His teaching and research career has spanned biological anthropology and kinesiology, and includes more than 800 publications to date. He has earned doctoral degrees in physical education (University of Wisconsin, Madison, 1963) and anthropology (University of Pennsylvania, Philadelphia, 1968), and honorary degrees (doctor honoris causa) from the Catholic University of Leuven, Belgium (1989), the Bronisław Czech Academy of Physical Education, Krakow, Poland (2001), the University School of Physical Education, Wrocław, Poland (2006), and the University of Coimbra, Portugal (2008).
His primary area of interest is the biological growth and maturation of children and adolescents in general and specifically in the context of motor development and performance, youth sports and young athletes, and the potential influence(s) of physical activity and training for sport. This interest in the growth and maturation of children extends to an ongoing study of the human biology of indigenous populations in Oaxaca, southern Mexico. The research includes studies of the growth, nutritional status, performance and physical activity of school age children, as well as of secular change in size and maturation of children, adolescents and adults. The research in Mexico began in 1968 and continues at present.