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UNIVERSITY OF NEVADA STUDY

Impact Forces During Running in a Novel Spring Boot

Jason Vance and John A. Mercer
Department of Kinesiology, University of Nevada, Las Vegas
E-mail: vance@nevada.edu
Web: http://www.unlv.edu/faculty/jmercer

INTRODUCTION
It has been reported that runners generally select a running style that optimizes oxygen cost of running but not impact magnitude (Hamill et al., 1995). Since impact magnitudes have been hypothesized to be related to running overuse injuries (Hreljac et al., 2000), it seems reasonable to suspect that if the impact magnitude can be reduced, the risk of overuse injuries could be reduced. Recently, a spring-boot (SB, Kangoo Jumps.) has been developed for general exercise use. Based on boot construction, a purpose of the SB is to reduce impact magnitudes during activities such as running. Since runners can change their running style (e.g. change stride length), and since running style affects impact attenuation (Hamill et al., 1995), it is not known if impact magnitudes are attenuated during SB running. Therefore, the purpose of this study was to determine if impact magnitudes are affected while running with Kangoo Jumps (Spring Boots).

METHODS
Seven healthy subjects (age: 23±2.5 years, height:168±8 cm, mass: 62±11 kg; male: n=4, female: n=3) were recruited from a university population. Informed consent was obtained prior to data collection. Subjects ran at three different velocities (2.2, 3.1, and 4.0 m×s-1) during two shoe conditions: 1) running shoes (RS, laboratory shoe); 2) Spring Boot (SB, Kangoo Jumps). The order
of shoe and velocity conditions were counterbalanced among subjects. All running trials were completed along a 20m runway with the right foot contacting a force plate (Kistler) mounted in the middle of the runway. GRF data were collected (1000Hz) for five acceptable trials for each shoe-speed condition. An acceptable trial was defined as the right foot entirely contacting the force platform with no obvious modification of stride (e.g., lunge or stutter-step), and velocity within ±5% of the target velocity. Velocity was monitored using sensors placed 3m before and 2m after the center of the force platform. Prior to testing, ample time was allowed for subjects to become comfortable with SB running. Impact magnitude (F1), active force peak (F2), average vertical force (Favg ), and contact time were each analyzed using repeated measures ANOVA with linear contrast follow-up testing (SPSS, version 10.1).

RESULTS AND DISCUSSION
An impact peak was observed in 15% of the trials during SB running, compared to 96% of the trials during RS running. Table 1 reports the percent of trials in which F1 was observed per condition.



Mean for F1 was not calculated across subjects since only a small number of trials contained an impact peak. However, one subject who had at least one F1 per condition, F1 magnitude ranged from 1.0-1.6 BW during RS and 1.5-2.0 BW during SB running. This suggests that for this user, impacts were not attenuated but for the majority subjects F1 was attenuated. The observation that frequency of F1 occurrence increased as velocity increased indicates that the effectiveness of the boots to attenuate impact peak was related to velocity.

F2 increased linearly across velocity for both RS and SB conditions (Figure 1, p<0.05) with F2 increasing 0.28 BW/m×s-1 during RS but only 0.17 BW/m×s-1 during SB. However, F2 was not different between shoes collapsing across velocity (p>0.05).



Favg increased linearly across velocity during RS (0.20 BW/m×s-1) and SB (0.13 BW/m×s-1) running (Figure 2, p<0.05). The Favg increase during RS was similar to the 0.21 BW/m×s-1 increase across similar velocities reported by Munro et al. (1987). Mean Favg was 6% greater during SB than RS across velocity (Figure 2, p<0.05).


Contact time was not different between RS and SB running (p>0.05) but did decrease linearly across velocity for both conditions (p<0.05). There is evidence indicating that runners adjust lower extremity stiffness to surface stiffness. Ferris et al. (1999) reported that runners adjusted lower extremity stiffness inversely proportional to the surface stiffness. They also reported that contact time and peak forces were similar during running across surfaces with different stiffness when runners adjusted lower extremity stiffness to the surface. Hardin et al. (2000) also reported that lower extremity stiffness was modified to surface-shoe changes. In the present study, the similarity in contact time and F2 between shoes across velocity suggests that subjects adjusted lower extremity stiffness to SB stiffness.

SUMMARY
Running with SB reduced the frequency of occurrence of F1 compared to RS, but F2, Favg and contact time remained similar between conditions across velocities tested. It is suggested that subjects were able to adjust lower extremity stiffness to the novel spring surface with the impact cost of running significantly reduced.

REFERENCES
Ferris, D.P. et al (1999). J. Biomech., 32, 787-794

Hamill, J. et al (1995). Hum. Movt. Sci., 14, 45-60

Hardin, E.C. et al. (2000). Proceedings of 24th Annual Meeting of ASB.

Hreljac, A. et al (2000). Med. Sci. Sp. Ex., 32, 1531-1666

Monroe, C.F. et al (1987). J. Biomech., 20, 147-155

ACKNOWLEDGEMENTS
The authors would like to thank Jump America, Inc. for providing Kangoo Jumps rebound exercise shoes (SB) for testing.