Ground reaction forces during downhill and uphill running
Introduction
Ground reaction force (GRF) data have been essential to our understanding of level locomotion biomechanics for over 70 years (Fenn, 1930). Today, we use GRF data to quantify impacts, understand propulsion and braking, compute muscle forces, and calculate mechanical energy fluctuations. In most geographic locales, runners encounter hills, yet there are scant published GRF data for running on declines or inclines and as a result, our understanding of hill running is limited.
Many researchers have quantified GRF values and patterns for running on the level (e.g., Cavanagh and Lafortune, 1980; Fenn, 1930; Munro et al., 1987). At a moderate pace of 3 m/s, for runners who land on their rear-foot, the vertical component of the GRF quickly rises and falls, forming the impact peak (≈1.6 BW). The vertical component of the GRF then more slowly increases to a second peak at mid-stance, termed the active peak (≈2.5 BW), before decreasing prior to toe-off (Fig. 1a). At foot strike, the horizontal component of the GRF is negative as a braking force is applied reaching a nadir (≈−0.3 BW) at about one-quarter of stance time before decreasing in magnitude and crossing zero at mid-stance. The horizontal component of the GRF is then positive as a propulsive force is applied reaching a zenith (≈0.3 BW) at about three-quarters of stance time before decreasing in magnitude prior to toe-off (Fig. 1b).
The dearth of GRF data for hill running is likely due to the difficulty of constructing a force platform runway on an angle or rigidly tilting a force treadmill. Given that challenge, Hamill et al. (Hamill et al., 1984) used tibia mounted accelerometers to measure leg shock. They found that leg shock increased by 30% during −5° downhill running and decreased by 24% during +3° uphill running. Dick and Cavanagh (1987) secured a force platform to a ramp, at −5° to compare downhill and level vertical force peaks. They demonstrated that during downhill running, vertical impact force peak increased by 14%, horizontal braking impulses increased by almost 200% and vertical active force peaks did not change compared to level running. Miller et al. (1988) used a heel insole transducer to examine the relationship between impact force peaks during distance running at a decline of −3° and an incline of +3°. They concluded that there was not a significant difference between impact force values during downhill and uphill running. Thus, they disagreed with Dick and Cavanagh (1987). Iversen and McMahon (1992) utilized a force platform embedded into a motorized treadmill to collect the normal GRF while subjects ran downhill and uphill. However, their device could not measure parallel forces. They reported that the normal active force peak was 2% larger during downhill running at −10° and 11% smaller during uphill running at +10° compared to level running. Thus, their active force peak data during downhill running contradicted the results of Dick and Cavanagh (1987). While these studies have advanced our knowledge of hill running, none quantified both the normal and parallel components of GRF for a range of downhill and uphill angles. Further, the data are inconsistent between studies.
Although there are few published GRF data, past research has answered numerous questions regarding hill running energetics, muscle actions, and mechanics. Margaria (1976) showed that during moderate downhill running, metabolic energy cost decreased and reached a minimum at approximately −8°, beyond which the energy cost curve inflected and began to increase curvilinearly. In contrast, during uphill running, energy cost increased linearly with the angle of the incline. Uphill running required an increase in the activity of leg muscles, particularly the vastus medialis, biceps femoris, and gastrocnemius (Sloniger et al., 1997; Swanson and Caldwell, 2000), that corresponded with the increase in external work rate during uphill locomotion. Buczek and Cavanagh (1990) quantified how much more negative work was absorbed at both the knee and ankle joints during downhill running. Klein et al. (1997) concluded that step length, contact time, and swing time did not significantly change during hill running at a fixed speed.
Questions remain unanswered that can only be addressed and clarified by collecting GRF data on a range of angles during both downhill and uphill running. For example, do impact forces change during downhill and uphill running so as to increase or decrease the probability of injury (Hreljac et al., 2000)? Do normal active force peaks increase during downhill running (Iversen and McMahon, 1992) or remain unchanged (Dick and Cavanagh, 1987)? Can the parallel propulsive force patterns explain the pattern of metabolic energy cost for hill running (Margaria, 1976)?
Our goal was to quantify the ground reaction forces during downhill and uphill running. We measured the forces both normal and parallel to the surface of the treadmill, analogous to the vertical and horizontal forces of level running. To do so, we mounted a force treadmill on wedges at several fixed angles. We tested three hypotheses: compared to level running, (1) normal impact force peaks would be larger during downhill running and smaller during uphill running, (2) normal active force peaks would be larger during downhill running and smaller during uphill running, (3) parallel braking force peaks would be greater during downhill running and parallel propulsive force peaks would be greater during uphill running. We predicted that these changes in parallel forces would be symmetrically opposite for downhill and uphill running at each angle.
Section snippets
Methods
Five men and five women volunteered (age=30.35±5.09 yr, height=1.72±0.06 m, mass=62.56±7.59 kg, mean±standard deviation). These healthy, recreational runners gave written informed consent following the guidelines of the University of Colorado Human Research Committee.
We constructed three sets of aluminum wedges to tilt our force treadmill (Kram et al., 1998) at 3°, 6°, and 9° (5.2%, 10.5%, and 15.8%). The base of each wedge was bolted to the mounting plate. Then, we bolted the force transducer box
Results
Compared to level running, the normal impact force peaks were dramatically larger for downhill running and smaller for uphill running (Fig. 4a). These results were partly due to subjects progressively altering their foot strike technique during uphill running. All ten subjects intiated foot strike with their rear-foot during the −9°, −6°, and −3° downhill, level, and +3° uphill conditions. However, at +6° uphill, three subjects switched to a mid-foot strike pattern and at +9° uphill, all
Discussion
The normal impact force peaks are larger during downhill running and smaller during uphill running. However, neither downhill nor uphill running affect normal active force peaks. Parallel braking force peaks are greater during downhill running whereas propulsive force peaks are greater for uphill running. Overall, our level running force data, for both normal and parallel components, are quite similar to comprehensive studies of the past (Cavanagh and Lafortune, 1980; Munro et al., 1987).
First,
Acknowledgements
This work was supported by a National Institute of Health Grant AR44688. We are indebted to Sid Gustafson for helpful advice on the construction of the force treadmill wedges. We also thank Claire Farley, Lawrence Greene and members of the University of Colorado Locomotion Laboratory for insightful comments and suggestions.
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