Introduction
Aerobic exercise training causes improved cardiopulmonary function and has well-established health benefits. Cardiopulmonary adaptations include, but are not limited to, increases in aerobic capacity, maximal stroke volume and cardiac output, oxygen diffusion capacity, flow-mediated dilation of conduit arteries, capillary density of activated skeletal muscle, and the number of mitochondria within skeletal muscle (13). Among numerous health benefits are reduced incidence of coronary artery disease, obesity, type 2 diabetes, and certain forms of cancer (17).
The 2008 Physical Activity Guidelines for Americans (PAG) recommended that adults could obtain similar health benefits from either 150 minutes per week of moderate-intensity activity or half that time of vigorous-intensity activity (16). The key variable was the total quantity of energy expended. However, a growing body of evidence now confirms that, even when total energy expenditure is equated, vigorous-intensity aerobic training as compared to moderate intensity causes greater improvements in aerobic capacity (6,11,15,18) and greater cardiovascular health benefits (14). Examples of greater health benefits from vigorous-intensity exercise training as compared to moderate intensity in research studies that equated total work between groups include a greater reduction in blood pressure (11), greater increase in insulin sensitivity (4), greater increase in flow-mediated dilation (15), and in heart failure patients, a greater increase in ejection fraction coupled with reversed cardiac dilation (18).
Although vigorous-intensity exercise training provides superior results to moderate intensity, practitioners may be hesitant to recommend vigorous-intensity exercise because of perceived negative effects, specifically, a greater risk of cardiovascular events such as myocardial infarction (MI) and sudden death and a greater risk of musculoskeletal injuries (5,16). Regarding the latter, the U.S. Department of Health and Human Services stated in the 2008 Physical Activity Guidelines, “Although physical activity has many health benefits, injuries and other adverse events do sometimes happen. The most common injuries affect the musculoskeletal system,” adding “Most people are not likely to be injured when doing moderate-intensity activities in amounts that meet the Physical Activity Guidelines” (16). The PAG went on to exemplify moderate intensity as walking and vigorous intensity as running.
The preference for moderate intensity over vigorous intensity in exercise prescription is based on studies reporting greater injuries from running than walking. In its 2011 position stand, the American College of Sports Medicine (ACSM) stated, “Walking and moderate-intensity physical activities are associated with a very low risk of musculoskeletal complications, whereas jogging, running, and competitive sports are associated with increased risk of injury” (5). However, an important distinction must be made. Walking and running differ in mode, not just intensity. Running has a flight phase between steps, resulting in the runner striking the ground with greater force than does a walker. We propose that the aerobic intensity of running is not responsible for its increased risk of musculoskeletal injuries; rather, its impact forces are; moreover, aerobic intensity and impact forces may be delinked through mode selection.
Aerobic intensity may be increased without substantially raising the risk of musculoskeletal injury using the proper mode of exercise. For example, the intensity of walking can be increased by going uphill, instead of switching to running. Does walking uphill at a vigorous intensity that equals the intensity of level running produce significantly greater orthopedic stress than does level walking at a moderate intensity? To investigate this question, we measured loading forces during walking and running on an instrumented treadmill. We hypothesized that vigorous-intensity incline walking would produce similar loading forces as moderate-intensity level walking and that both would produce lower loading forces than level running at a vigorous intensity.
Methods
Experimental Approach to the Problem
A randomized clinical trial design was used to test the hypothesis. One trial was horizontal walking at a moderate intensity (gross MET level of approximately 4, where 1 MET is the “metabolic equivalent” of resting energy expenditure or oxygen consumption), 1 trial was horizontal running at a vigorous intensity (approximately 8.5 MET gross), and 1 trial was uphill walking at a vigorous intensity matched to that of the running. Although the impact forces of walking and running on treadmills have been studied in the past, no previous study has matched the exercise intensities. The three trials were performed in a counter-balanced order, such that each trial was performed first, second, or third by one-third of the subjects.
Subjects
Twenty healthy young adults (10 male and 10 female; age 22.8 ± 0.5 years, range 20–30 years; 170 ± 2 cm; 70.1 ± 2.8 kg) were recruited to participate in this study. As a matter of convenience, subjects were recruited from exercise science classes at the university. Inclusion criteria were being able to comfortably run at 8 kph (self-reported) and being of low risk for cardiac events during exercise according to ACSM guidelines (i.e., participants had no known cardiopulmonary disease or diabetes, no symptoms of cardiopulmonary disease, and no more than 1 major risk factor for cardiopulmonary disease (2)). Also, female subjects were excluded if they believed they might be pregnant. The study was approved by the University Institutional Review Board, and all subjects gave written informed consent before participation.
Procedures
After screening and consent procedures were completed, subjects were measured for height and mass and then performed three exercise trials on a treadmill: horizontal walking, uphill walking, and horizontal running. The trials were completed in a counter-balanced order in a single session. First, the subject stood quietly for 5 minutes, then walked for 3 minutes at 4.8 kph as a warm-up, and then performed the three trials for 10 minutes each, with 10 minutes of rest between each trial. The three trials were 5.5 kph and 0% grade for horizontal walking, 5.5 kph and 11% grade for uphill walking, and 8.0 kph and 0% grade for horizontal running. Based on metabolic equations (12), these trials were expected to produce net oxygen consumptions of approximately 8.8, 26.4, and 26.8 ml·min−1·kg−1, respectively; i.e., the uphill walking and horizontal running are similar and are approximately three times more vigorous than the horizontal walking. Before testing, each subject was fitted with a chest strap heart rate (HR) monitor (Polar FT1, Kempele, Finland) and a mouthpiece and nose-clip to collect exhaled air for the measurement of oxygen consumption (V[Combining Dot Above]O2), carbon dioxide production (V[Combining Dot Above]CO2), and respiratory exchange ratio (RER) with a calibrated metabolic cart (Parvo Medic TrueOne 2400, Sandy, UT, USA). The mouthpiece and nose-clip were removed during the 10-minute rest breaks between trials. V[Combining Dot Above]O2 measured during the final 5 minutes of each trial was gross V[Combining Dot Above]O2. V[Combining Dot Above]O2 measured during the 5 minutes of standing rest was subtracted from gross V[Combining Dot Above]O2 to yield net V[Combining Dot Above]O2.
For each participant, gait measures were recorded during walking and running on an automated treadmill (H/P/Cosmos Mercury Med 4.0) with an installed pressure plate (FDM-T Zebris Medical GmbH, Germany). The specific measures related to the spatio-temporal features of the gait pattern (i.e., stride length and time, step length and time, swing time, stance time, and step frequency) and the vertical ground reaction forces (i.e., peak VGRF and average VGRF loading rate). This treadmill provides step by step analysis of these variables. All gait measures were averaged over the last 5 minutes of each trial for each subject. All subjects wore a single pair of their own self-selected athletic shoes during the entire data collection involving all three trials.
Statistical Analyses
A 1-way repeated-measures analysis of variance (ANOVA) was used to compare each measured variable across the three trials. The data used were the average values from each subject collected over the last 5 minutes of each trial. All values are reported as mean ± SE. For analyses where the ANOVA revealed a significant p value, Tukey post hoc testing was performed to determine which mean differed from others. Linear regression was used to examine correlations and the Student's t-test to determine significance of Pearson R values. Significance for all tests was judged at the 0.05 level.
Results
Cardiometabolic Variables
V[Combining Dot Above]O2 during standing rest averaged 4.2 + 0.1 ml·min−1·kg−1. Net V[Combining Dot Above]O2, RER, and HR during the three trials are presented in Table 1. For all three variables, a significant trial effect was found (p < 0.001). Post hoc testing revealed that running and uphill walking were not different from each other but were both significantly greater than horizontal walking (p < 0.001).
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Table 1: Cardiopulmonary variables during the exercise trials.*
Spatio-Temporal Gait Variables
Table 2 provides spatio-temporal gait values for the 3 trials. A significant trial effect was observed for stance time (p < 0.001), swing time (p < 0.001), and stride time (p < 0.001). Post hoc analyses revealed that all differences were seen between the running trial and both walking trials. The step frequency was also significantly different between trials (p < 0.001). Post hoc analysis revealed that step frequency was significantly greater (p < 0.001) during running compared with both walking trials.
