceived exertion, and thermal sensation. Results: There was no difference in the 10-km time between the placebo (53.2 [8.0] min), 3-mg·kg⁻¹ (53.4 [8.4]), and 6-mg·kg⁻¹ (52.7 [8.2]) conditions (P = .575, ${\eta }_{\mathrm{p}}^2=.060$). There was not a main effect of average heart rate (P = .406, ${\eta }_{\mathrm{p}}^2=.107$), rating of perceived exertion (P = .151, ${\eta }_{\mathrm{p}}^2=.189$), or thermal sensation (P = .286, ${\eta }_{\mathrm{p}}^2=.130$). There was a significant interaction for core temperature (P = .025, ${\eta }_{\mathrm{p}}^2=.170$); the moderate-dosage caffeine condition showed a higher rate of rise in core temperature (0.26 [0.08] °C·km⁻¹ vs 0.20 [0.06] and 0.19 [0.10] °C·km⁻¹ in the low-caffeine and placebo conditions, respectively). Conclusion: The results support previous research showing a thermogenic effect of caffeine, as the moderate-dosage condition led to a greater rate of heat storage and no performance benefits.