Date Published: February 15, 2019
Publisher: Public Library of Science
Author(s): Joyce Ndabi, Alan M. Nevill, Gavin R. H. Sandercock, Maciej S. Buchowski.
Comparisons of physical fitness measures between children or within group measures over time are potentially confounded by differences in body size. We compared measures of strength (handgrip) and aerobic fitness (running-speed [20m shuttle-run]) of 10.0–15.9 year-olds from Dar es Salaam, Tanzania (n = 977) with schoolchildren from England (n = 1014) matched for age and sex. Differences in fitness were analyzed using general linear models, with allometric scaling for body size (mass and stature) and further adjustments for physical activity. Mean handgrip of Tanzanians was lower than English youth (F = 165.0, P<0.001, ηp2 = .079). The difference became trivial when run-speed was scaled for body size (ηp2 = .008). Running-speed of the English children was higher than in Tanzanians (F = 16.0, P<0.001, ηp2 = .014). Allometric scaling for accentuated this between-county difference in running-speed (ηp2 = .019) but when adjusted for physical activity between-country differences in running-speed were trivial (ηp2 = .008). These data contradict those studies showing poor muscular fitness in African youth and highlight the need for appropriate scaling techniques to avoid confounding by differences in body size. In contrast to those from rural areas, our sample of contemporary urban Tanzanians were less aerobically fit than European youth. Differences were independent of body size. Lower aerobic fitness of urban Tanzanian youth may be due to reported physical activity levels lower than those of English youth and lower still than previously reported in rural Tanzania.
Economic growth and urbanization means the historical risks from under-nutrition have been replaced by those from changes in lifestyle indicative of nutritional and physical activity transition[2–4]. Between 1990 and 2010 Dar es Salaam was the third fastest growing urban area in Africa with an annual growth rate of 4.7%. Economic development, population growth and migration have driven rapid urbanization in Dar es Salaam which increased in size from 9 km2 in 1945 to become an urban conurbation covering 1000 km2 today.
The University of Essex Institutional Review Board approved the study and parents gave written informed consent. The study was conducted according to the principles expressed in the Declaration of Helsinki. We adopted the same methodology as the East of England Healthy Hearts Study (EoEHHs) and obtained permission to approach schools from the Ministry of Education and Vocational Training in Dar es Salaam.
We calculated means (standard deviations) for anthropometric measures in boys and girls separately and assessed the between-country difference by calculating effect size using Cohen’s d. We interpreted values of d to indicate; small (d>0.2), medium (d>0.5) or large (d>0.8) effect sizes. We visually inspected scatterplots of outcomes against indices of body size to assess the nature of association. Figs 1 and 2 show the association of stature with handgrip (kg) and running-speed (expressed as total shuttles) respectively. Figs 1 and 2 both show heteroscedascity; with increased error terms at higher measurements values. Handgrip and running-speed can be adjusted for body size allometrically using a multiplicative model with allometric exponents for mass and stature[14, 15, 19–21]. These exponents were derived from general linear models including data from both countries.
The unadjusted means in Table 1 show that Tanzanian boys and girls had shorter stature and lower body mass compared with English children of the same age. There were medium differences in BSA of Tanzanian and English participants of both sexes. Tanzanians children also had a lower BMI, lower mean values for BSA and RPI. English boys and girls of all ages were more physically active than Tanzanians. There were main effects for age (F = 138.5, P<0.001, ηp2 = .088), sex (F = 138.5, P<0.001, ηp2 = .088,) and country (F = 138.5, P<0.001, ηp2 = .088) and a small age-by-country interaction effect (F = 5.0, P<0.001, ηp2 = .013). This sample of Tanzanian youth was taken from the rapidly growing urban areas of Dar Es Salaam; a region in nutritional transition. To our knowledge, there are no comparable data describing fitness of contemporary Tanzanian youth from urban areas. Concurrent with findings of studies from across sub-Saharan Africa[10, 12, 15, 17] we found Tanzanian youth children had lower body mass and shorter stature than Europeans of the same age. As differences in body size potentially confound comparisons of fitness between populations we will discuss unadjusted and adjusted values to illustrate the importance of appropriate scaling cross-cultural comparisons of children’s physical fitness. These data present a challenge to the consensus that handgrip strength of African children is inferior to that of Europeans and highlight the need for appropriate scaling when comparing fitness between groups of different body size [15, 20, 22, 25]. Source: http://doi.org/10.1371/journal.pone.0211414