Biomedical Engineering Reference
In-Depth Information
The shock-absorbing feature in the soles of footwear has been used effectively in the preven-
tion of foot/ankle injuries. Sports shoes should be equipped with an effective cushioning system to
attenuate potentially injurious foot-ground impact forces. A wide range of footwear sole materials
is available, differing in density, elasticity, and viscoelasticity. The optimal design of cushioning
materials and structures is imperative in order to meet complicated and individual functional and
clinical demands (Fong, Hong, and Li 2007, 2009). The efficiency and effectiveness of a design are
the main concerns for designers and manufactures. The following evaluation relies on information
obtained from available experiments, such as materials tests, cadaveric experiments, kinematic and
kinetic analysis, plantar pressure measurement, and muscle activity monitoring.
5.1.1 e
xperimental
a
pproacH
Experimental results show that the heel pad has the capacity to attenuate the heel strike peak
acceleration transmitted to the lower leg by 80% (Noe et al. 1993). Polymeric shock-absorbing materi-
als can augment this capacity depending on the compliance of the foot/shoe structures. The reduction
in the peak accelerations due to shock-absorbing inserts may be around 18% in the presence of an
intact compliant heel pad, but may rise to 83% in the absence of a heel pad. The rearfoot peak ground
reaction force is significantly greater for a harder midsole during a step-off landing with basketball
shoes (Zhang et al. 2005). Shock absorption capability has also been reported to be significantly
greater in the calcaneal heel pad than in external shock absorbers (Jorgensen and Bojsen-Moller 1989).
5.1.1.1 Pendulum Impact experiments
Human pendulum impact tests were widely used to quantify lower limb biomechanical responses
during walking, running, and trauma injury. A pendulum approach was used to simulate and quan-
tify the shod heel region under impact loading, and it was found that peak loadings for the soft-
and hard-soled feet differed significantly (Aerts and De Clercq 1993). However, impact tests can
offer heel-pad deformation information without providing a true stress-strain response (Aerts and
De Clercq 1993). It was also found that changes in peak muscle forces with simulated footwear con-
ditions did not reflect changes in the peak impact force, and those responses to footwear conditions
varied widely between subjects (Wright et al. 1998). A significant decrease in peak tibial accelera-
tion and acceleration slope following fatigue using a vertical force plate on 24 healthy women was
reported (Flynn, Holmes, and Andrews 2004). The venous plexus was also found to contribute to
the damping properties of the heel pad during walking using three different impact velocities (0.2
to 0.6 m/s) (Weijers, Kessels, and Kemerink 2005).
5.1.1.2 accelerometer measurement
Tibial axial acceleration was shown to be more sensitive for gauging footwear cushioning than
ground reaction force measurement (Lafortune and Hennig 1992). Accelerometers positioned on
the tibia have been used to evaluate the impact attenuation properties of different footwear during
locomotion (Hamill 1999). During running, marked reductions were found in the higher frequency
components of tibial shock signals (>20 Hz) by using specific midsole materials in the footwear
(Shorten, Valiant, and Cooper 1986). Higher mean power frequencies were associated with more
severe impacts (Johnson 1986). It was shown in animal studies that bone tissue development was very
sensitive to the frequency content of transmitted impact shocks (Rubin, Mcleod, and Bain 1990).
5.1.1.3 material test
Previous material tests focused on the effectiveness of reducing the magnitude of peak impact force
and shock wave caused by heel strike. Besides transient force reduction, the evaluation of cushion-
ing properties included material responsiveness to repetitive impacts (Sun et al. 2008). Long-term
shock absorption characteristics could be as important as initial shock absorption responses in shoe
designs (Jarrah et al. 1997). Plantar soft tissue demonstrated nonlinear stress-strain characteristics,
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