Exercise can influence bone size, geometry, and density, thus decreasing the potential for osteoporosis and increasing resistance to fracture (14). Exercises that promote osteogenesis are of particular importance for those who are at an increased risk of impaired bone health (8). The nature of the exercise prescription is likely to affect osteogenic potential (10). Although it is difficult to assess the osteogenic potential of exercise in vivo, ground reaction force (GRF) data are often used as an indirect measure (1,13). The osteogenic potential of exercise has been proposed to be a function of the magnitude and rate of force development (RFD) (13) and has been evaluated using peak vertical GRF (1,4,13) and eccentric RFD (4) during resistance training exercises.
Some types of exercise may be better than others for promoting osteogenesis. For example, ground-based resistance training exercise programs produced greater gains in the development of subjects' femoral neck bone mineral density (BMD) compared with an exercise program focused on joint reaction forces, typified by open kinetic chain exercises (9). Similarly, Cussler et al. (3) found the back squat to be the most effective exercise at increasing BMD at the trochanter of the femur, compared with open chain exercises (3). The back squat is a common resistance training exercise that is believed to increase athletic ability and strength and enhance ligament and bone strength (2). The back squat has been compared with other modes of training including loaded squat jumps, depth jumps, running, and walking in an attempt to quantify the kinetic osteogenetic potential via GRF (4). The kinetic characteristics of some variations of the back squat exercise have been previously evaluated (5,6). Typically, researchers used loads up to 80% (11) or 5RM when comparing the osteogenic potential of the back squat to other modes of exercise (4). However, no study has compared exercise load conditions at 1 repetition maximum (1RM), or in excess of the 1RM, performed with reduced range of motion, to assess the osteogenic potential.
During dynamic exercises such as the back squat, the external torque is a function of the magnitude of the external load and the moment arm through which the load is expressed. The moment arm changes throughout the range of motion of dynamic exercise (7). For example, during the back squat, the knee joint moment is the longest at approximately the same point when the knee flexion angle is the greatest (6). Thus, the external torque is greatest during the deeper portion of the back squat. This observation suggests that subjects must use a lower training load when performing the back squat with greater compared with less depth. On the other hand, when performed with less range of motion, exercisers can handle larger back squat loads. As a result, limiting the squat depth allows for a greater magnitude of the load and therefore osteogenic potential. To date, no study has examined the osteogenic potential of back squats using supermaximal loads.
The purpose of this study was to assess the peak vertical GRF, GRF normalized to body weight, and RFD for both the eccentric and concentric phases of the back squat exercise performed at 80, 100, and 120% of the subject's 1RM, to assess the osteogenic potential of submaximal, maximal, and supermaximal back squat loading variations.
Experimental Approach to the Problem
This study assessed the hypothesis that GRF and RFD would be different during back squats of various loading conditions. Independent variables included the back squat load condition including 80, 100, and 120% of the subject's 1RM and the eccentric and concentric phases of the back squat. Dependent variables included the subject's GRF and RFD during each of these load conditions and exercise phases.
The subjects included 12 men (mean ± SD; age = 22.42 ± 2.54 years; height = 175.05 ± 7.18 cm; body mass = 83.75 ± 15.25 kg; squat 1RM = 157.10 ± 28.61 kg). These subjects had participated in 2.67 ± 1.07 high school sports for an average of 3.75 ± 0.87 years and 1.08 ± 1.78 college sports for an average of 1.67 ± 1.07 years. These subjects participated in 3.83 ± 0.93 resistance training sessions per week, 1.21 ± 1.53 plyometric training sessions per week, and 1.92 ± 1.74 aerobic training sessions per week. Inclusion criteria consisted of men who participated in lower body resistance training at least twice per week for 8 weeks preceding the study. Exclusion criteria included any orthopedic lower limb pathology that restricted athletic functioning, known cardiovascular pathology, or inability to perform exercises with maximal effort. All the subjects provided informed consent before the study, and the university's internal review board approved the study.
The subjects participated in a habituation and test session. Before each session, the subjects warmed up for 3 minutes on a cycle ergometer. The subjects also performed 5 slow bodyweight squats, 10-yd forward walking lunge, 10-yd backward walking lunge, 10-yd walking hamstring stretch, 10-yd walking quadriceps stretch, 20-yd skip, and 5 countermovement jumps of increasing intensity. The subjects then rested for 2 minutes.
