Our study on urban (U) and rural (R) 9-18 year children from 3 states of India revealed that U boys had the highest GS and R girls the lowest; GS was higher at all ages in boys than in girls. GS increased with age in both genders; urban boys had higher GS than rural boys at all ages. In girls, the difference between the U and R girls was less than in boys (no difference in mean GS: 14.9 vs 13.8 kg) and the two curves for girls GS merged at around 16 years. After correcting for muscle mass (ratio of GS/MM), the difference between genders reduced (but still remained, figure 3) especially so between R boys and U and R girls till 15 years; the GS/MM in girls was again much lower after 15 years. After correcting for body size through BMI, difference in GS of U and R boys and girls reduced further. Muscle mass, body size, and life style factors (dietary protein and zinc, physical activity and sunlight exposure) were variably associated with GS in the four groups. MM was the chief determinant of GS in urban children and rural boys; contribution of lifestyle factors to GS (more so in the rural girls) was higher in girls.
Higher grip strength in U in comparison with R boys was probably due to better nutrition, higher moderate physical activity and increased sunlight exposure. Reports suggest that U children enter puberty earlier than their rural counterparts [26, 27], and this may also have contributed to these differences; later puberty in R boys may explain the reduction in differences in GS in U and R boys after 16 years. To our surprise, the picture was quite different in girls. Overall, the differences in U and R girl’s GS were less than in boys and at 15 years, the mean GS was the same in both sets of girls. Once again, later puberty in rural girls may have resulted in the rural girls catching up by 15 years.
Gender differences in GS as observed in our study have also been reported by other researchers [1, 3, 4, 7, 28–31],. In line with our observations, Serrano et al. and Cohen et al. have also reported a significant sexual dimorphism from the age of 12 years in Spanish and English children [3, 31, 33], while Newman et al. report that girls manifest a linear increase in GS up to 13 years of age after which it remained constant . Studies suggest that on adjusting for MM, gender differencing in GS were nullified . However, we found that although the differences in U and R girls GS/MM ratio were lower and girls’ mean GS/MM ratio was closer to the boys curves till 15 years (particularly to the R boys), after this age, the GS/MM ratio for both U and R girls was much lower. Thus, gender differences in GS in our study population could not be explained by MM alone.
Interestingly, although the mean muscle mass percentage was higher in R boys, their GS was lower (Muscle Mass % & GS: U:78%, 21.1 kg R:85%, 18 kg). Even though the differences in GS in U and R boys reduced after correcting for MM (more so after 16 years, possibly once again due to later puberty and later acquisition of MM in R boys), they reduced even further on adjusting for body size. Our results thus imply that nutrition acting through higher total muscle mass and increased body size contributed to the higher GS in U boys, further, the differences were also possibly contributed to by differences in the timing of puberty. After adjusting for body size, the difference between the U and R girls decreased, and the decrease was more pronounced.
Minimal differences in MM and body size corrected GS between U and R girls was an intriguing finding of our study. Like the U boys, U girls also had higher dietary intakes (energy and protein intake/day: UB: 2126 Kcal, 56g; UG: 1806 Kcal, 47 g vs RB: 1762 Kcal, 43 g, RG: 1417 Kcal, 34 g), moderate physical activity, anthropometric parameters and total muscle mass, however, the differences in U and R girls GS (absolute GS, GS/MM and GS/ body size) were minimal, particularly after 15 years. Thus, unlike in boys, a combination of nutritional and lifestyle factors did not result in higher GS in U girls post 15 years, implying that there were other reasons for this lack of U/R differences in girls (variance of grip strength by body composition and lifestyle factors was explained the least in urban girls on the regression analysis). We speculate that socio cultural influences, including but not limited to reduced physical activity after achieving menarche (moderate to vigorous physical activity mean pre vs post menarche [32, 34, 35]: U Girls: 1.4 vs 1.1 hours/day, R Girls: 1.0 vs 0.9 hours/day) may possibly explain reduced GS/MM and GS/body size ratio in girls in comparison with boys and also lack of differences in U and R girls.
We compared mean GS of our study children with reports from other studies with similar methodology from United States of America and Chile (Figure 5a, 5b) [32, 34, 35]. Dodds et al have noted that differences in GS exist with respect to different countries and ethnicities. They also demonstrate in a review that the average grip strength measurements are significantly lower in developing countries as compared to the developed world . Further, in addition to body composition, nutrition, physical activity and sunlight exposure, studies propose that GS may also be determined by genetic factors [4, 36]. We observed that the mean GS of urban boys from our study was close to the Chile boys. However, they were lower than the American children with rural boys having the least GS. Mean GS of urban girls was close to the Chile girls till the age of 12 and was lower after that (figure 5a, 5b) with the rural girls having the lowest GS. The GS in our girls was lower than the American girls at all ages.
Limited literature is available describing the relationship of lifestyle factors and anthropometric parameters with GS in children and adolescents. The correlation of GS with muscle mass and lifestyle factors in urban and rural boys and girls observed in our study indicates that the body composition and lifestyle factors may affect GS differently in urban and rural children. Some studies have reported correlations of age, anthropometric parameters and body composition with GS in children [7, 28, 29, 37–40]. Wayako et al have reported an association of GS with vitamin D concentrations in school going children. Although we do not report the vitamin D concentrations, sunlight exposure as a surrogate (of vitamin D concentrations and physical activity) was a determinant of GS especially in urban children. Researchers have also reported significantly greater GS values in children who were physically active than in non-active children[42, 43].
To the best of our knowledge, ours is the first Indian multicentric study to assess GS in children from urban and rural areas. There are limited data describing regional difference in GS within a country . Given the differences in nutrition and body composition in urban and rural children, we were able to explore important relationships between GS, body composition and lifestyle factors in urban and rural children of both genders. Our study has important public health implications; while nutrition seems to be driving the differences in GS in U and R boys, in addition to nutrition, socio cultural factors may play an important role in girls. Thus, strategies to improve GS in rural boys and girls need to be different.
One of our limitations is that as this was a school based study, we were not able to perform sexual maturity staging, puberty affects body composition and consequently muscle mass and strength. We did record the date of menarche which was earlier in urban girls than in rural girls (11.5 years vs 12.3 years), however, no other puberty related data were available. Further, the physical activity and sunlight exposure data were collected using questionnaires, thus, objective methods of assessment of these parameters were not available. Also, our study was cross-sectional; describing increase in GS with age would be more appropriate using longitudinal data.
To conclude, our multicentre study on Indian children revealed that urban boys had the highest grip strength and rural girls the lowest. Nutrition working though body size and composition was largely responsible for the differences in GS in boys, while, in addition to nutrition, socio cultural factors also possibly impact GS in girls. Strategies to improve nutrition and address socio cultural factors, especially in rural girls are critical.