Exercise is believed to enhance bone growth and bone strength, although the optimal prescription for osteogenesis has yet to be determined. The essential stimulus for bone growth includes compressive strain and the hydrostatic stress that it produces (12). Exercises offering high strain rates and magnitudes are believed to be optimal for bone adaptation (9,12), and ground reaction forces (GRFs) have been proposed as an estimate of the skeletal response to compressive strain (13). However, the mode of exercise that offers the highest GRFs and the best kinetic profile to promote osteogenesis has yet to be determined.
Some modes of exercise and their osteogenic potential have been studied. Results of these studies reveal that exercise such as walking produces little effect on bone development (6). On the other hand, some evidence demonstrates that jogging results in small increases in lumbar spine bone mineral density (BMD) (10). Literature reviews indicate that lower intensity endurance exercise such as running may be less effective (11) compared with strengthening and weight-bearing exercises, which may promote bone development if the overload is sufficient (3).
Participation in athletic activities such as weightlifting is thought to be useful for the development of bone mineral content (BMC) and BMD, as evidenced by cross-sectional studies (11). Training studies examining the effect of exercise on bone development typically used some form of resistance or jump training and evaluated the effect on subject BMC or BMD (3,5,9). Results of these studies are mixed with respect to the effectiveness of the training interventions (3,5). Resistance training has been demonstrated to increase lumbar spine BMD by an average of 0.98% (5). However, a limitation of some training studies (3-5) was the incorporation of open kinetic chain exercises that would not be likely to offer high magnitudes and rates of compressive strain, which are thought to be necessary for bone development (9,12,13). Other studies have demonstrated small but significant increases in BMD in specific regions of the body, without demonstrating increased total BMD. Some of these studies used resistance training with vests that weighed a percentage of the subject's body mass (16). Thus, it is likely that the exercises employed in some studies failed to attain either a high level of strain magnitude or compressive loading to optimize bone development, potentially constraining these studies to suboptimal results. In addition to resistance training, training studies used combinations of activities such as stomping, walking, running, jumping (14), incorporated plyometrics (16), or compared running and resistance training (10). Results demonstrate small gains in bone circumferences and strength (14), BMC, or BMD (10,16).
A limited number of studies have examined kinetic variables such as tibial strain during jumping and running (8) and GRFs of select exercises such as plyometrics (1,2,7), for the purpose of quantifying exercise intensity or the potential for osteogenesis. Previous research has also compared a variety of exercise modes and their effect on measures of athletic performance such as power (15). However, no study has compared a variety of exercise modes for the purpose of evaluating the magnitude and rate of mechanical loading and their relative potential as an osteogenic stimulus. The purpose of this study was to assess the impulse (I), peak GRFs, and rate of force development (RFD) during a variety of exercise modes including walking and jogging, as well as during plyometrics, maximum power training, and resistance training, represented by the depth jump, jump squat, and back squat, respectively.
Experimental Approach to the Problem
This study used a randomized repeated measures research design to compare 5 modes of exercise and their resultant kinetic characteristics. The research hypothesis was that there were differences between the modes of exercise for each of the variables assessed. The independent variable included the exercise modes assessed that consisted of walking, jogging, depth jump normalized to the subject's countermovement jump height, jump squat with a load equal to 30% of the subject's back squat 1 repetition maximum (RM) load, and the back squat with the subject's 5 RM load. The dependent variables included I, landing GRF, and landing RFD.
Twenty-three subjects (mean ± SD, age 21.2 ± 1.4 years; body mass 77.8 ± 16.2 kg) who were recreationally fit participated in this study. Inclusion criteria required subjects who were 18-27 years old, participated in high school or college sports, were without orthopedic lower limb or known cardiovascular pathology, and who had no contraindications to resistance or plyometric training. All subjects provided written informed consent, and the study was approved by the university's internal review board.
Subjects participated in a pretest habituation session where their 5 RM back squat load and countermovement jump height were assessed and the test exercises were practiced. After at least 72 hours of recovery, subjects returned for the test session and performed 2 repetitions of each of the randomly ordered test exercises including the back squat with the subject's 5 RM load, jump squat with 30% of the subject's estimated 1 RM back squat load based on 5 RM testing, depth jump from a box height equal to the subject's countermovement jump, and jogging and walking across the force platform. Subjects were instructed to perform back squats and jump squats at maximal volitional velocity. Depth jump landings were performed at self-selected stiffness, although subjects were instructed to land and perform a countermovement jump as fast as possible. Walking and jogging were performed at a self-selected pace. Range of motion and velocity of each of the test exercises were not further controlled because doing so would potentially diminish external validity.
Depth jump box heights were individualized to countermovement jump heights using a commercially manufactured plyometric box with 0.75-in. layers of pieces of rubber flooring added to the top of the box to equal each subject's specific jump height.
Prior to the pretest habituation and test sessions, subjects warmed up for 5 minutes on a cycle ergometer and performed dynamic stretching exercises including 5 repetitions of each of the following: slow and fast bodyweight squats; forward, backward, and lateral lunges; walking quadriceps and hamstring stretches; and 5 countermovement jumps of increasing intensity.
