Reliability and Running Speed Effects of In-shoe ...

FWT 81 ANKLEJNTERNATKINAL Copyright O 2000 by the American Orthopaedic Foot & Ankle Society, Inc.

Reliability and Running Speed Effects of In-shoe Loading Measurements During Slow Treadmill Running 'W. Kernozek, P ~ . D . ' BK.A. ~ , Zimmer, PT' La Crosse, WI

ABSTRACT In-shoe measurement systems ailow the clinician and researcher to examine the loading parameters within the shoe. This study sought to investigate the test retest dlability and speed effects of in-shoe loading parameters using the Pedar System (Novel GMBH Munich) during slow treadm!ll running, The results indicated good to excellent test retest reliability between the two days tested, lntraclass correlation coefficients (ICC's) ranged from 0.84-0.99 depending on the plantar region and variable analyzed. All plantar loading variables increased (peak pressure, peak pressure time impulse, peak force, and force time impulse) with the exception of contact area when treadmill running speed was increased from 2.24 m/$ to 3.13 d s . Results indicate that control of running speed is essential in obtaining reproducible data using this system to measure in-shoe loading data. Key words: Pressure distribution, plantar loading, reliability, in-shoe loading, jogging, locomotion. INTRODUCTION

The interface between the foot and shoe is of interest to the clinician and researcher. There are various methods available for examining the loading parameters under the foot including force plates, pressure ptatforms, and in-shoe measurement de~icesl,~. Early in-shoe devices had limitations due to the use of discrete sensors3. Sensors from these systems had a tendency to migrate during activity due to shearing

Qepariment of PhysicalTherapy, Univ%fs&y of Wisconsin-La Cross, La Crosse, W1, USA 2GundersenLutheran Sports Medicine, La Crosse, WI, USA Corresponding Author: Thomas W. Kernozek, Ph.5. University of Wisconsin - La Crosse Physical Therapy Department Health Science Center La Crosse, WI 54601 Phone 608-785-8468 Fax 608-785-M60 e-mail: [email protected]

forces piaced upon them and provided limited information since the sensors had to be placed in predetermined locations. Discrete sensors may also alter performance due to their ability to act as foreign objects within the shoe. More recently, matrices of sensors have been built into insoles to measure loading parameters over the entire plantar surface. However, these sensing systems have some limitations. In-shoe sensors are under constant, repetitive loading when worn predisposing them to breakdown. A warm and humid in-shoe environment may also decrease sensor performance. The sensors have to conform to curved surfaces of the foot, shoe, and/or orthotics. Finally, current in-shoe sensor matrices are incapable of measuring shear loads. This final limitation of in-shoe measurement techniques may be the critical in documenting plantar loading during various gait conditions. When using sensor technology in either a clinical or research setting, it is important that the researcher calibrate each sensor throughout the measurement range to ensure accuracy4. in a sensor system, it is important to calibrate and verify measurements of each individual sensor to a known load due to the differences in characteristics of each individual sensor in the matrix. These differences in sensor characteristics manifest themselves in plantar loading data that is greater or less than the load actually applied. High reliability ensures consistency of measurements across and within time to best enable quality, objective judgements to be made5. Previous research has found excellent in-shoe loading reliability (Intraclass correlation coefficients greater than 0,9)for most plantar loading variables with the Pedar in-shoe measurement system from successive steps during treadmill walkingi. Little is known about the reliability of in-shoe loading parameters during running. There is a need to evaluate in-shoe measurements under other conditions besides walking. The purpose of this study was twofold: 1) to assess the reliability of Pedar system during slow treadmilt running at two different speeds and 2) to examine how in-shoe loading parameters change with increasing running speed.

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METHODS Subjects

Seventeen healthy college-age students (age 24.58 +. 4.70 yrs.) volunteered for participation in this study. Ail were reported to be free from any gait abnormalities or lower extremity pathologies that could affect running ability. lnstrumentaticrn

The Pedar in-shoe measurement system (Novel GMBH Munich) was used for data collection. The system utilizes pairs of sensor insoies of varying size. Each insole consists of 99 capacitance sensors and all were calibrated throughout the measurement range (O600 KPa) prior to data collection. After the calibration procedure, the insoles were checked in the same pressure chamber at a static toad of 300 KPa prior to each testing day. If the insoles recorded a static pressure of 300*10 KPa when loaded to that pressure in the pressure chamber they were not calibrated again for that test. This same procedure was used prior to each pre and post testing session. Each insole is flexible, approximately 2mm thick, and can easily be placed into the bottom of the shoe between the foot and the shoe itself. The insoles were connected by leads to an 8-bit analog-to-digital converter (ADC) box, which was secured to participants' waists via a storage belt. Velcro straps secured the leads to participants' legs at both the ankles and mid-thighs to minimize lead movement while running. The ADC was connected to a laptop computer used to collect data. The system was set to coiiect data from the right insole at a sampling rate of 150 Hz. Twenty-eight sensors in the mid-foot were deactivated to allow the increased sampling frequency to be used, The mid-foot region was associated anatomically within the arch region of the plantar surface of the foot. These sensor regions were determined from a standing weight bearing measurement of each participant. Procedure

Prior to testing all participants read and signed an informed consent form approved by the Institutional Review Board at the University. The tester (KZf recorded age, body weight, and running shoe type, Each participant used their own type of distance running shoe that had mileage of less than 100 miles during the time of participation. A five minute warm-up by walking preceded the running assessment to allow time for participants to become acquainted to the treadmill and equipment. The order of treadmill speed was randomized and assigned to each participant. Each participant then ran at both 2.24 m/s and 3.13 m/s for four minutes and the data were collected for approximately 20 seconds. No rest period was allowed between running speeds.

