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REDUCING GROUND IMPACT FORCES DURING JOGGING:
Robert U. Newton, Brendan J. Humphries, & I. Barry Ward

Center for Exercise Science and Sport Management Southern Cross University, Lismore, Australia.
Running Head Shoes with springs
Date of Submission October 30, 1995
Corresponding Author Robert Newton, Centre for Exercise Science and Sport Management, Southern Cross University PO Box 157, Lismore NSW 2480, Australia · Telephone Int + 61 66 203 234 · Facsimile Int + 61 66 203 880 · Email RNEWTON@SCU.EDU.AU

ABSTRACT
Impact forces experienced during running have been recognised as a source of injury Kangoo Jumps are a revolution in shoe design which includes a spring attached to the sole of the shoe.

The aim of this study was to investigate the effects of wearing Kangoo Jumps on the vertical ground reaction forces produced during the support phase of jogging.

Seventeen male and seventeen female subject participated in the study.

Each completed three trials jogging over a force plate wearing his or her normal training shoes and three trials wearing the KangooJumps.

Vertical ground reaction force was measured at 750Hz and normalised to the subject?s Body weight.

MANOVA with repeated measures followed by univariate ANOVA tests were used to determine the effects of the shoe type.

A criterion alpha lever of p £ 0.01 was used there was a significant multivariate effect of shoe type and subsequent univariate analysis revealed that the mean force and 50 ms impact decreased 5.4 ± 9.3 % and 46.9 ± 14.8 %respectively.

The total impulse, time to peak force, and contact time increased 16.5 ± 7.5 %.24.2 ± 12.5 % and 49.9 ± 24.8 % respectively.

Peak force was not significantly different between the conditions.

It was concludes that the Kangoo Jumps significantly reduced the impact forces experienced while jogging.

Wearing such shoes may have application for the recreational jogger, cross training for competitive athletes, or as a rehabilitiation modality for patients with lower limb injuries.

Key words:
Shoe design, shock absorption, rehabilitation, Kangoo Jumps

INTRODUCTION

The current strategies adopted for sport shoe constrution are to limit ground reaction forces associated with repetitive landings (McNeill-Alexander, 1987; Nigg & Segesser, 1992) and optimise energy return (Nigg & Segesser, 1992). Under normal circumstances of running, jumping and bounding, the elastic compliance of a sports shoe and the internal structures of the foot, such as the arch and supporting tendons, reduce impact forces but provide only 40 to 65% energy return (Mc Neill-Alexander, 1987). Typically, the average runner impacts the ground at 2-4 times their own bodyweight with the initial impact producing damaging effects on the internal tissues and skeletal structures (Nigg, Denoth & Neukomm, 1981).

Invertigations with regards to impact forces have reported that between 27% to 70%.

Of runners or joggers are injured in first year after commencement of running as a result of external forces (clement et al., 1981). Other studies have also shown that between 21% and 50% of tennis players are injured per season (Kuhlund et al., 1979) as a result of excessive forces. Additionally, research involving the military found an increased frequency of overuse injuries for soldiers in relation to the low absorbancy in the hell pad of standard issue army boots (Joergensen & Hansen, 1989).

Typically, when running, bounding or jumping, the musculature and joint mechanism involved in the landing process dampen the impact forces placed on the body. However, it has been reported that the neuromuscular system has a limited reaction time response, between 50 to 75 ms, to forces applied to the body (Nigg, Denoth & Neukomm, 19781; Devita & Skelly, 1992). When a force is applied to the musculature prior to the response of the neuromuscular system, the muscles involved are unable to absorb the shock of landing. This initial sharp peak in the ground impact force has possible implications for injuries to the lower limbs related to Landings (Munro, Miller & Fuglevand, 1987; Ricard & Veatch, 1990). Possible injuries occurring as a result of impact landings include cartilage degeneration (Radin, et al., 1973), fatigue fractures, shin splints (Andreasson & Peterson, 1986) and achilles tendon problems (Joergensen, 1985).

