85 Journal of Applied Biomechanics, 2013, 29, 85-97 © 2013 Human Kinetics, Inc. Andrea Biscarini (Corresponding Author) is with the Department of Surgical, Radiological and Odontostomatologic Sciences, Medical Physics Section, University of Perugia, and with the LAMS Laboratory, University of Perugia, Perugia, Italy. Fabio M. Botti is with the LAMS Laboratory, University of Perugia, and with the Department of Internal Medicine, Human Physi- ology Section, University of Perugia, Perugia, Italy. Vito E. Pettorossi is with the Department of Internal Medicine, Human Physiology Section, University of Perugia, Perugia, Italy. Joint Torques and Joint Reaction Forces During Squatting With a Forward or Backward Inclined Smith Machine Andrea Biscarini, Fabio M. Botti, and Vito E. Pettorossi University of Perugia We developed a biomechanical model to determine the joint torques and loadings during squatting with a backward/forward-inclined Smith machine. The Smith squat allows a large variety of body positioning (trunk tilt, foot placement, combinations of joint angles) and easy control of weight distribution between forefoot and heel. These distinctive aspects of the exercise can be managed concurrently with the equipment inclina- tion selected to unload specifc joint structures while activating specifc muscle groups. A backward (forward) equipment inclination decreases (increases) knee torque, and compressive tibiofemoral and patellofemoral forces, while enhances (depresses) hip and lumbosacral torques. For small knee fexion angles, the strain-force on the posterior cruciate ligament increases (decreases) with a backward (forward) equipment inclination, whereas for large knee fexion angles, this behavior is reversed. In the 0 to 60 degree range of knee fexion angles, loads on both cruciate ligaments may be simultaneously suppressed by a 30 degree backward equip- ment inclination and selecting, for each value of the knee angle, specifc pairs of ankle and hip angles. The anterior cruciate ligament is safely maintained unloaded by squatting with backward equipment inclination and uniform/forward foot weight distribution. The conditions for the development of anterior cruciate liga- ment strain forces are clearly explained. Keywords: Smith squat, joint load, cruciate ligaments, line of gravity Squatting, a fundamental strengthening exercise, is an integral part of many training programs in sports and ftness, and is commonly prescribed in rehabilitative inter- ventions. Squat biomechanics have been the subject of extensive research studies with particular focus on muscle activity and safety for knee structures. 1–12 A variety of different squat techniques and equipment has been devel- oped over the years to comply with the postural, joint, and muscular needs and demands of individual people and special populations. These squat variants are primarily characterized by different modalities of administration of the resistance load, which is represented by body weight, and by the gravitational overload directly provided by weighted barbells, dumbbells, and belts, or transmitted to the body by lever and/or cable/pulley systems. The resistance load system, together with the instantaneous values of the kinematical parameters, determines muscle forces, and the joint reaction forces resulting from bony contact forces and ligament tensions. Knowledge and control of muscular and joint forces occurring during exercises is of fundamental importance in the design of suitable rehabilitation and conditioning programs, as well as for injury prevention. 13,14 Different equipment imposes specifc degrees of mechanical constraint to the squat exercise. For example, in the free barbell squat, the center of mass (C) of the system consisting of the user’s body and the weighted barbell should fall between forefoot and heel, and conse- quently hip, knee, and ankle joint angles take “in-phase” values which, within small inter-subject variability ranges, are tightly related to each other. Thus, each phase of the exercise is characterized by a well-defned joint torque distribution, i.e., by specifc ratios of the torques at different joints. The Smith machine (Technogym, Cesena, Italy) is a popular piece of equipment used in weight training and particularly in squat exercises (Smith squat). It consists of a barbell constrained to move up and down sliding along rectilinear steel tracks (Figure 1). In the Smith squat, the tracks’ reaction forces (  R S in Figure 1) acting on the barbell compensate forward or backward imbalances of C determined by backward or forward foot displacements or trunk tilts. Therefore, the value of the joint angles may be changed independently of the other, enabling a wider range of exercise positions and, concurrently, a wider An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH