This article written 30 years ago provides substantial support for the Applied Functional Science® “truths of human movement” proposed by Dr. Gary Gray over the past 40 years. Many subsequent articles by researchers trained under Dr. Zajac at Stanford University have used mechanical modeling with inverse and forward dynamics to understand the role of muscles during function. The difference between the functions ascribed to muscles based purely on anatomy and the roles of muscles during activity-specific, multi-joint movements is monumentally important to training and rehabilitation.