Table 2: Spatio-temporal gait variables during the exercise trials.
Vertical Ground Reaction Forces
An example of the typical vertical GRF profile during each of the 3 movement conditions is shown in Figure 1. Significant trial effects were observed for both the peak VGRF (p < 0.001) and average loading rate of VGRF (p < 0.001). Post hoc analyses revealed that, for the peak VGRF, the running trial was significantly greater than both walking trials (p < 0.001). For the rate of loading, differences were seen between all 3 trials, with the rate during running significantly greater than both walking trials (p < 0.001), and the rate during horizontal walking greater than uphill walking (p = 0.011). Peak VGRF and VGRF loading rates as a function of the three different trials are shown in Figure 2. No differences in any of the force metrics were observed between the left and right legs.
Figure 1: Representative example of the vertical ground reaction forces during the 3 trials. All data were obtained from the same subject during a single session.
Figure 2: Peak vertical ground reaction force (GRF) (upper panel) and average vertical GRF loading rate (lower panel) as a function of the three different trials. Error bars represent 1 SE of the mean. *Horizontal running significantly greater than both walking trials (p < 0.001). #Horizontal walking significantly greater than incline walking (p = 0.011).
Peak VGFR was directly and significantly related to body mass, with p < 0.001 for all three conditions (Figure 3). Average loading rate of VGRF was also significantly related to body mass (not shown), although with lower correlations (R = 0.627, 0.492, and 0.591 for horizontal walking, uphill walking, and horizontal running, respectively, and with p = 0.003, 0.028, and 0.006, respectively).
Figure 3: Correlation of peak vertical ground reaction forces (peak VGRF) with the subjects' body mass during the 3 conditions.
Discussion
This study has demonstrated that incline walking at a vigorous aerobic intensity produced no greater loading forces on the lower limbs than did horizontal walking at a moderate intensity. However, horizontal running performed at the same vigorous intensity as the incline walking produced substantially greater loading forces. Peak vertical ground reaction force was 49% greater per impact during running than during incline walking. Moreover, the frequency of impact (steps per minute) during running was 32% greater than during incline walking, compounding the potential orthopedic stress. Thus, the product of peak VGRF and step frequency was 97% greater during running than walking uphill at the same aerobic intensity.
The vigorous intensity trials in this study elicited a net V[Combining Dot Above]O2 of 25–26 ml·min−1·kg−1, which was 2.5-fold greater than the net V[Combining Dot Above]O2 observed during moderate-intensity walking (10.5 ml·min−1·kg−1). In gross terms, these were equivalent to 8.7 MET for uphill walking, 8.4 MET for level running, and 4.2 MET for level walking. The PAG defines moderate intensity as 3.0–5.9 MET and vigorous intensity as 6.0 or more MET (16); thus, the intensities used in this study were typical of those recommended.
To the best of our knowledge, this is the first study to directly compare loading forces between running and uphill walking while controlling exercise intensity. Previous research of level walking and running has demonstrated that peak VGRF increases with speed and is much greater during running than walking (7). Other studies have examined the effect of grade on loading forces during walking, with mixed results. One study found that peak VGRF increased with grade; however, the subjects were allowed to self-select their speed, and speed increased with grade (9). Consequently, the increased peak VGRF with increased grade can likely be attributed to the increased speed. Another study reported mixed results for different measures of loading force with increasing walking grade, concluding that grade had little overall effect (8). In this study, peak VGRF was the same between level and uphill walking, and the shape of the VGRF profiles was similar (as shown in Figure 1). However, there was a lesser rate of loading force development during uphill than level. None of the previous studies reported loading rate, and this finding bears more investigation. Because of the complex loading force profile during walking, the peak loading rate (initial slope of the force curve) may be a useful indicator.
