To the authors' knowledge, this is the first study to investigate the interplay between biomechanical, anthropometric, and psychological variables and their ability to predict back squat 1RM strength. Contrary to our hypothesis, multiple regression revealed that anthropometric factors were best able to predict squat performance (Table 2).
Height has been proposed to play an important role in determining squat strength, as those who are taller, or have longer femurs, may have longer resistance moment arms (37). However, it has been argued that both internal and external moment arms scale isotropically with segment lengths (57); as such, those with longer femurs may not be as disadvantaged in the squat as previously thought. This is further evidenced by height having no (49,75), or even a positive (26,63), correlation with strength performance in previous studies. Although height was found to have a positive correlation with performance in this study (r = 0.563), this finding is confounded by fat-free mass. After controlling for fat-free mass, this relationship reverses (r = −0.500), as is also the case for thigh length, whose correlation drops from r = 0.384 to −0.191 after controlling for fat-free mass. These data suggest that height may indeed be detrimental to squat proficiency.
With regards to isometric strength, the finding that the sum of the hip and knee moments was not found to have a statistical contribution to the multiple regression must be interpreted with caution. It might seem to be in contrast to previous work (77); however, these findings simply suggest that the strength variables may be redundantly correlated or do not account for enough of the variance to be contributory to the regression model. In isolation, however, it seems that single-joint strength variables are strongly correlated with squat performance (r = 0.533–0.672), and these correlation coefficients were not statistically different from one another. From these data, it is unclear if strengthening a single muscle group (e.g., knee extensors) would increase squat strength, as the strength of 1 muscle group does not seem to relate more strongly to squat strength than another. Furthermore, there are nuances to the squat that may complicate this relationship, which are discussed herein.
The complexity of the squat as a multi-joint exercise likely plays a role in the findings of this study. Hip, knee, and spinal extensors must be able to overcome the flexion moment imposed on each respective joint. However, the isometric strength of the hips and knees working in isolation may not be an extraordinarily accurate predictor as to what the bottleneck is in each individual's squat. During simultaneous hip and knee extension, the biarticular hamstrings are used to produce a hip extension moment (7,19,72), but concomitant with this utilization of the hamstrings is a larger knee extension moment because of the hamstrings' knee flexion moment (7,72). Thus, net joint moments observed during the squat do not approach 100% of what each joint can produce in isolation (9). Whether or not a single muscle group is the bottleneck in a 1RM squat for an individual is predicated on not only individual kinematics, but also the other muscles involved. The relationship between individual joint strength and multi-joint strength may be highly nonlinear and paradoxical, owing to the increased degrees of freedom of multi-joint movements. For example, in theory, if an individual has an unusually strong bi-to-monoarticular hip extensor ratio, but squats with what is traditionally considered “hip dominant” kinematics (2,11,27,45,77), then despite having a small knee extensor net joint moment for a given amount of knee flexion, the internal resistance moment that the knee extensors must overcome may be quite large because of the reliance on the hamstrings to overcome the hip extension net joint moment (77).
The maximum isometric knee and hip extension net joint moments measured in this study are greater than the knee (633 vs. 449.5 N·m) and hip (726 vs. 604.0 N·m) extension net joint moments linearly extrapolated from Swinton et al. (67) (r = 0.97), whose participants were heavier (100.2 vs. 73.76 kg) and used greater loads (220.2 vs. 126.47 kg). One should be cautious when interpreting these data, though, as these are net moments and thus describe the minimum muscular effort, or the sum of all muscles acting on the joint in a movement where cocontraction is not negligible (8). However, isometric net joint moments are collected in isolation and with minimal cocontraction. When performing simultaneous hip and knee extension, neither the rectus femoris, hamstrings, nor gluteus maximus reach maximal electromyography (EMG) amplitude (19,29,78), potentially at least partially because of reciprocal inhibition (18). Another possible consideration as to why these muscles do not reach maximum EMG amplitude is that these muscles could not produce the necessary endpoint wrench (i.e., forces and moments that interact with the environment) if they were to do so (69), as each lower extremity muscle provides a unique contribution in wrench space (36,41). Although one would expect EMG amplitudes to increase proportionally with one another until one reaches a ceiling, such an outcome may not be possible while achieving the necessary endpoint wrench (68). However, EMG observations may be confounded by electrode placement, in that the biarticular muscles that cross the thigh have multiple innervation zones (33,59,76) with corresponding functional implications (48,59). Notwithstanding these limitations, it seems that one may not be able to maximally activate the biarticular muscles of the thigh during simultaneous hip and knee extension, which suggests that humans may be less than the sum of their parts when it comes to multi-joint force production.
