IMPACT FORCES WHEN EXERCISING ON THE FREEBOUNDER® IN COMPARISON TO WALKING AND RUNNING ON A TREADMILL AND USING A MINI-TRAMPOLINE
John P. Porcari, Ph.D., Megan Thiel, B.S., and Abigail Ryskey, M.S.
University of Wisconsin-La Crosse
March 1, 2018
When choosing a weight bearing activity to incorporate into an exercise regimen, impact forces may dictate which exercise, piece of machine, or surface an individual chooses. Impact forces can range from body weight up to 12 times body weight, depending on the exercise (Hreljac, 2004). Modes of exercise that generate low impact forces may be useful for individuals looking to avoid jarring movements or athletes who are looking to rehabilitate an injury. Additionally, loading rate (i.e., the rate at which those impact forces are transmitted to the body) represents a strong predictor of injury risk (Davis, Bowser, & Mullineaux, 2015).
Rebounding has been of interest as an exercise modality since its emergence as a training tool for elite athletes in the 1970s (Esposito & Esposito, 2009). Trampoline exercise has also been used as a rehabilitation device. Improvements in mobility and activities of daily living were found in stroke patients who trained on mini-trampolines compared to patients who participated in similar exercises on level ground (Miklitsch, Krewer, Freivogel, & Steube, 2016). Exercises included the patients lifting their heels and walking in place while on the mini-trampolines.
Rebounders are considered low-impact because they are a compliant surface. Rebounders absorb and decrease the amount of impact endured by the joints, particularly in the lower extremities (McGlone, Kravitz, & Janot, 2002). Plyometric exercises, such as depth jumps and counter movement jumps occurring on a mini-trampoline, or rebounder, and the ground were studied by Crowther, Spinks, Leicht, and Spinks (2007). They found the compliant surface resulted in a reduction in impact forces during jump training.
Several studies have investigated the cardiovascular response to exercising on mini-trampolines. A study conducted by NASA in the 1970s concluded that aerobic training on a trampoline was as effective at improving VO2max as running (Bhattacharya, McCutcheon, Shvartz, & Greenleaf, 1978). It was also determined that heart rate and VO2 showed similar linear relationships while running on a treadmill and jumping on a mini-trampoline. Burandt, Porcari, Cress, Doberstein, & Foster (2016) found that mini- trampolines offered sufficient intensity to improve cardiorespiratory fitness. The study also found that exercise on a mini-trampoline fall burned the same number of calories as running 6 miles per hour on flat ground (Burandt et al., 2016).
The FREEBOUNDER® Fitness and Rehab Machine, invented by John Louis (Northfield, IL), came on the market in February of 2017. It consists of a spring loaded platform attached to a metal frame and has rebounding characteristics similar to those of mini-trampolines. The FreebounerTM is marketed as low-impact because it purportedly reduces the impact forces put on the body during an aerobic workout.
Although, numerous studies have investigated the cardiovascular response to rebounder exercise, there is very little data on the forces acting on the lower extremities during this type of exercise. The purpose of this study was to determine the impact forces put on individuals when exercising on the FREEBOUNDER® and compare those to impact forces when walking and running on a treadmill and double-leg bouncing on a mini- trampoline.
Eighteen apparently healthy male and female volunteers between 19-28 years old were recruited from the University of Wisconsin – La Crosse. All subjects completed a PAR-Q to screen for known cardiovascular and orthopedic contraindications to exercise. Eligible subjects provided written informed consent prior to participating in the study. The study was approved by the University of Wisconsin – La Crosse Institutional Review Board for the Protection of Human Subjects.
Subject’s height and weight were measured and dominant foot was determined prior to the start of the study. Dominant foot was determined by asking subjects with which foot they would kick a soccer ball. Subjects then completed 4 conditions; walking on a treadmill, running on a treadmill, double-leg bouncing on a mini-trampoline, and double-leg bouncing on a FREEBOUNDER®. Each condition was 1-minute in duration and the order of conditions was randomized. For the treadmill test, subjects completed one session of walking and one session of running on a motorized treadmill. Walking was conducted at 3 miles per hour, while running was at 6 miles per hour. The pace remained constant throughout both walking and running sessions and treadmill incline was set at 0% for both trials.
For the mini-trampoline test, subjects were shown the double-leg bounce on the mini-trampoline and then were allowed to practice until they felt comfortable. The pace was set at 80 beats per minute. Once subjects were deemed proficient on the mini- trampoline, they were tested. On the FREEBOUNDER®, the spring tension of the
FREEBOUNDER® was adjusted based upon body weight, where each spring added equals approximately 7.5 pounds of resistance. Subjects were shown the double-leg bounce exercise, known as active recovery, and subjects were allowed to practice until they felt comfortable on the machine. Once subjects became proficient, the measurement session began. The pace was set at 60 beats per minute.
For all testing, plantar forces were collected using Loadsol® in shoe sensors (Novel Electronics, Inc, St. Paul, MN) within the subjects’ dominant shoe. Data was recorded during the last 10 seconds of each trial, with the 5 most representative strides being analyzed for ground reaction force (GRF) and loading rate (LR). All sessions took place in the Human Performance Laboratory (HPL) on the UW – La Crosse campus.
Figure 1 depicts a GRF graph with a heel-strike pattern. The impact peak, also referred to as the passive peak in some literature, is the initial force at heel contact. The slope of the line from ground to impact peak is used to calculate LR. The steeper the line, the higher the loading rate, and the quicker the force is applied to the exerciser. This is in comparison to a less steep slope, where it takes longer for the force to be applied to the exerciser. The active peak is the function of the force applied by the foot and supported by body weight during mid-stance.
