Kathleen D. Little

Physical Activity and Bone Health

Weight-bearing physical activity has beneficial effects on bone health across the age spectrum. Physical activities that generate relatively highintensity loading forces, such as plyometrics, gymnastics, and high-intensity resistance training, augment bone mineral accrual in children and adolescents. Further, there is some evidence that exercise-induced gains in bone mass in children are maintained into adulthood, suggesting that physical activity habits during childhood may have long-lasting benefits on bone health. It is not yet possible to describe in detail an exercise program for children and adolescents that will optimize peak bone mass, because quantitative dose-response studies are lacking. However, evidence from multiple small randomized, controlled trials suggests that the following exercise prescription will augment bone mineral accrual in children and adolescents:

Effect of recreational exercise on pregnancy weight gain and subcutaneous fat deposition

This study was designed to test the hypothesis that continuing a regular regimen of recreational endurance exercise alters the time-specific rate of maternal weight gain and subcutaneous fat deposition during pregnancy. Serial measurements of body mass and 5-site skinfold thickness were obtained from 44 women before and during pregnancy who continued their preconceptional exercise regimen throughout pregnancy and from women who voluntarily stopped their preconceptional exercise regimen either before conception (N = 31) or reduced it below baseline fitness levels in very early pregnancy (N = 4). In the first and second trimester, the rate of weight gain and change in skinfold thicknesses were unrelated to exercise performance. However, those who continued exercise had a reduced rate of weight gain and change in skinfold thickness at specific sites in the last trimester of pregnancy. Overall weight gains were (mean +/- SEM) 13.0 +/- 0.5 kg and 16.3 +/- 0.7 kg in the exercise and control groups, respectively, and the increases in the sum of skinfolds were 22 +/- 2 mm and 31 +/- 2 mm, respectively. We conclude that continuing a regular exercise regimen throughout pregnancy does not influence the rate of early pregnancy weight gain or subcutaneous fat deposition but decreases both in late pregnancy. However, overall pregnancy weight gain remains well within the normal range.

Validity of "generalized" equations for body composition analysis in male athletes.

Equations by Durnin and Womersley [(D-W), Br. J. Nutr. 32:77, 1974], Jackson and Pollock [(J-P), Br. J. Nutr. 40:497, 1978], and Lohman [(L), Human Biol., 53:181, 1981] for estimating body density (BD) purportedly overcome the problem of specificity by accounting for age and/or the curvilinear relationship between skinfolds (SF) and BD. Their equations were validated on 265 male athletes against percent fat measured by underwater weighing [(UWW); mean +/- SD = 9.2 +/- 4.4%]. Equations by Sloan [(S), J. Appl. Physiol. 23:311, 1967], Katch and McArdle [(K-M), Human. Biol. 45:445, 1973], and Forsyth and Sinning [(F-S), Med. Sci. Sports 5:174, 1973] were included as linear regression models to compare to the curvilinear models of J-P, D-W, and L. Differences between UWW and estimated mean values ranged from -1.1 to +5.9%; correlations ranged from 0.58 to 0.85; SEE ranged from +/- 2.41 to +/- 3.61% and total error (E) ranged from 2.38 to 6.97%. The seven D-W equations overestimated mean percent fat by from 3.9 to 5.9%. The K-M, S, and L equations overestimated by 1.3, 0.5, and 1.7%, respectively. The F-S equations overestimated by 2.4 to 3.8%. Of the 21 equations evaluated, only 3 by J-P gave estimates not significantly different from UWW percent fat. Regression analyses of the relationship between UWW (y) and estimated (x) percent fat values from those equations were: y = 1.037x - 0.08 +/- 2.38, E = 2.38, r = 0.84; 0.869x + 1.36 +/- 2.45, E = 2.51, r = 0.83; 1.107x - 1.14 +/- 2.51, E = 2.53, r = 0.82.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of regular maternal exercise on erythropoietin in cord blood and amniotic fluid

