Growth of Specific Muscle Strength Between 6 and 18 Years in Contrasting Socioeconomic Conditions Maciej Henneberg, 1 * Gerry Brush, 2 and Geoffrey A. Harrison 2 1 Department of Anatomical Sciences, University of Adelaide, Adelaide, 5005 South Australia, Australia 2 Institute of Biological Anthropology, University of Oxford, Oxford 2OX 6QS, UK KEY WORDS “Cape Coloured” males and females; Africa; skinfold; arm circumference; nutrition ABSTRACT The influence of sex, age, and socioeco- nomic conditions on specific grip strength of 6 –18-year-old individuals was studied among 1,704 males and 1,956 females belonging to the so-called “Cape Coloured” com- munity in the western part of South Africa. Half of the participants of both sexes came from communities in the Greater Cape Town area where living conditions are com- parable to those of middle-class First World communities (high SES). The other half came from the poorest rural communities of Klein Karoo (low SES). Arm circumfer- ences, triceps skinfold thickness, and grip strength of the right and of the left hand were greater in individuals from high SES at all ages. Females within each SES group had skinfolds thicker than males, especially at older ages, and were weaker. Specific grip strength (SS), estimated as grip strength per unit area of cross section of the fat-free arm, increased with age in each group, was greater in males, and was significantly lower in low SES groups, than in the high SES ones, especially during and after puberty. It seems that SES difference in SS will persist into adult- hood. Sexual differences in SS can be attributed to hor- monal differences; while the SS increase with age and the difference between SES groups find no clear explanation in current theories of muscle growth and development. Since the speed of neuromuscular reaction observed in our participants is slower among low SES individuals, it seems that the difference in neuromuscular control of strength may be responsible for our findings. Differences in muscle metabolism and hormonal regulation must also be considered. Am J Phys Anthropol 115:62–70, 2001. © 2001 Wiley-Liss, Inc. It is generally believed that the contractile prop- erties of human skeletal muscles become mature early in infancy (Malina and Bouchard, 1991), and that changes in muscle strength are a direct result of muscle hypertrophy (Lieber, 1992). Thus, changes in isometric force as measured by grip strength dur- ing childhood are attributed mainly to increases in muscle mass, with some role played by the changing quality of muscle tissue and/or maturation of neural control of volitional exertion of maximum force (Ma- lina, 1986). The growth of muscle fibers, directed by their use and by maturation of the nervous system, is still poorly understood (Vogler and Bove, 1985). Data on changes in muscle composition during childhood and adolescence are scarce. They indicate small, if any, changes in the relative contribution of type I and type II fibers, and an increase in the amount of intracellular protein and DNA. Due to small sample sizes and sampling problems, how- ever, these data should be treated with caution (Ma- lina, 1986). Besides, it seems that the slow- and fast-twitch fibers generate similar specific tensions (Fitts et al., 1991), and thus changes in the propor- tions of various fiber types during growth should not affect strength. Although muscle architecture, espe- cially the angle of fibers to the tendon, affects power output (Morris, 1948), there seems to be little change in the basic morphology of muscles during later childhood and adolescence. Since the size of muscle fibers seems to be the major factor responsible for the development of strength, differences in strength between children growing in different socioeconomic conditions have typically been attributed to the differential growth of muscle mass (Benefice, 1990; Malina and Bou- chard, 1991). However, it is possible that other fea- tures of muscle morphology and physiology also play a significant role (Malina et al., 1987). Particularly important would be variation in muscle quality as evidenced, for example, in specific strength, i.e., force exerted by a unit of muscle size. Since strength is one of the basic determinants of human physical performance, and thus the capacity for manual labor, it is important to ascertain Grant sponsor: Medical Research Council (South Africa); Grant sponsor: Foundation for Research Development; Grant sponsor: Uni- versity of Cape Town; Grant sponsor: University of Witwatersrand. *Correspondence to: M. Henneberg, Department of Anatomical Sci- ences, University of Adelaide Medical School, Adelaide, South Aus- tralia 5005, Australia. E-mail: maciej.henneberg@adelaide.edu.au Received 8 April 1999; accepted 31 January 2001. AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 115:62–70 (2001) © 2001 WILEY-LISS, INC.