The test exercise modes were assessed with a 60 × 120-cm force platform (BP6001200; Advanced Mechanical Technologies Incorporated, Watertown, MA, USA), which was bolted to the laboratory floor according to manufacturer's specifications and mounted flush in the center of a 122 × 244-cm weightlifting platform. The force platform was calibrated with known loads to the voltage recorded prior to the testing session. Kinetic data were collected at 1,000 Hz, real time displayed, and saved with the use of computer software (BioAnalysis 3.1; Advanced Mechanical Technologies, Inc.) for later analysis. Impulse, GRF, and RFD were calculated from the force-time records consistent with methods previously used (2). All values were determined as the average of 2 trials for each exercise. Impulse was defined as the area under the curve of the force-time record for the positive acceleration phase and the landing phase from the point of ground contact to the time when the GRF equaled body mass. Peak GRF occurred during the landing phase of each exercise mode and was defined as the highest value attained. The RFD was defined as the first peak of GRF minus the initial GRF on landing divided by the time to the first peak of GRF minus the time of initial GRF (1) and normalized to a second.
The statistical analyses were undertaken with SPSS 16.0 (SPSS, Inc., Chicago, IL, USA) using a repeated measures analysis of variance to assess differences in I, GRF, and RFD between the exercise modes. Significant main effects were further analyzed with Bonferroni-adjusted pairwise comparison to identify the specific differences between the exercise modes. A Pearson correlation analysis was conducted to assess the potential relationship between walking and jogging velocity and I, GRF, and RFD. Assumptions for linearity of statistics were tested and met. An a priori alpha level of p ≤ 0.05 was used with power and effect size represented by d and ηp2, respectively.
Significant main effects were found demonstrating differences between the exercise modes for each variable assessed including I (p ≤ 0.001, d = 1.00, ηp2 = 0.79), GRF (p ≤ 0.001, d = 1.00, ηp2 = 0.78), and RFD (p ≤ 0.001, d = 1.00, ηp2 = 0.64). Bonferroni-adjusted post hoc analysis demonstrated that I and GRF were different between all the exercise modes. Post hoc analysis also revealed that RFD was different between all exercise modes except for the back squat and jogging. The specific differences in I, GRF, and RFD, for each exercise mode, are shown in Figures 1-3. Subject's walking and jogging velocity averaged 1.47 ± 0.14 and 2.65 ± 0.26 m·s−1, respectively. Walking velocity did not correlate with walking I (p = 0.75), GRF (p = 0.61), and RFD (p = 0.56), respectively. Similarly, jogging velocity did not correlate with jogging I (p = 0.68), GRF (p = 0.42), and RFD (p = 0.13), respectively.
This is the first known study to quantify kinetic data of a variety of exercise modes to estimate the magnitude and rate of mechanical loading and the potential of each exercise mode as an osteogenic stimulus. This study demonstrated that the back squat produced the highest I, while the depth jump elicited the highest GRF and RFD, of the exercise modes assessed. The jump squat produced the second highest I, GRF, and RFD. Thus, based on this analysis, the back squat, depth jump, and jump squat each appears to offer the greatest potential as an osteogenic stimulus and a mixed mode training approach may be useful. Jogging and walking are likely to be less effective osteogenic stimuli based on this analysis.
Ground reaction forces have been proposed to estimate the skeletal response to an exercise stimulus (13). In the present study, analyses of kinetic variables such as I and GRF were used to estimate strain magnitude, whereas RFD was used to quantify strain rate because the magnitude and rate of mechanical loading have been proposed to be important for bone development (9,12). However, it is recognized that strain rate and magnitude are not mutually exclusive because mechanical loading varies strain rate in direct proportion to strain magnitude (13).
Previous research used data derived from GRFs to quantify the intensity of specific types of exercise, including various jump landings (1,2,7). Depth jumps have been demonstrated to produce the highest GRF and landing RFD of a variety of jumping exercises assessed (2,7), and their kinetic characteristics have been defined for the purpose of establishing force magnitude and loading rates (1). For these reasons, the depth jump was included for analysis in the present study and it is not surprising that it produced the highest GRF and RFD. However, unlike previous studies (1,2,7), the present study sought to compare the depth jump with a variety of other exercise modes to provide a more comprehensive understanding of which modes may have the best osteogenic potential. While most studies do not evaluate differences between exercise modes, one study compared the depth jump and running, by assessing tibial strain, compression, and tension with strain gauge bone staples (8). Results indicate no significant difference between strains during running and depth jumps, although compression and tension were significantly higher during running. These findings differ, in part, from the present study, which demonstrated that depth jump produced I, GRF, and RFD that were 66.0, 63.6, and 75.5% higher, respectively, than those produced during jogging. Some of these differences between the studies may be attributable to the outcome measures used as well as differences in depth jump box height, which was normalized to subject's countermovement jump height in the present study and was greater on average than the 52-cm box used by all subjects in the study by Milgrom et al. (8). No data are available with respect to the relationship between this box height and subject's jumping ability (8). Normalizing depth jump height to individual ability is required because using the one box height for all subjects potentially produces a stimulus that may be more or less than individual ability, and any decision about what height to use would be arbitrary. The use of higher boxes would inflate the GRFs, whereas lower box heights would reduce those forces. Additionally, in this present study, the squat and jump squat were normalized to individual ability, thus the depth jump needed to be normalized to individual ability as well. Finally, the jogging velocity in the present study was lower than running velocities used in other studies, which may lead to comparatively lower kinetic values (8).