OBJECTIVEnOur purpose was to test the hypothesis that continuing regular, high-intensity exercise until the onset of labor produces significant fetal hypoxemia, as evidenced by elevated erythropoietin levels in the fetal compartment.nnnSTUDY DESIGNnErythropoietin levels were measured in samples of amniotic fluid and cord blood obtained from fetuses born to 31 exercising women and 29 matched controls.nnnRESULTSnErythropoietin levels (mean +/- SEM) in amniotic fluid obtained at the time of membrane rupture (9 +/- 2 vs 11 +/- 2 mU/ml) and in cord blood (38 +/- 6 vs 53 +/- 16 mU/ml) and amniotic fluid at delivery (9 +/- 1 vs 24 +/- 12 mU/ml) were no different in women who exercised regularly until the onset of labor. In both groups the majority of elevated cord blood levels (> 50 mU/ml) could be explained by labor events. Amniotic fluid erythropoietin levels correlated directly (r = 0.52) with cord blood hematocrit and increased slowly during labor.nnnCONCLUSIONnWe conclude that the initial hypothesis is incorrect and speculate that cord blood erythropoietin reflects fetal oxygenation during labor, whereas amniotic fluid erythropoietin primarily reflects the adequacy of oxygenation before the onset of labor.

The interaction between regular exercise and selected aspects of women's health

The public health initiative to increase womens participation in regular recreational exercise to the 90% level raises multiple theoretic concerns about its impact on the reproductive health of women. However, at all points in a womans life the overall effect of regular exercise to appetite appears to be beneficial rather than harmful, and in the absence of other stressors, exercise performance must significantly exceed usual recreational levels to have an adverse effect on any aspect of a womans reproductive life. Therefore even in elite athletes abnormalities of any part of the reproductive process (puberty, menstrual cyclicity, pregnancy, lactation, and menopause) should not be attributed solely to exercise without complete evaluation. While generally beneficial, the interaction between exercise and skeletal integrity is influenced by hormonal status and multiple exercise variables. Thus, whereas regular exercise at all ages appears to provide both short- and long-term benefit, the characteristics of the exercise regimen need to vary at different time points.

Fetal heart rate response to sustained recreational exercise.

OBJECTIVEnWe aimed to test the hypotheses that fetal heart rate increases during and after sustained exercise and that the magnitude of the increases is related to gestational age and the duration, intensity, and type of exercise.nnnSTUDY DESIGNnMaternal oxygen uptake and fetal heart rate were monitored in 120 regularly exercising women in association with routine 20-minute workouts between 16 and 39 weeks gestation.nnnRESULTSnIn 97% of the studies fetal heart rate increased during and after exercise. This was significant at all gestational ages and with all forms of exercise with an overall increase of 15 +/- 11 beats.min-1 at 60% +/- 12% of maximal aerobic capacity (mean +/- SD). The magnitude increased with gestational age (10 +/- 8 to 20 +/- 11 beats.min-1), exercise intensity (8 +/- 7 to 21 +/- 13 beats.min-1), and exercise duration (8 +/- 4 to 16 +/- 7 beats.min-1).nnnCONCLUSIONnWe concluded that the hypothesis is correct and speculate that these changes represent a maturing fetal response to a reduction in Po2.

Self-selected recreational exercise has no impact on early postpartum lactation-induced bone loss

UNLABELLEDnAlthough exercise is known to positively impact bone mineral density (BMD), its effect on lactation-induced BMD loss has not been previously evaluated in a case-control study.nnnPURPOSEnThe purpose of this study was to compare lactation-induced bone changes in women who engaged in regular, self-selected, recreational exercise versus those who refrained from such during early postpartum.nnnMETHODSnSubjects were 20 healthy, lactating women who either exercised regularly (exercise, E; N = 11) or refrained from such (control, C; N = 9) during the first 3 months postpartum. Although preconception VO2max was significantly higher in E than C (E = 54.1, C = 36.9 mL.min-1.kg-1), no significant group differences were observed for parity, age, height, weight (WT), % body fat, dietary calcium intake, lactation calcium loss, and serum estradiol. Total body (TB), lumbar spine (LS), and femur neck (FN) BMD were measured within 2 wk of parturition and repeated at 3 months postpartum by dual energy x-ray absorptiometry.nnnRESULTSnAlthough TB was unchanged, BMD decreased significantly from baseline in both groups at LS (C = -5.4, E = -4.1%) and FN (C = -2.7, E = -2.8%). WT decreased significantly over time but was not significantly correlated with BMD loss. No significant group by time interactions were observed for WT or BMD changes.nnnCONCLUSIONnThese results suggest that regular, self-selected, recreational E has no impact on early postpartum lactation-induced BMD loss.

