Modified vector coding analysis of trunk and lower extremity kinematics during maximum and sub-maximum back squats
Introduction
The squat is a highly utilized, multi-joint resistance training exercise that requires coordinated efforts spanning the trunk, pelvis, and lower extremity musculature. The loads for the hip and knee joints comprise 80–90%, whereas the ankle contributes 0–20%, of the total lower extremity moment to lift the system mass during the upward (concentric) portion of back squats (Escamilla, 2001, Flanagan et al., 2015, Flanagan and Salem, 2008, Fry et al., 2003, Hirata and Duarte, 2007, Lorenzetti et al., 2012). Previous research has delineated the concentric (upward) portion of the back squat into three distinct regions: Acceleration, when vertical bar velocity is increasing from zero to peak positive, Sticking/Failure Region, vertical velocity decreases to a local minima, and finally the Strength and Deceleration Region which ends when the person is fully standing erect (Escamilla et al., 2000). The “Sticking Region” has garnered a great deal of attention, as research aims to understand how the body overcomes the external resistance (Bryanton et al., 2012, Kubo et al., 2018), leading to a success instead of failure (Kompf and Arandjelović, 2017, Kompf and Arandjelović, 2016, van den Tillaar, 2015, van den Tillaar et al., 2014). Failure of a maximum load back squat is most frequently blamed on physiological parameters (Cramer et al., 2015, Duncan et al., 2014, Eslava et al., 2006, Mathew et al., 2016). Largely, discrete variables have been the focal point of biomechanical assessments of the back squat. However, there is no consensus within the current literature of a primary/singular cause of failure to complete back squats. For example, Yavuz, et al. identified a slightly more forward trunk lean in a 1RM back squat compared to a 90% 1RM back squat, while muscle activation at both conditions were not different in any lower extremity muscle observed (Yavuz and Erdag, 2017). However, van den Tillaar, et al. identified decreased biceps femoris and increased soleus muscle activation between the Acceleration and Sticking Regions of a maximum back squat (van den Tillaar et al., 2014). Van den Tillaar et al. also found increased rectus femoris and decreased gluteus maximus muscle activation between the Acceleration and Sticking Regions in a follow-up study, with no differences in the biceps femoris (van den Tillaar, 2015). Escamilla, et al. hypothesized that the decrease in vertical velocity creates an insurmountable resistance that the muscles are unable to overcome (Escamilla et al., 2001). Based on the lack of unanimous results of the current literature, there is a need for further research examining factors that impact maximum/supramaximum lifts.
Although a vast majority of previous literature has described back squats using discrete variables (Escamilla, 2001, Escamilla et al., 2001, Kritz et al., 2009, Schoenfeld, 2010), an analysis of the trunk/lower extremities as a coordinated functional unit has yet to be attempted (i.e. dynamical systems approach). The dynamical systems approach recognizes that the human body is a complex system with many components that influence movement and incorporates that understanding into evaluation (Glazier et al., 2003). Analyses of joint/segment coordination, a subset of dynamical systems, has advanced our understanding of movement in multiple areas, including running injuries (Hamill et al., 2012, Heiderscheit et al., 2002), increasing speeds (Hafer and Boyer, 2017), anticipated/unanticipated dynamic tasks (Weir et al., 2019), and the injury susceptibility during landings (DiCesare et al., 2019). Considering the back squat requires complex interactions between the joints/segments of the lower extremities and trunk, a dynamical systems approach could elicit further insights into the coordination required for success with maximal loading.
Angle-angle plots allow for examinations of the coupling of segments between limbs, between segments or within a segment. Modified vector coding has been implemented to analyze coordination of joints and segments during different movement patterns (Hamill et al., 2012). For example, vector coding has been used to analyze intra-limb coordination (Needham et al., 2015) during cutting maneuvers (Samaan et al., 2015), coordination variability (Ferber et al., 2005, Miller et al., 2010), and inter-segment coordination (Chang et al., 2008, Needham et al., 2014). Although back squats are quite different from the previous analyses, joint coupling should likely exist given the need for simultaneous extension of the hip, knee, and ankle joints to raise the trunk/weight and lower extremities during the upward phase (Escamilla et al., 2001). Thus, a technique such as modified vector coding could be useful for analyzing the complex coordination patterns during back squats.
Therefore, the purposes of this study were to 1) describe coupling of the shank and thigh, thigh and trunk, and knee and hip and 2) present the effects of bar load on segment and joint coupling during the concentric phase of back squats. We hypothesized that 1) both thigh-trunk movements and knee-hip movements would be uncoupled during the Sticking Region, demonstrating a heavy focus on raising the trunk and hips and 2) segment and joint uncoupling would increase with greater bar loads.
Section snippets
Participants
This study was approved by the Institutional Review Board. Fourteen healthy adults (Table 1) were recruited to participate in this study. To be included in this study, participants had to be 18–55 years, have no history of knee injuries, have at least one-year experience back squatting at or near maximal loads, and have performed weighted squats at least one-day per week. Exclusion criteria included any lower extremity injuries in the past three months, knee pain in the past six months, a
Results
Statistical significance level was reached for the overall MANOVA (F = 6.79, p = 0.011; ηp2 = 0.982). Reports from statistically significant within-subjects ANOVAs are provided in Table 2.
Discussion
During the dynamic movement provided by the back squat, the shank and thigh act in a closed chain manner, while the trunk is free to move as an open chain segment. While our first hypothesis was supported (thigh-trunk and knee-hip were not coupled during Sticking Region), our second hypothesis was only partially supported. Increased loads demonstrated coupled thigh rising and trunk falling, whereas the Submax condition was almost completely focused on thigh-rising only during the Sticking
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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