Research Review By Jessica Sleeth©


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Date Posted:

November 2011

Study Title:

Energetics and Biomechanics of Inclined Treadmill Walking in Obese Adults


Ehlen KA, Reiser RF, Browning RC

Author's Affiliations:

Department of Health and Exercise Science, Colorado State University, Fort Collins CO, USA.

Publication Information:

Medicine & Science in Sports & Exercise 2011; 43(7): 1251-1259.

Background Information:

In the United States, the prevalence of obesity has increased to over 30%. Obesity is associated with numerous diseases and is the main preventable risk factor for large-joint osteoarthritis (1). Moderate-intensity physical activity for 30 minutes per day, 5 days a week is recommended for most obese individuals to improve health (2). Walking is commonly recommended because of convenience and low rate of injury.

A positive association exists between level walking speed and lower-extremity joint loading, estimated using net muscle moments (NMM), joint reaction forces, and joint loading rates. The combination of obesity and a brisk walking speed may produce large loads in the lower-extremity joints and thus increase the risk of osteoarthritis. Hootman et al. found that, over a 12-month period, approximately 25% of habitual overweight walkers endured a significant musculoskeletal injury, with many subsequently quitting their exercise program (3). A slower pace could prevent injury, however, it would also reduce the cardiovascular benefits of walking.

The authors of this study questioned whether obese individuals could maintain the appropriate intensity by walking uphill at a slower pace. According to the American College of Sports Medicine, walking 0.75m/s on a 6 degree incline elicits similar oxygen consumption as 2.1m/s on a level surface. Yet, the research is very limited on the biomechanics of incline walking for obese individuals.

The purpose of this study was to quantify the energetics and biomechanics of uphill versus level walking in moderately obese adults. The authors hypothesized that slower walking on moderate inclines, compared with faster level walking, would provide similar physiologic stimulus and reduce lower extremity net muscle moments and peak vertical loading rates for moderately obese persons.

Pertinent Results:

Mean VO2max was 9.9W/kg (W/kg = net metabolic rate) and mean standing metabolic rate was 1.2W/kg. Net metabolic rate did not differ between uphill at 0.50m/s (9 degrees) and 0.75m/s (6 degrees) compared with level walking at 1.50m/s, but was 29.8% and 14.4% greater during level walking at 1.75m/s and 1.25m/s, respectively compared with level walking at 1.50m/s. All trials were performed at moderate intensity and required between 49% and 60% VO2max.

Temporal stride kinematics were significantly different across the speed-grade combinations. As walking speed increased and grade decreased, stride frequency and stride length increased. Step width was significantly greater than level walking at the slowest speed.

Knee and ankle joint angles differed across all speed-grade combinations. Increased walking speed and decreased incline resulted in less flexion at the knee during the early stance. Mean peak knee flexion in early stance was 70% greater (39 versus 23 degrees) at 0.50m/s (9 degrees) versus 1.75m/s (0 degrees), respectively. A steeper incline resulted in more dorsiflexion in the ankle throughout the gait cycle.

During early and late stance, peak normal ground reaction forces (GRFs) were greater in trials with faster speeds and lower grades. Peak anterior-posterior GRFs in early stance were greater in the faster, level speeds and there was no braking force at the steepest incline. Greater medial-lateral GRFs occurred at faster speeds and lower grades compared to lower speeds and moderate inclines. Maximum normal force loading rate increased with walking speed. Significant main effect of speed-grade occurred on peak NMM (flexion and extension) about the hip (P < 0.001), knee (P < 0.002), and ankle (P < 0.001). Peak NMM increased as speed increased and incline decreased. Peak extensor NMM at the hip, during early stance, and ankle, during late stance, were significantly lower when walking at 0.75m/s (6 degrees) versus both 1.50m/s (0 degrees) and 1.75m/s (0 degrees). Peak knee abduction/adduction muscle moments increased with speed (P = 0.005). Peak knee abduction moment decreased 54% and 26% when walking at 0.50m/s (9 degrees) and 0.75m/s (6 degrees) versus 1.50m/s (0 degrees), respectively.

Total positive joint work during stance (sum of hip, knee, and ankle) was similar, but negative joint work was smaller during slower, incline walking compared to faster, level walking. Total net joint work (sum of positive and negative work) during stance was greater when walking uphill than faster, level walking. Hip and knee positive work was similar across the trials while ankle positive work increased with faster/level speeds.

Clinical Application & Conclusions:

Slower walking, with moderate incline, requires similar physiologic stimulus and energy expenditure compared to faster, level walking for moderately obese individuals. Incline walking requires increased mechanical work in the lower-extremity muscles compared to level walking. Therefore, hip, knee, and ankle net joint work (sum of positive and negative work) during stance was greater when walking uphill than faster, level walking. The increase in net joint work during uphill walking was due to a decrease in negative joint work – thus, the use of stored energy was reduced during the uphill trials.

The authors found that moderate obesity did not alter the stride length-frequency relationship during uphill versus level walking. Lai et al. report that obese individuals remain in stance and double support longer than non-obese individuals. The study in review here did not have a non-obese control group, however the authors support this finding because of consistency with the level walking results (4). It is reported that the obese participants in this study had similar knee flexion as reported by other studies with non-obese and obese adults (5). Other studies that reported decreased knee flexion and consequently more erect postures were conducted with severely obese participants – which may be the reason for the difference in results. Ehlen, Reiser, and Browning (2011) reported that uphill walking required increased knee flexion and ankle dorsiflexion throughout the stance compared to level walking. This may be due to the walking speed during the uphill trials – the authors recommend further research into the muscle forces and joint movements in uphill versus level walking.

