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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
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).
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
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.
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.
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,
Monroe, C.F. et al (1987). J. Biomech., 20, 147-155
The authors would like to thank Jump America, Inc. for providing Kangoo
Jumps rebound exercise shoes (SB) for testing.