Figure 1. Depiction of a ground reaction forces graph (Arthur, Loy, Porcari, Aminaka, & Foster, 2018).
Standard descriptive statistics were used to summarize the data. Ground reaction force and LR were compared between the four conditions using one-way ANOVA with repeated measures. When there was a significant F-ratio, pairwise comparisons were made using Tukey’s post-hoc tests. Alpha was set at .05 to achieve statistical significance. Data were analyzed using SPSS version 25.0.
The descriptive characteristics of the subjects who participated in the study are presented in Table 1. The age range of subjects was 19-28 years. Table 1. Descriptive characteristics of subjects (N=18).
The responses to the FREEBOUNDER® Fitness and Rehab Machine are presented in
Table 2. It was found that GRF and LR were significantly lower for the FREEBOUNDER®
compared to walking, jumping on a mini-trampoline, and running.
Table 2. Ground reaction force (GRF) and loading rate (LR) during the four different exercise conditions.
The comparison of GRF, relative to body weight, during the four conditions is displayed in Figure 2. The comparison of LR, relative to body weight per second, during the four conditions is displayed in Figure 3.
The purpose of this study was to examine plantar force in the lower extremities associated with exercising on the FREEBOUNDER® Fitness and Rehab Machine in comparison to walking and running on a treadmill and bouncing on a mini-trampoline. The current study found that subjects had lower GRF when exercising on the FREEBOUNDER® compared to all other conditions. The GRF (Newtons) on the FREEBOUNDER® were 63% lower than walking, 151% lower than bouncing on a mini-trampoline, and 196% lower than running, respectively. Relative to body weight, the GRF when exercising on the FREEBOUNDER® was .75 times body weight; walking was 1.2 times body weight, bouncing on the mini-trampoline was 1.9 times body weight, and running was 2.2 times body weight.
The results of this study are consistent with findings of Porcari and Foster (2000) who found that running on a treadmill generated GRF approximately 2.5 times body weight. In that study, the GRF for using an elliptical was approximately equal in magnitude to the subject’s own body weight. Burnfield, Jorde, Augustin, Augustin, and Bashford (2007) compared the plantar pressures when exercising on five different cardiovascular machines. Comparable to Porcari and Foster (2000), they found the elliptical to have lower pressures than those found in running. The results of the abovementioned studies are similar to what was found with the FREEBOUNDER®.
Lower impact forces are considered beneficial, since high GRF can result in a higher risk of injury, especially in runners (Lopes, Hespanhol, Yeung, & Costa, 2012). Hrelaj (2004) concluded that runners who acquire training habits that reduce impact forces and minimize the effect forces have on the body could be at a lower risk for
developing injuries. The spring-loaded platform of the FREEBOUNDER® can be beneficial in minimizing injury or for rehabilitation since the platform acts as a compliant surface where the platform absorbs some of the force applied to it and therefore is not applying as much force back to the user.
Similar to GRF, LR was lower on the FREEBOUNDER® compared to all of the other conditions. The LR (Newtons/second) on the FREEBOUNDER® was 926% lower than walking, 1,339% lower than bouncing on the mini-trampoline, and 2,709% lower than running on the treadmill, respectively. In terms of body weight, the LR of the FREEBOUNDER® was .65 body weight per second (BW/s); walking was 7.2 BW/s, bouncing on the mini-trampoline was 10.2 BW/s, and running was 19.4 BW/s. Dixon, Collop, and Batt (2000) found that there was a lower LR in runners who ran on a rubber- modified surface compared to conventional asphalt. As part of their testing, the researchers dropped a weighted sphere onto the surfaces and found that the rubber- modified surface had greater impact absorption than the asphalt surface. Again, this impact absorption mirrors the spring-loaded platform of the FREEBOUNDER®, which acts as a compliant surface and slows down the forces that are applied to the exerciser.
A lower LR when exercising is also considered beneficial because it signifies a reduced speed at which forces impact the body. Davis et al. (2015) examined runners who had a history of injury and those who had not sought medical treatment for injury. It was found that vertical average LR was lower in the runners who had never been injured compared with those who had and sought medical treatment. Thus, decreasing LR should help minimize tissue stress to the user. Since the FREEBOUNDER® is not applying forces to
the body as quickly compared to running, walking, or bouncing on a mini-trampoline, the injury potential should be reduced compared to those modalities.
A possible limitation in the current study was that the active recovery exercise that was used during testing was not easy for all subjects to learn because it was not a common motion that most individuals use when exercising on traditional cardiovascular equipment. Even though subjects were required to practice on each modality before they were tested, with more practice it is possible subjects would feel more relaxed with performance of the active recovery exercise. Whether this would have affected the force data is unknown.
To our knowledge, this is the first research project to be conducted relative to the impact forces when exercising on the FREEBOUNDER®. Future research may want to evaluate potential improvements in balance and coordination associated with exercising on the FREEBOUNDER®. Additionally, the enjoyment of exercising on the FREEBOUNDER® should be compared to other cardiovascular equipment, as exercise enjoyment is a strong factor in exercise adherence.
In summary, it was found that the GRF and LR of the FREEBOUNDER® Fitness and Rehab Machine were lower when compared to walking and running on a treadmill and double-leg bouncing on a mini-trampoline. These findings suggest that the FREEBOUNDER® is an excellent low-impact option for individuals looking for an alternative compared traditional modes of exercise.
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