The authors accept the hypothesis that decreased walking speeds with moderate inclines reduces lower-extremity net muscle moments compared to faster, level walking – as the walking speed decreased and the incline increased, hip, knee, and ankle NMM decreased. Uphill walking at slower speeds reduced the peak early stance knee abduction NMM. The reduction in knee abduction may result in decreased osteoarthritis progression because of less medial compartment knee loading and varus misalignment. Thus, the load distribution on the medial compartment of the knee is reduced during slower incline walking.

In conclusion, uphill walking at slower speeds, compared to faster level walking, may reduce load, load distribution, and loading rate across the lower-extremity joints while still providing adequate cardiovascular/physiologic stimulus for health benefits. Health practitioners and fitness/rehabilitation professionals should keep this in mind when designing programs for obese individuals.

Study Methods:

  • 30 individuals were recruited with 12 (7 females) participating in the study.
  • Subjects had no known acute/chronic diseases or physical activity limitations, and were currently participating in less than 3 hours of physical activity per week.
  • Subjects also had stable body mass, reporting less than 2.5 kg net change in the previous 3 months.
  • They were also not taking any medications known to alter metabolism.
Experimental Protocol:
  • Each participant completed 3 experimental sessions.
  • First session: followed a 12-hour fast, physical examination, body composition, and anthropometric measurements. Participants then completed standard graded exercise stress test to determine VO2max
  • Second and third sessions: followed a 4-hour fast, collected metabolic and biomechanics data as participants stood and walked at 16 speed-grade combinations (8 per visit). Trials included 0.50m/s (9 degrees), 0.75m/s (6 degrees), 1.25m/s (3 degrees), 1.50m/s (0 degrees), and 1.75m/s (0 degrees). Trials were 6-minutes in duration with 5-minutes rest between trials.
  • Participants were familiarized to the treadmill at the beginning of the second session by walking for 10-minutes (0 degrees) at a self-selected pace.
  • Physical health and activity: each participant completed a health history form and was assessed by a physician (physical exam and blood work).
  • Body composition: measured participant’s body composition with dual-energy x-ray absorptiometry. Determined percent body fat and lean mass for whole body, thigh, shank, and foot.
  • Maximal oxygen uptake: Used modified Balke treadmill protocol to determine each participant’s VO2max. Participant’s heart rate and blood pressure were measured in supine, sitting, and standing positions to test for orthostatic intolerance. Participant’s warmed up for 5-minutes, then treadmill speed was increased until Rate of Perceived Exertion (RPE) reached moderate (~11), then speed was held constant and grade was increased by 1% each minute, until exhaustion. Physiologic response to exercise was monitored using heart rate, blood pressure, and RPE every 3-minutes.
  • Energetic measurements: collected during second and third sessions, while participants walked on treadmill at varying speeds and inclines. Metabolic rate was determined using rate of oxygen consumption and carbon dioxide production. Measured standing metabolic rate for 6-minutes, then allowed 4-minutes during trial for participants to reach steady state, and then calculated average VO2max and VCO2 (ml/s) for the final 2-minutes of each trial.
  • Biomechanics measurements: used three-dimensional motion capture system and a dual-belt, inclinable, force-measuring treadmill. Data was collected for 30-seconds during the final minute of each trial.
  • Statistical analysis: repeated-measures ANOVA used to determine effects of various speed-grade combinations on metabolic rate, temporal gait characteristics, mid-stance joint angles, peak NMM, and peak loading rate. If significant main effect observed, post hoc comparisons using the Holm-Sidak method were performed. If data failed the normality test, a multiple-comparison Tukey test on ranks was used. A criterion of P < 0.05 defined significance. SigmaPlot version 11.0 was used to perform statistical analysis.

Study Strengths / Weaknesses:

  • The authors used the ACSM metabolic rate prediction equation to select speed-grade combinations, however, the equation overestimated metabolic rate during slower, incline walking by approximately 10% and underestimated faster, level walking by up to 28%. Thus, caution must be used when estimating energy expenditure and intensity with these equations.
  • The study sample size is relatively small (12 participants) and only included moderately obese individuals. Thus, the results may not be generalizable to severely obese individuals.
  • The exercise prescription may be limited to treadmill walking as walking uphill in the outdoors inevitably includes downhill walking, which is known to increase joint loading.

Additional References:

  1. Powell A, Teichtahl AJ, Wluka AE, et al. Obesity: a preventable risk factor for large joint osteoarthritis which may act through biomechanical factors. Br J Sports Med. 2005;39: 4-5.
  2. Haskell WL, Lee IM, Pate RR, et al. Physical activity and public health: updated recommendation for adults from the American College of Sports Medicine and the American Heart Association. Med Sci Sports Exer. 2007;39(8): 1423-34.
  3. Hootman JM, Macera CA, Ainsworth BE, et al. Epidemiology of musculoskeletal injuries among sedentary and physically active adults. Med Sci Sports Exerc. 2002; 34(5): 838-44.
  4. Lai PP, Leung AK, Li AN, et al. Three-dimensional gait analysis of obese adults. Clin Biomech. 2008;23(1 suppl): S2-6.
  5. Browning RC, Kram R. Effects of obesity on the biomechanics of walking at different speeds. Med Sci Sports Exerc. 2007;39(9): 1632-41.