Exercise professionals routinely prescribe resistance training to clients with varied goals. Therefore, they need
to be able to modify the difficulty of a variety of exercises and to understand how such modifications can alter
the relative joint loading on their clients so to maximise the potential for positive adaptation and to minimise
injury risk. This paper is the first in a three part series that will examine how a variety of biomechanical
principles and concepts have direct relevance to the prescription of resistance training for the general and
athletic populations as well as for musculoskeletal injury rehabilitation. In this paper, we start by defining the
terms moment (torque), moment arms, compressive, tensile and shear forces as well as joint stress (pressure).
We then demonstrate how an understanding of moments and moment arms is integral to the exercise
professionals’ ability to develop a systematic progression of variations of common exercises. In particular, we
examine how a variety of factors including joint range of motion, body orientation, type of external loading,
the lifter’s anthropometric proportions and the position of the external load will influence the difficulty of each
exercise variation. We then highlight the primary results of several selected studies which have compared the
resistance moment arms and joint moments, forces or stresses that are encountered during selected variations
of common lower body resistance training exercises. We hope that exercise professionals will benefit from this
knowledge of applied resistance training biomechanics and be better able to systematically progress exercise
difficulty and to modify joint loading as a result. The two remaining articles in this series will focus on the
neuromechanical properties of the human musculoskeletal system and better understanding the biomechanical
implications of a variety of alternative resistance training techniques, respectively.