Loading forces are directly affected by body mass. The peak VGRF is a function of the person's mass and acceleration when running or walking on the treadmill, as seen in Figure 3. The lower (though still significant) correlation of loading rate with body mass is likely due to individual variations in how quickly subjects load their mass onto the plate; striking more rapidly (with greater limb acceleration) produces a greater rate of loading than would lowering the leg in a more controlled fashion. Clearly, individuals with greater mass due to excess body fat would suffer from greater loading forces. However, those forces should be no more than those of a leaner individual with the same total mass. Although body fat was not examined in this study, the female subjects, with presumed higher body fat percentages, had the same relative loading forces as males; i.e., males were one-third heavier than females on average, and male loading forces were also approximately one-third higher than those of females.
As noted in the Introduction, there are significant advantages of vigorous-intensity exercise over moderate intensity for purposes of cardiometabolic health. However, one must also consider the risk of cardiovascular events such as MIs and sudden cardiac arrest (SCA). Myocardial infarction and SCA can be triggered by exercise-induced elevations of sympathetic drive in individuals with pre-existing disease. Mittleman et al. (10) found that among patents presenting with an MI, the risk of MI was significantly greater during or after vigorous-intensity exercise as compared with during rest. However, that risk varied with the individuals' usual frequency of vigorous-intensity exercise. Those who normally did not engage in vigorous-intensity exercise experienced a 105-fold increased risk of MI when attempting it, whereas those who regularly performed vigorous-intensity exercise experienced only a 2.4-fold increased risk. Albert et al. (1) reported similar results for SCA, with a 74-fold increased risk for sedentary victims and an 11-fold risk for the most active. Clearly, although vigorous exercise can elicit a cardiovascular event, the risk of it doing so is greatly reduced by regular training. A likely explanation of this adaptation was demonstrated by Billman et al. (3) in dogs that were given an MI and at a later date given an exercise plus coronary ischemia challenge. Dogs that experienced SCA were defibrillated and then placed into either a sedentary group or a training group for 6 weeks. The exercise plus ischemia test was repeated and 7 of 8 sedentary dogs experienced SCA again, whereas none of the 8 trained dogs did. The trained dogs also exhibited a significantly improved autonomic response to a pressor challenge. Vigorous-intensity exercise is safe from a cardiovascular perspective in those without disease and, in those with disease, the risk is significantly ameliorated by training.
Although vigorous-intensity exercise is routinely noted to cause increased musculoskeletal injuries, this is normally associated with running as the mode (5). This study has examined the loading forces caused by moderate intensity and vigorous-intensity walking and found that vigorous-intensity walking imparts substantially less loading force on the lower extremities than does running of the same intensity. Intriguingly, walking uphill at a vigorous intensity produces a lower rate of force loading than does level walking of a moderate intensity. This is a novel finding that bears further investigation.
The subjects in this study were young adult males and females (20–30 years of age) who were self-described as being physically active. All were able to run comfortably at 8 kph for 10 minutes. Thus, our results apply to this specific population. We do not expect that the loading forces examined in this study would be affected by factors other than body mass, speed, and mode of ambulation. However, specific populations, such as those with pre-existing orthopedic or gait problems, might have somewhat different results.
Practical Applications
Substantive loading forces are valuable for stimulating the maintenance or improvement of bone mineral density. Exercise prescriptions for the general public should include modes of exercise that cause significant impact forces. For bone health, running (including many ball sports) is better than walking, and walking is better than low-impact machines or swimming. When loading forces need to be reduced in populations with clinical orthopedic concerns, practitioners are routinely advised to not use vigorous-intensity activities that cause high-impact forces. However, if clients or patients fail to incorporate vigorous-intensity aerobic exercise in their overall exercise routine, they will be denied important cardiovascular benefits. Thus, they should be prescribed vigorous-intensity exercise with the use of lower impact activities. Incline treadmill walking has been demonstrated in this study to be an excellent choice, providing no greater impact forces than level walking, but substantially less than incurred by running. Bicycling and elliptical trainers would provide even less impact and swimming the least, when orthopedic limitations are great.
Acknowledgments
This study was partially funded by a grant from the Old Dominion University Honors College. The authors have no professional relationships with companies or manufacturers who might benefit from the results of the study. The results of the study do not constitute endorsement by the National Strength and Conditioning Association.
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