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Impact force characteristics in HealusTM footwear in comparison to conventional running shoes.
In recent years running has become a popular physical/recreational activity (De Wit et al 2000). Running is a convenient low cost pass-time which contributes to a decreased mortality rate, as well as a reduction in morbidity and the development of disabilities in older adults (Curfman 1993, Fries et al 1994, Paffenbarger et al 1993 and Sadvick et al 1993). Unfortunately however, epidemiological studies analyzing the prevalence of running injuries suggest that overuse injuries are a prominent complaint for both recreational and competitive runners (Hreljac 2004).
Typical distance running speeds may result in as many as 300 foot strikes per leg per kilometre, often on hard surfaces (Valiant 1990). The interaction between the foot and ground during each foot strike induces high peak vertical impact forces, generating a transient shock wave that is transmitted through the musculoskeletal system (Shorten 2000). There is considerable evidence to suggest that transient forces are harmful (Whittle 1999).
Repeated impact loading of the body during gait has been linked to the development of degenerative osteoarthritis (Radin et al 1972, Radin et al 1991, Yiling et al 2000 and Dekel and Weismann 1978), lower back pain (Voloshin and Wosk 1982), tendinitis (Lysholm and Wiklander 1987 & Van Mechelen 1992) and stress fractures (Watson and Di Martino 1987).
In addition in vivo analyses have revealed that shoe cushioning mechanisms do not influence impact force parameters. It is hypothesized that the cushioning properties of modern footwear creates a perceptual underestimation of impact severity (Frederick 1986). Under load plantar feedback is important for the perception of impact and neuromuscular adaptations in kinematics. Thus wearing shoes with cushioning systems may reduce this plantar sensory mechanism, resulting in a sharp reduction in shock attenuating behaviour (Frederick 1986).
Previous investigations have demonstrated that the impact shock during landing when running is predominantly determined by the movement strategy a person adopts (Hartveld and Chockalingham 2004). Based on these observations a new type of running shoe the HealusTM technology has been developed. The aim of the HealusTM shoe is to induce kinematic adjustments that will in turn facilitate positive alterations in kinetics (Hartveld 2005, 2006a, 2006b & 2006c). The HealusTM technology incorporates a compliant slant sole, which is designed to encourage the runner to strike the ground with a more plantar flexed ankle (Hartveld 2005, 2006a, 2006b & 2006c). The rationale behind this plantar flexion movement during the stance phase can provide a more appropriate deceleration of the body hitting the ground, thus decreasing the vertical ground reaction force parameters on foot strike.
Ten male participants with varied running experience volunteered to take part in this study. All were injury free at the time of data collection and completed an informed consent form. The mean characteristics of the participants were; Age 19.90 + 1.10 years, Height 178.10 + 5.20 cm and mass 76.79 + 8.96 kg. The procedure was approved by the ethical board of the University of Central Lancashire.
Participants were tested whilst running in conventional and HealusTM running shoes, at a velocity of 4.0ms-1. The three orthogonal vectors of the GRF were calculated as participants ran over a KistlerTM force plate (sampling at a frequency of 1000 Hz), mounted flush with the floor at centre of a 20m runway. Recorded forces were normalized to the body weight of the subjects by dividing by body weight. Average loading rate was calculated by dividing the impact peak magnitude by the time to the impact peak. Peak loading rate was quantified as the maximum increase in Fz between frequency intervals. Stance time was measured during the time that 5 N of vertical force or greater was applied to the force plate. GRF data was left unfiltered in accordance with the Aquinaldo and Mahar (2003) guidelines to preserve the high frequency components that may indicate the presence of transient forces.
Running velocity was quantified using infrared photocells mounted at chest height on both sides of the force plate 2.0m from the centre of the plate. In accordance with the guideline set by De Wit et al (2000) a maximum deviation of +5% from the set velocity was allowed.
Participants were allocated time in which to complete an adequate warm up and familiarize themselves with the experimental running velocities. Runners completed a minimum of five accepted trials. A successful trial was defined as one within the specified velocity range, with the right foot landing totally on the force plate with no evidence of targeting (Challis 2001 and White and Lage 1993). In accordance with the guidelines established by Nigg (1986) a key criterion for an acceptable trial was an equal anterior and posterior impulse during stance.
The order in which participants wore the different shoes was randomized, whichever shoe was selected to be worn first all of the necessary trials were completed in that shoe prior to running in the other.
UK sizes) and consisted of a conventional running shoe (Saucony Grid Tangent 3) and the HealusTM technology.
Given that forefoot or mid-foot strikers characteristically lack an impact peak, in order to control for differences in foot strike, only participants with a natural heel-toe gait pattern were included in the data analysis. Differences between the two footwear conditions were analyzed using paired samples T-tests with significance accepted at the p 0.05 level. All statistical procedures were conducted using SPSS 17.0 software.
The aim of this study was to provide a biomechanical comparison of HealusTM and conventional cushioned running shoes. Differences in kinetics between the two footwear conditions were examined to gain an understanding of the adaptations runners make when running in the HealusTM footwear. Due to the relatively recent launch of the technology running shoes, this study represents the first comparative study concerning the kinetic characteristics of HealusTM footwear.
The transient shockwave in running is capable of generating significant forces in the joints and other structures of the lower limbs, and is linked to the aetiology of a variety of bony and soft tissue disorders (Whittle 1999). It is important to acknowledge the link between these forces and pathological conditions, since the magnitude of these forces and by implication; the incidence of these conditions can be reduced by attenuating the impact magnitude (Whittle 1999).
The double peaked GRF time history predominantly exhibited whilst running in the Healus running shoe highlights a potential problem when using conventional methods of quantifying the average loading rate. Until such time when the foot-ground interface is modelled appropriately. This study thus further emphasizes the conception that peak loading rate is a more appropriate measure than the average loading rate. Peak loading rate is a more suitable measure of the shock attenuation properties of running footwear as it provides a more accurate representation of the transient shock wave associated with footstrike. Future studies may wish to report peak loading rates to quantify the shock attenuation properties of running shoes.
The HealusTM running shoe is designed to reduce the vertical GRF parameters associated with footstrike. The results of this study support the hypothesis in that the magnitudes of the vertical impact peaks and peak vertical rates of loading were significantly p 0.05 lower in the HealusTM technology shoes compared to the conventional cushioned footwear. This agrees with the findings of Hartveld et al (2005) who reported significantly lower vertical impact peaks and rates of loading when running in the HealusTM running shoes compared to a conventional training shoe. Such loading rate patterns represent the capacity of the running shoes to attenuate the magnitude and rate at which the impact shock is applied to the lower extremities (Logan 2007). These findings lead to the conclusion that the HealusTM technology running shoes have the potential to reduce impact related injuries (Misevich and Cavanagh 1984).