During the habituation session, the subject's age, body mass, height, and history of athletic participation were assessed. The subjects also performed their back squat 5RM down to a knee angle of 90° to determine their estimated 1RM and determine their loads for the testing session. Before the 5RM test, the subjects warmed up with 2 sets of 3 reps at approximately 75 and 90% of their self-assessed maximum ability. The subjects also performed 2 sets of 1 repetition in the supermaximal condition, characterized by a load of 120% of the 1RM, at a knee angle of 65° to become familiar with this testing condition. All knee angles were determined by a manual goniometer.
The subjects then returned for the testing session. During this time, they warmed up using the same warm-up protocol as the one used in the habituation session. After 5 minutes of rest, the subjects performed 2 sets of 1 repetition of the back squat in the randomly ordered test conditions with 5 minutes rest between sets. Test conditions included the back squat performed at 80, 100 and 120% of the subject's estimated 1RM. These load conditions were used because previous research investigating osteogenic potential used loads that were limited to 80% of the subject's 1RM (11) or 5RM (4) back squat load. Thus, the loading conditions in this study assessed loads similar to what had been studied and also compared those loading conditions with maximal loads and supermaximal loads, in conditions that increased by equal increments of 20% up to the 120% of the subject's 1RM.
The subjects performed the loading conditions of 80 and 100% of the subjects' estimated 1RM loads at approximately 90° of knee flexion. The set at 120% of estimated 1RM load was performed at 65° of knee flexion, because it was not possible to perform the exercise to 90° of knee flexion with the supermaximal load.
All exercises were performed on a force platform (BP6001200, Advanced Mechanical Technologies Incorporated, Watertown, MA, USA), which was calibrated with known loads to the voltage recorded before the testing session. Kinetic data were collected at 1,000 Hz, real time displayed, and saved by using computer software (BioAnalysis 3.1, Advanced Mechanical Technologies, Inc., Watertown, MA, USA) for later analysis.
Kinetic data were analyzed for peak vertical GRF, GRF normalized to body weight, and RFD for both the eccentric and concentric phases of each back squat condition. All the values were averaged using 2 test trials. Peak vertical GRF was defined as the highest value attained during the eccentric and concentric phases of each exercise. The RFD was defined as the peak vertical GRF minus the vertical GRF occurring 100 milliseconds before the peak vertical GRF and normalized to a second for both the eccentric and concentric phases. Figure 1 shows a sample force-time record for the squat performed in the 80% condition.
Data were evaluated with SPSS 18.0 for Windows (Microsoft Corporation, Redmond, WA, USA) using a 2-way repeated measures analysis of variance to determine statistical differences in kinetic data between the exercises and the interaction between GRF and RFD and the eccentric and concentric phase. Significant main effects were further evaluated using Bonferroni adjusted pairwise comparisons. Assumptions for linearity of statistics were tested and met. Statistical power (d) and effect size (η2) are reported, and all data are expressed as mean ± SD.
Analysis of GRF demonstrated significant main effects for both the eccentric (p ≤ 0.001, η2 = 0.92, d = 1.00) and concentric (p ≤ 0.001, η2 = 0.93, d = 1.00) phases, indicating differences in GRF, among the 3 squat loading conditions. There was no significant interaction between GRF and eccentric and concentric phases (p = 0.11, η2 = 0.10, d = 0.46). Analysis of GRF normalized to body weight demonstrated significant main effects for both the eccentric (p ≤ 0.001, η2 = 0.91, d = 1.00) and concentric (p ≤ 0.001, η2 = 0.91, d = 1.00) phases, indicating differences in GRF normalized to body weight among the 3 squat loading conditions. There was no significant interaction between GRF normalized to body weight and eccentric and concentric phases (p = 0.17).
Analysis of RFD demonstrated no significant main effects for the eccentric (p = 0.09, η2 = 0.20, d = 0.48) and concentric (p = 0.38, η2 = 0.08, d = 0.20) phases, indicating no differences in RFD among the 3 back squat load conditions, though the eccentric RFD was approaching significance. There was no significant interaction between GRF and eccentric and concentric phases (p = 0.33, η2 = 0.10, d = 0.23). Significant main effects were further evaluated for both the eccentric and concentric phases and are described in Tables 1 and 2. Individual GRF data are shown for each subject in each back squat loading condition for the eccentric (Figure 2) and concentric phases (Figure 3). Descriptive RFD data are shown in Table 3.
