Static loads on the knee and ankle for two modalities of the isometric smith squat

Abstract

Introduction: The back squat is a popular strength training exercise that recruits approximately 75% of the muscular system. However, knowledge of muscular and joint loads incurred when performing two variations of the back squat, namely the high bar and the low bar isometric parallel-depth Smith squat, is limited. Therefore this study aims to determine the lower limb muscle forces and the compressive and shear joint forces at the knee and ankle incurred in these two subtle variations of the one repetition maximum (1RM) isometric Smith squat.
Method: Eight healthy male 400-m sprinters participated in the study. The participants performed the two modalities of the squat using a 7° backward-inclined Smith machine. The bottom of the squat corresponded to a position in which the thighs are parallel to the ground. The mean ± SD 1RM external load for the eight participants was 100.3 ± 7.2 kg. During the squat, the participants paused for 2-3 s at the bottom of the squat. This was, therefore, considered a static position for the calculation of isometric muscle forces and joint loads using static mechanical analysis. Moment arms, and joint and segmental angles were calculated from video images of the squat obtained at 25 Hz. Internal forces were computed using a geometrical model of the lower limb.
Results: Quadriceps muscle and knee joint forces were higher in the high bar squat; where, the mean patellofemoral joint reaction force was 3.7 body weights (BW). The ankle extensor muscle and ankle joint forces were larger in the low bar squat; whereby, the mean compressive force at the ankle joint was 3.0 BW.
Discussion: The high bar squatting modality may be avoided in the rehabilitation of ACL injury. Conversely, the low bar technique may be discouraged in conditions of ankle joint instability, strained Achilles tendon, and damaged gastrocnemius and soleus muscles. The findings of the static biomechanical evaluation provide an in-depth understanding of the musculoskeletal loads associated with the two squat modalities in isometric conditions and offer a foundation for the dynamic modelling of the high bar and low bar Smith squat. Further, the knowledge gained can be used for the prevention of injury in strength training and in the design of rehabilitation programs that control muscle recruitment and joint loads.