Training studies have assessed the effect of resistance training on bone development, finding no changes in total body BMD (3,5) or small changes in lumbar spine BMD (5). A meta-analysis yielded mixed results with respect to bone adaptation to resistance training, with some evidence that training is effective (5). Previous research has used open kinetic chain upper-body resistance training exercises such as the bench press, biceps curls, supine flys, “lat” pull-downs, military press, and wrist curls as well as open kinetic chain lower-body exercises such as the leg press, leg curl, and leg extension, demonstrating no change in total body BMD and small but significant changes in lumbar spine and femur trochanter BMD of 1.9 and 2.0%, respectively (4,5). Others have included weighted vest training in addition to plyometric training, demonstrating no changes in total body bone mass but a significant increase in greater trochanter BMC compared with controls (16). There is a lack of theoretical support for the inclusion of exercises or exercise loads that fail to maximize compressive strain. In fact, low-intensity resistance training is not likely to be effective (3). Results of the present study suggest that resistance training exercises offering high compressive strain, such as the back squat performed with 5 RM loads, warrant inclusion in research and clinical training programs due to the high levels of I produced.
In the present study, the depth jump offered the highest GRF and RFD, demonstrating why plyometric training has shown some potential for increasing BMD (16) and bone circumference (14). Previously, loaded jumps such as the jump squat were found to have the highest power output compared with exercises such as back squats and depth jumps (15). In the present study, the jump squat produced the second highest I, GRF, and RFD, which is most likely due to a combination of exercise characteristics including the added mass and the acceleration of gravity upon landing from the jump. Thus, in addition to potential athletic performance enhancement via power production, the jump squat deserves consideration for inclusion in programs designed to potentially maximize osteogenic potential.
Based on the results of this study, walking and jogging appear to demonstrate less osteogenic potential than the other modes assessed. Research examining the role of walking on bone development demonstrated no significant changes in lumbar spine and femoral neck BMD (6). This finding is not surprising given the comparatively low I, GRF, and RFD demonstrated during walking in the present study. No previous research has compared the acute kinetic differences between walking, jogging, and other modes of training. Previous research has shown 1.3 and 1.2% increases in BMD in response to both running and weight training programs, respectively (10).
The results of this study and the implications for osteogenesis are most applicable to the axial skeleton through which the load is distributed regardless of the number of lower limb contact points. While the depth jumps, jump squats, and back squats were performed in bilateral conditions, kinetic data during walking and jogging were obtained in the unilateral condition. Thus, the I, GRF, and RFD are experienced through 1 leg. To equate the bilateral and unilateral distributions of GRF, the values for some of the outcome variables such as I and GRF should be divided in half for the depth jump, jump squat, and back squat to allow for comparison with unilateral exercises such as jogging and walking to better understand the potential effect of these exercise modes on the appendicular skeleton.
The results of this study are most germane to exercises performed with the loads used in this study. Performing exercises such as the back squat or loaded jump squat with different percentages of the RM than those used in the present study will likely change the I, GRF, and RFD and potentially change the ranking of each of the exercise modes with respect to these variables. Subjects were instructed to perform the back squat, jump squat, and depth jump at maximal velocity and to walk and jog at a self-selected pace. Further controlling exercise velocity and range of motion may have increased the internal validity but at a cost of decreased external validity. It is believed that in applied settings, there is variability in the performance of these exercises. Thus, external validity was prioritized in this study.
Future training and research designed to promote bone development via exercise should include a mixed mode training strategy using exercises offering compressive strains (12) including high load exercises such as the back squat and high rate of loading such as the depth jump. Additionally, exercises previously referred to as maximal power training such as the loaded jump squat should be considered in program design because this exercise offers a relatively high combination of I, GRF, and RFD.
This study quantifies kinetic variables associated with a variety of exercise modes for the purpose of approximating the strain rate and magnitude and thus each mode's potential as an osteogenic stimulus. Results demonstrate that exercises such as the depth jump produced the greatest GRF and RFD, while the back squat created the highest I. Jump squats produced the second highest levels of I, GRF, and RFD. Walking and jogging demonstrated the lowest I, GRF, and RFD. Based on this information, practitioners and researchers should prescribe a mixed mode training strategy including exercises such as the depth jump, back squat, and jump squat in an attempt to maximize potential bone development.
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