This study demonstrates that performing the back squat with loads of 120% of the estimated 1RM, through a range of motion of approximately 65° develops higher GRF than when performing the back squat with maximal or submaximal loads of 100 and 80% of the 1RM, respectively. Thus, performing this exercise with supermaximal loads may be useful for bone development because the magnitude of the load is believed to be osteogenic (2,4,13). Previous research found increased lumbar spine and femoral neck BMD during exercise protocols with resistance training loads up to 80% (11). No previous study has examined resistance training with the use of loads exceeding 100% of the 1RM. The results of this study suggest that supermaximal loads may be a more potent osteogenic stimulus. In this study, a mean increase in back squat load of 20%, from the condition of 100% of the 1RM to 120% of the 1RM, resulted in a mean increase in eccentric and concentric GRFs of approximately 9 and 13%, respectively. Minimal individual variation was observed and data from each subject (Figures 2 and 3) show that most subjects responded similarly with large power values and effect sizes found for all statistically significant findings.
Although the relationship between back squat load and peak GRF may be intuitive, it has not been previously investigated across a loading continuum or during supermaximal loading conditions. In fact, previous research demonstrated that high load back squats offered less GRF compared with lower load exercises such as jump squats (4), and research examining the osteogenic potential of resistance training exercises has not exceed loads of 80% of the 1RM (11).
Back squat range of motion, and the concomitant increase in the length of the moment arm of the resistance force throughout the range of motion, likely reduces the possible manageable training load, and thus GRF, because the moment arm of the resistance is greatest as a subject descends during the exercise (6). Programs designed to optimize osteogenesis may benefit from the inclusion of exercises such as the back squats with loads in excess of 100% of the 1RM but through a smaller range of motion. Previous research has also demonstrated that other modes of training including loaded squat jumps and depth jumps, which produce high GRF and RFD, may be valuable training modes for the potential promotion of osteogenesis as well (4). Thus, a multimodal exercise prescription is likely to be optimal for bone development as it is for other types of training adaptation such as the development of explosive muscular power, as has been proposed by Newton and Kraemer (12).
From the perspective of sport specificity, training for athletic development may require performing the back squat with a range of motion that is greater than the 65° of knee flexion that was used in this study, particularly for sports that require athletes to function in lower positions (2). Thus, training that seeks to maximize osteogenesis with limited range of motion and supermaximal loads should be supplemented with larger range of motion back squats for sport specificity as well.
In this study, RFD was not significantly different between exercise conditions. Significant subject variability was present with respect to the speed of the eccentric and concentric phases despite the fact that all the subjects in this study were instructed to perform the test exercises at maximal volitional velocity in all test conditions. Nonetheless, the eccentric RFD approached but did not reach statistical significance during the back squat with the 100% of the estimated 1RM, which demonstrated the highest mean value. The concentric RFD demonstrated a pattern similar to the mean eccentric RFD data, with the highest mean RFD occurring during the back squat condition that employed 100% of the estimated 1RM.
The back squat condition at 120% of the estimated 1RM demonstrated mean RFD values that were slightly greater than the condition at 80% of the estimated 1RM. These data suggest that the RFD may not be associated with the lightest load. In contrast, previous research demonstrated that eccentric RFD was greatest during depth jump landings and was progressively lower during jump squats at 30% of the subject's back squat 1RM and back squats performed with 5RM loads (4). The results of this study are most generalizable to exercisers who are most similar to the subjects used in this study, which included fit and healthy young men.
Exercises that produce large GRF are likely to be potent osteogenic stimuli. Performing the back squat at loads of 120% of the estimated 1RM, accomplished with reduced range of motion, results in a higher GRF than does the back squat performed at 80 or 100% of the 1RM. Thus, supermaximal loads may have potential as part of a resistance training program designed to promote osteogenesis. This form of training is most likely useful for exercisers most similar to those used as the subjects such in this study. The use of supermaximal loads should be limited to healthy subjects and if prescribed, exercisers should be gradually introduced to loads exceeding 100% of the 1RM, consistent with the principle of progressive overload.
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