Free Weight versus Smith Machine Squats
Lifting techniques are defined by very specific joint actions for safe and effective performance. Therefore proper biomechanics is one of the most important factors to take into account when selecting strength exercises. One classical example is the use of traditional free weight squats (TBS) and Smith machine squats (SMS) as interchangeable lower body exercises. Several research studies have shown that the kinematics differences between both exercises promote specific muscle activation patterns and consequently, different training adaptations. The basic understanding of how the load are applied on the body or more specifically, how the moments are applied about the knee and hip joints, may provide the explanation to the differences found between these two exercises. As a basic reminder, the further the resistance is held from the body, the longer the resistance arm (distance between the lifted weight and the joint) consequentially increasing in the resultant torque of the joint.
In a traditional squat, the center of gravity of the system (defined by the trainee and weight lifted) must constantly remain over the feet of the exerciser, otherwise the system will rotate, and in this case, tip over and the trainee will fall. Therefore, in order to maintain balance all force must be applied to the center of gravity (cg) vertically. On the other hand, Smith machines allow variations in the anterior-posterior foot placement due to the linear restriction of the bar motion along the vertical axis. Individuals who have difficulty attaining proper eccentric range while maintaining appropriate knee hip relationship may gravitate to the Smith machine for better form. While trainees position their feet forward to enable bar lowering without knee translation, variations on the loads applied on the body occur, and the angular moments applied to the hip and knee joints will be highly affected (figure 2).
In 2002, Albelbeck et al evaluated the effect of six different foot placement in the torque applied at the hip and knee joints level using SMS. They found that placing the feet closer under the body resulted in greater moment about the knee (meaning more work done by the quadriceps) while the placement of the feet farther in front of the body generated greater moment about the hip joint (meaning more work done by the gluteus and hamstrings). The difference in muscle activity between both exercises was further studied (Anderson 2005 and Schwanbeck) and it was found that TBS increased the muscle activity for the gastrocnemius by 34 %, biceps femoris by 26 % and vastus lateralis by 49% compared to the SMS. Although the combined increase in prime movers and stabilizers muscle activity was around 43%, no further differences were found between trunk stabilizers muscles. Finally, exercise selection can also influence the maximal force production (1 RM)(Cotterman, 2009) depending on the prime mover used. When comparing 1 RM using free weight or Smith machines exercises, the weight lifted in a 1RM squat test was higher using Smith machine than free weights, while the weight lifted in the 1RM bench press exercise was higher using free weights.
Personal trainers should recognize that the biomechanical characteristics of Smith machine squats exercise will lead to training adaptation in a stable condition, meaning that less balance will be needed due to the lower activation of the stabilizer muscles. This will have limited or poor implication on activities of daily living (ADL’s) or athletic related movements since there is a lack of specificity on the angles and muscle used during the exercise. Additionally, it is important to recognize that the higher loads applied to the knee joint while placing the feet further away from the resistance force may increase the risk of injury if the joint is not prepared to tolerate this additional loading.