Although most previous research on the squat has focused on the hips and knees, few studies have investigated the role of the spinal extensors (12,67). By extrapolating previously reported traditional back squat data using a linear regression (r = 0.99), the projected L5/S1 net joint moment is 423.3 N·m at 100% 1RM (67). Because the participants in this study were much lighter than those in the study by Swinton et al. (67) (100.2 vs. 73.76 kg) and their 1RMs were much less (220.2 vs. 126.47 kg), it is likely that the L5/S1 net joint moments of the participants in this study were much lower. Nevertheless, the L5/S1 net joint moments reported by Swinton et al. (67) are lower than the maximum L5/S1 net joint moments observed in this present investigation (485 vs. 423.3 N·m). Therefore, although stronger squatters seem to have stronger spinal extensors, it does not seem that this strength would be a limiting factor in the back squat. Rather, this component may arise because of the relatively linear relationship between load and spinal extension demands (67). Indeed, the knees and hips are affected by the cocontraction of biarticular muscles and unpredictable, nonlinear changes in joint moments. By contrast, the spinal erectors need only resist the net joint moment as well as a small abdominal cocontraction, which does not necessarily increase with load. It is possible that exerting a spinal extension moment on a dynamometer differs from doing so in the back squat, perhaps due to the coordination required in the squat, while a dynamometer provides a stable testing environment. Finally, it is unlikely that abdominal cocontraction would increase the spinal extension moment requisite in the squat to a significant degree because abdominal EMG amplitudes seem to be low (3,6,74).
Although Crural index has previously been shown to be predictive of success in elite powerlifters (46), this was not the case in this study (Figure 2). In theory, Crural index may be an indicator of the “strategy” used by the lifter. McLaughlin et al. (51) found that among national-level powerlifters, the more highly skilled squatters tended to favor a more “knee dominant strategy” with less trunk lean—a strategy that may be less viable for lifters with a small Crural index. These kinematic limitations, or lack thereof, have direct kinetic implications; a larger Crural index may allow for a greater number of movement options than those with a smaller Crural index, and thus, lifters with larger Crural indices may better be able to “choose” a kinematic “strategy” that results in kinetics that are more feasible to overcome. This theory has recently been brought into question, however, as Fuglsang et al. (28) did not find a correlation between Crural index and trunk angle during the parallel squat (r ≈ −0.350). Thus, the findings of Lovera and Keogh (46) may be specific to elite lifters.
Although previous studies have elucidated the importance of PSE in strength and physical performance (23,55,56,64,73), the correlations observed in this study are unremarkable. In fact, the only remarkable correlations observed pertaining to PSE and its constituents involved experiential variables (r = 0.469–0.612) (Figure 2). Self-efficacy is predicated on previous experience (61,62,80); as one trains and gains experience with squatting, they should also become more confident in their ability to squat. The interplay between experience and PSE may explain why self-efficacy has been shown to be important in within-subject and homogeneous between-subject studies (23,55,56,64,73). This study, however, used a heterogeneous population. Self-efficacy in and of itself cannot increase the potential for an individual to generate a joint moment. However, as per previous studies, self-efficacy seems to increase the ability of an individual to make use of the muscles that one possesses.
There are several limitations to consider when interpreting the results of this study. First, testing conditions were not necessarily the same for every individual, as investigators were unable to control who was in the weight room at the time of testing or what songs were playing, which has been considered to be detrimental to internal validity (31); however, such an environment may be considered to be more ecologically valid than a laboratory setting. Second, sticking points were determined in 2D, using Dartfish and a webcam rather than motion capture with retroreflective markers, but angles obtained are similar to those previously reported (21). Third, the net joint moments obtained using the dynamometer were collected isometrically rather than dynamically. This static assumption may be justified by the fact that accelerations in maximal squats are negligible (42) and the sticking point was defined to be where velocity was minimal. However, the reductionist nature of obtaining single-joint net joint moments in the dynamometer ignores the complexity of multi-joint efforts (19). Fourth, dynamometry fails to capture 3D joint angles and moments, which likely interact with those that were measured in 2D (e.g., simultaneous hip extension, abduction, and external rotation). With respect to psychological variables, it must be noted that self-efficacy is a complex phenomenon. Although our survey has been validated as an instrument to assess this outcome, it may not wholly encompass the relevance to task specificity; the self-efficacy questionnaire used was not specific to the barbell back squat exercise. This study investigated recreationally active college students, not powerlifters or Olympic lifters, and therefore, cannot be extrapolated to such populations, as squat kinematics differ (53) and because such populations are more homogenous. Finally, participants were not allowed to wear a weight belt or knee wraps/sleeves during testing, which may have impacted their self-efficacy.
After examining psychological, anthropometric, and biomechanical factors, it was found that squat strength is primarily predicated on anthropometric variables (fat-free mass relative to height), and in a heterogeneous population, biomechanical and psychological variables seem to play secondary roles. Scientists can apply these exploratory findings in hypothesis-driven research; that is, to understand if these cross-sectional measures hold longitudinally. Practically, these results confirm the importance of weight classes in strength sports and also provide practitioners with an improved understanding of the emergent nature of back squat strength.
The authors gratefully acknowledge the contributions of Sean Butler, Michael DeJesus, Osvaldo Gonzalez, Ramon Jimenez, and Jona Kerluku in their indispensable role as research assistants in this study. The authors have no conflicts of interest to disclose.
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