The initial impact forces that occur within this 50 ms time frame of landing are referred to as passive impact forces (Ricard & Veatch, 1990). It is these passive impact forces that have been speculated to resut in injuries to the musculature and skeletal systems (Ricard & Veatch, 1990). The impact forces that the body sustains from landing appear to be influenced by the inter-relationship between several mechanical parameters. The height of the stride, jump of bound (Stacoff, Kaelin & Stuessi, 1988; Dufek & Bates, 1990) determines the velocity at impact (Valiant & Cavanagh, 1983; Munro, Miller & Fuglevand, 1987; McNitt-Gray, 1991; Devita & Skelly, 1992) and therefore the loading applied to the body (Frederick & Hagy, 1986; Scott & Winter, 1990). Subsequently it is the momentum attained prior to ground contact that influences the impact loads upon landing (McNitt-Gray, 1991). Landing technique (Frederick & Hagy, 1986; Scott & Winter, 1990; Steele, 1990) and landing surface (McNeill-Alexander, 1987) have been shown to effect the impact forces by changing the time period over which the negative work of absorbing the downwards momentum is reversed to the positive work leading to toe off. Any device which could attenuate the impact forces associated with foot strike bears investigation for its possible use for reducing injury risk during recreational activities and also rehabilitation from injury.

Recently, there has been a fun and fitness shoe developed, the Kangoo Jumps shoe (Figure 1), that would appear to attenuate the damaging effects of high impact forces. The shoe is constructed in a similar fashion to that of an inline roller skate with the exception of wheels and bearings. Under the shoe is an eliptical arch stretching from the toe to the heel of the shoe that is bisected with a removeable band. At each depression of the arch the band absorbs the energy and returns it in the form of positive work. The design of these shoes is quite revolutionary and as such the implications for use in training and rehabilitation need to be investigated. Injury to the lower extremities an back as a result of chronic exercise involving ground impact is prevalent in a wide rage of sports and recreational activities. Further, rehabilitation from injury aims to improve muscle function, coordination, and cardiovascular fitness without impeding recovery or exacerbating the injury. The problem is to provide ground based exercise while maintaining a low impact environment.

Therefore, the purpose of this study was to investigate the effect of wearing the Kangoo Jumps on the ground impact forces associated with jogging. Particular attention was focused on the initial impact impulse and impact spike which have been shown to be associated with injury.

METHODS

Subjects
Seventeen healthy males and seventeen healthy females volunteered to take part in the study. The subjects were all undergraduate students and none reported any significant musculoskeletal disorder. The Subject?s mean (±SD) age, height and weight were 22.6 ± 6.6 yrs 1.73 ± 0.10 m and 70.7 ± 9.2 kg respectively. The study was approved by the Ethics Committee of the Southern Cross University, and all subjects signed an informed consent document prior to the commencement of testing.

Testing Procedures

The subject?s age, height, weight, and shoe size were recorded. The subject was then instructed to run at his or her normal jogging speed over the force plate while trying to ignore the force plate, that is, without attempting to target the plate. All subjects began their approach from a minimum of 10 m from the force plate and continued to jog for 10 m past the force plate. This was to ensure a consistent gait pattern when crossing the plate. If the subject missed the plate or the foot contacted the edges of the plate, the starting position was moved back by the distance that the foot missed the centre of the plate and the trial was repeated. Each subject completed three trials wearing the Kangoo Jumps and three trials wearing their normal training shoes. As none of the subjects had worn the Kangoo Jumps previously each was allowed an unlimited amount of time to familiarize themselves with jogging in the boots. Not until the subject was comfortable did they begin the test trials. The order of the conditions was randomized between subjects to reduce the possible confounding effects of learning, fatigue of boredom. During each trial vertical force data from the force plate was collected and stored for later analysis.

Equipment

Normal Training Shoes. The subjects were instructed to wear his or her normal training shoes. That is , the shoes they would normally wear when jogging or playing sport. These shoes will be referred to in the remainder of this paper as normal training shoes.

Kangoo Jumps. The Kangoo Jumps (Kangoo Jumps, Switzerland) (Figure 1) consist of a rigid boot similar to that used in a pair of inline skates with a leaf spring configuration attached to the sole. A plastic band stretches between the hing points of the leaf springs to provide added stiffness and elastic recoil. Different band thickness are available and were selected based on the subject?s body mass and the manufacture?s recommendations. The heavier the subject, the greater the thickness of band used.

Force Measurement System. Vertical ground reaction force was measured by means of a force platform (Type 9287) (Kistler, Switzerland) mounted level with the surface of an articifial running track. The amplified signals from the charge amplifiers were passed to a DT01EZ analog to digital card (Data Translation, Marlboro, MA, USA) in an 80486DX computer running Windows 3.11. The digitized data were stored on computer disk for later analysis. The force measurement system was calibrated prior to each testing session. Vertical ground reaction force was sampled at a frequency of 750 Hz.

