Date of Award

December 2018

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Exercise Science


Tom D. Brutsaert


accelerometers, birth weight, fetal programming, muscle strength, physical activity

Subject Categories

Medicine and Health Sciences


Appropriate fetal growth and development largely depends on the health and nutritional status of the mother. Exposure to a stressful intra-uterine environment leads to a series of adjustments, known as fetal programming, which have both short- and long-term implications. Immediately, the growth and development of the fetus is altered, resulting in intra-uterine growth restriction and a small size at birth. Long-term, individuals who experience fetal programming are at higher risk of developing certain chronic diseases.

In the current study, we examined the relationship between size at birth (an indicator of intra-uterine growth restriction and thus, fetal programming) and two main outcomes, physical activity behaviors and muscle strength, in a cohort of healthy young adults. Birth weight was adjusted for gestational age to create a standardized birth weight (SBW). Physical activity behaviors included sedentary behavior and moderate-to-vigorous physical activity, which were both measured using two different accelerometers over a seven-day period. Muscle strength was measured four times: dominant handgrip maximal voluntary contraction (MVC), non-dominant handgrip MVC, left leg MVC, and right leg MVC. Additional measurements included height, weight, body composition, and maximal oxygen consumption (VO2max) as a measure of cardiorespiratory fitness.

A total of 124 participants completed the study. A subset of 75 participants completed the accelerometer measurements and performed a VO2max test. Using data from this subset, no relationship emerged between SBW and time spent in either moderate-vigorous physical activity (B = 5.642, p = 0.088) or sedentary behavior (B = -14.571, p = 0.422) among the full cohort of healthy young adults. However, the relationship between SBW on time spent in MVPA depends on age. Participants aged 18 – 21 (N = 42) years had an increase of 7.02 minutes of MVPA for each unit increase in SBW (p = 0.017). Participants aged 22 – 40 years (N = 33) had a decrease of 10.8 minutes of MVPA for each unit increase in SBW (p = 0.021). Furthermore, there was a non-significant trend toward a sex effect on the relationship between SBW and time spent in SED. Among male participants (N = 15), time spent in SED increased by 26.7 minutes with every 1 unit increase in SBW (p = 0.203). Among female participants (N = 60), time spent in SED decreased by 13.5 minutes for every 1 unit increase in SBW (p = -0.250).

A subset of 100 participants performed the muscle strength testing. Participants born small for gestational age had lower muscle strength in their dominant hand compared to adults born at a normal size for gestational age (B = 1.533 kg/1 SD increase in SBW, p = 0.004). After controlling for sex and lean body mass, SBW explained 8.4% of the variance in dominant handgrip MVC. LBM had a significant indirect effect on the relationship between SBW and dominant handgrip MVC, confirmed by mediation analysis using the Sobel test (p = 0.04), partial posterior p-value (p = 0.025), and hierarchical Bayesian confidence interval (95% CI = 0.063, 1.401). These results suggest that individuals born small not only have muscles that function less well, but also tend to have smaller muscles.

In conclusion, muscle strength is directly related to size at birth among healthy young adults. The effect of FP on later-life physical activity behaviors remains unclear, but may be influenced by age and sex. The results of the current study confirm that permanent physical and physiological effects of FP on skeletal muscle development and performance. More research is needed to determine the role of intra-uterine stress on physical activity behaviors.


Open Access

Available for download on Thursday, January 14, 2021