The physiological response of instructors and participants to three aerobics regimens.

This study was designed to test the hypothesis that there are regimen-specific differences in the thermal, metabolic, and cardiovascular responses of nonpregnant women when they perform their chosen type of aerobics at their usual performance level. Thirty-six instructors and 53 participants were studied while performing their chosen aerobics regimen under simulated field conditions using oxygen consumption as the index of exercise intensity. Under these conditions no significant differences were observed in the physiological response to low impact, high impact, or step regimens. However, although their ratings of perceived exertion were lower, the participants worked at a much higher exercise intensity than the instructors (76 +/- 1 vs 62 +/- 2% VO2max). As a result they attained a higher respiratory exchange ratio (0.96 vs 0.90), rectal temperature (38.62 vs 38.12 degrees C), and blood levels of glucose (5.70 vs 4.95 mmol.l-1), lactate (3.65 vs 1.60 mmol.l-1), and norepinephrine (2656 vs 1191 pg.ml-1). We conclude that both intensity and physiological response to aerobics are individual specific not regimen specific and that participants consistently underestimate their level of performance.

Comparison of five‐site skinfold thicknesses in late pregnancy vs early postpartum

To test the hypothesis that the sum (∑) of five skinfold thicknesses in the immediate postpartum period is significantly less than that obtained in the last month of pregnancy in healthy American women, a standardized approach was used to measure skinfold thicknesses at five sites in 81 healthy, active women. Measures were obtained in the last month of pregnancy and again in the first month postpartum. Prior to pregnancy the subjects ranged in age from 24–39 years and weighed between 45–87 kg. The ∑ of five skinfold thicknesses ranged between 34 and 162 mm. During pregnancy, weight gain ranged between 6.5–24.6 kg with a mean gain of 14.5 kg. The ∑ of five skinfold thicknesses peaked between 8–36 weeks gestation and the peak increase ranged from −19 mm to +63 mm above preconceptual levels. In late pregnancy, the ∑ of five skinfold thicknesses ranged between 34.1 and 166.8 mm (mean = 91.7 mm), and postpartum, the range and mean values were similar (38.1–156.4 mm, and 91.8 mm). When values obtained in late pregnancy are compared with those obtained postpartum, the r2 value was 0.9122. There was also good correlation at each of the skinfold sites (r2s = 0.8001–0.9239), except the abdomen (r2 = 0.6049). In conclusion, the hypothesis is incorrect; the ∑ of five skinfold thicknesses in the immediate postpartum period is as acceptable a measure of late pregnancy subcutaneous fat deposition and/or retention as that obtained in the last month of pregnancy in present day healthy American women. Am. J Hum. Biol. 10:299–305, 1998.

The effect of sustained exercise on follicular phase levels of 17β-estradiol in recreational athletes

OBJECTIVEnOur purpose was to test the hypothesis that sustained exercise elevates circulating levels of 17 beta-estradiol in an intensity-dependent manner.nnnSTUDY DESIGNnBlood samples were obtained in the follicular phase of the menstrual cycle from 75 female recreational athletes before and immediately after 20 minutes of aerobics or running at their usual exercise intensity.nnnRESULTSnThe levels of 17 beta-estradiol rose after exercise 97% of the time. At exercise intensities between 50% and 88% of maximum capacity there was a direct linear relationship (r = 0.57) between exercise intensity and magnitude of increase in estradiol levels. A similar relationship was not present for cortisol.nnnCONCLUSIONnSustained exercise produces an intensity-dependent increase in the levels of 17 beta-estradiol that probably reflects decreased hepatic clearance caused by the fall in splanchnic blood flow. Thus the magnitude of the increase in the level of 17 beta-estradiol can be used as a rough index of the exercise-induced decrease in splanchnic blood flow.