Data Analysis
The force data were normalized by dividing by the subject?s body weight in Newtons. The following variables were then calculated to assess the impact characteristics of the foot contact phase. Peak force was measured as the highest force recorded. Mean force was calculated as the average force over the entire contact phase. Total impulse was calculated byx integrating the force time curve for the contact phase. The impact over the initial 50 ms was calculated by integrating the force time curve over the 50 ms following the start of contact. The time to reach the peak force was measured from the start of contact to ehen the highest force was recorded. Contact time was the time that the foot was in contact with the plate. The mean of the three trials for each variable was then calculated and used in the statistical analysis.

Statistical Analysis
The results for peak force, mean force, total impulse, 50 ms impact, time to peak force, and contact time were compared using MANOVA with repeated measures and univariate ANOVA for follow-up comparisons. The criterion level for significance was set at p =<0.01.

RESULTS

The MANOVA revealed no significant effect of gender on the dependent variables so the data was pooled for all subsequent analysis. There was a significant multivariate effect of shoetype and subsequent univariate analysis revealed that the mean force and 50 ms impact decreased 5.4 ± 9.3 % and 46.9 ± 14.8 % respectively (Table 1). The total impulse, time to peak force, and contact time increased 16.5 ± 7.5 %, 24.2 ± 12.5 %, and 49.9 ± 24.8 % respectively (Table 1). There was no significant difference in peak force (Table 1). Figure 2 contains a graph of the force time data for a representative subject which shows that the forces exerted during the contact phase increase more gradually and are applied over a longer time period for the Kangoo Jumps compared to the normal training shoe.

Table 1: Impact forces and force time characteristics foot strike when wearing Kangoo Jumps vesrus normal training shoes.
* indicates significant difference between normal shoes and the kangoo Jumps at the 0.01 level.

Figure 2: Ground reaction force during the stance phase of jogging for a representative subjet wearing normal training shoes and the Kangoo jumps

DISCUSSION
The purpose of this study was to investigate the ground reaction force characteristics when jogging in a pair of Kangoo Jumps compared to normal training shoes. The results clearly show that the Kangoo Jumps result in considerably less impact forces being transferred

To the lower limbs compared to the normal training shoes. In particular, the impulse over the first 50 ms following initial ground contact was reduced some 47 % which should translate into reduced risk of impact force induced injury as a result of jogging in the Kangoo Jumps.

Figure 2 clearly shows that the contact phase is lengehened such that the forces are applied over a longer period of time. The force rises more gradually after impact and as can be seen for the representative subject, the impact spike is not observed in the Kangoo Jumps trial. Further, the time to achieve th peak force is increased. Overall, the impact of foot strike is considerably more gentle than that resulting from normal training shoes.

The peak force was not significantly different between the two shoes and this was somewhat unexpected, particularly given that the contact time was lengthened suggesting that a lower overall force was being produced over greater time, requiring less magnitude of force. The overall force was lower as indicated by the lower mean force, however, the total vertical impulse was greater for the Kangoo Jumps. This indicates that the subjects may have adopted a more bouncing gait pattern with increased vertical oscillation between strides. Thus, the greater emphasis on vertical motion may help to explain the fact that the peak force was not significantly different. The effect of the Kangoo Jumps on normal gait patterns requires further investigation.

This preliminary study shows promise for the use of Kangoo Jumps for low impact exercise such as recreational jogging and cross training for athletes. The uninjured person may be able to continue with a high volume of ground based activity without the impact injury risk inherent in their usual activity or sport. Further, there may be application of these shoes in rehabilitating patients with lower limb injuries. The patients could perform activities similar to normal running, recovering muscle strength, and maintaining cardiovascular fitness, but in a low impact situation. Jogging with Kangoo Jumps may be a good intermediate step in the rehabilitation process between non-support work and fully loaded running. Such training also has the advantages of being functionally similar to a normal gait pattern.

CONCLUSIONS
The wearing of Kangoo Jumps rather than normal running shoes when jogging significantly reduces the impact characteristics experienced during foot contact which have been reported to result in injuries to the lower extremities. Therefore, the Kangoo Jumps may have application for recreational jogging as well as providing a safer environment for rehabilitiation of patients with lower extremity injuries. Further research work is required to assess the changes in gait pattern that the Kangoo Jumps may produce, energy efficiency of jogging in the Kangoo Jumps, and the efficacy of using them in the rehabilitiation setting.

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*indicates significant difference between normal training shoes and the Kangoo Jumps at the p

<= 0.01 level