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Slower running speeds were measured since the sampling rate of the in-shoe measurement system was 150 Hz. Since this sampling rate was low compared lo standard force platform techniques, the researchers tested the participants at a jogging speed rather than a higher running speed, This enabled the researchers to collect more samples of data from all sensors used during the stance phase of the running cycle. A faster running speed would mean fewer data points for used to generate plantar loading parameters. As a result, the tack of an adequate sampling rate may mean that the loading information was simply not captured due to the direct violation of the sampling theorem. A self-determined time for cool-down followed all testing. Participants returned two weeks later for a second testing session with the same footwear and the same testing protocol. All participants again wore their own similar type of running shoe during second testing session. Since this study examined the reliability of in-shoe loading in a test retest situation and also examined the loading differences within speeds identical shoe types were not necessary since each participant served as their own control in this repeated measures experimental design. Data was collected in an air conditioned laboratory facility where the average temperature ranges between 19.4 and 22.8 degrees C (67 and 73 degrees F). Data Analysis

NovelwinTMsoftware (Novel GMBH Munich) was used to calculate the following loading parameters for the four regions of the foot: maximum force (Mff, peak pressure (PP), force-time impulse (FTl), peak pressure time impulse (PTI), and contact area (CA). Software allowed the pressure measurement to be divided into the following four regions: heel (HLf, medial forefoot (MFFf, central forefoot (CFF), and lateral forefoot (LFF). The MFfz measured loading parameters under the head of the first metatarsal, the CFF measured the area under the second and third metatarsal heads, and the LFF measured the area under the fourth and fifth metatarsal heads. Eight consecutive footfalls were analyzed and the means for each of the plantar loading variables were caiculated for each participant for the pre and post test. lntraclass correlation coefficients (tCC's) reflect the both the degree of correspondence and agreement among ratings5. To examine the reliability of each dependent variable, an ICC (2,l) was performed between the two days for each variable! A series of repeated measures multivariate analysis of variance (RM MANOVA) were performed (one on each plantar region) to compare the loading variables for each plantar region between the running speeds (p < 0.05). Follow-up univariate tests were performed to examine each specific plantar loading variable between speeds.

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EFFECTS OF IN-SHOE LOADING MEASUREMENTS 751

RESULTS

lntraclass correlation coefficient (ICC) values obtained are presented in Table 1. The reliability ranged from 0.84 to 0.99 for plantar loading measurements at both of the running speeds examined between the days tested. The LFF region was the least reliable region of the plantar surface analyzed while the HL, MFF and CFF were more reliable. Running speed did not seem to alter the reliability of the plantar loading parameters at the speeds tested.

that running speed did not have a significant effect on CA. Thus, the forces between the foot and shoe were not influenced by more of the plantar surface in contact with the sensor insole in the regions analyzed. Since in the midfoot region the sensors were inactivated, it is unknown how the arch region of the plantar surface is influenced by increases in treadmill running speed. MF increased an average of 10.67% PP increased an average of 10.9g0/~,PTI increased an average of 11.27% and FTI increased an average of 10.97%. Table 2 has the means and standard deviation for each variable for each speed and plantar region analyzed.

Speed

DISCUSSION

Reliability

Results of the RM MANOVA found differences in MF, PP, PTl, and FT1for all plantar regions between treadmill running speeds. Follow up univariate tests indicated

Table 1. lntraclass correlation coefficient values jetween days for 2.24 m/s and 3.13 m/s running ;peed by anatomical region and loading variable, JlF indicates maximum force, PP is peak pressure, CP s contact area, PTl is peak pressure time impulse, anc TI is force time impulse. H L denotes the heel region JlFF the medial forefoot region, CFF the central foreoot region and LFF the heel region. Region

Variable

HL MFF CFF LFF

MF(%BW)

HL MFF CFF LFF

PP(KPa)

HL MFF CFF LFF

CA (cm2)

HL MFF CFF LFF

PTI (KPa*s)

HL MFF CFF LFF

FTI (%BW*s)

Speed (mls) 2.24 3.13

Reliability

The results indicate that the PEDAR system has good to excellent reliability for maximum force (MF), peak pressure (PP), contact area (CA), peak pressure time impulse (PTI) and force time impulse (FTI) at the running speeds of 2.24 meterslsec and 3.13 meterslsec for all plantar regions analyzed. The area under the lateral forefoot (LFF) appears to be the least reliable of the regions analyzed. These good to excellent results are similar to studies that examined the reliability of the PEDAR system during treadmill walking". This study provides evidence that the PEDAR in-shoe measurement system may yield good to excellent test-retest results for slow treadmill running. Variability in plantar loading measurements that influence the reliability is comprised of human variability and measurement error of the instrumentation used in this experiment. Therefore, it is unknown which may have affected our findings. Previous research has demonstrated that gait is variable (Kernozek et a12). Since LFF region was the least reliable it may be that participant may utilize this region of the foot to control body position on the treadmill during running. It is unknown how different plantar regions function with changes in body position and slight directional changes. It may be that the heel region is inherently a more reliable measure while forefoot measurements are inherently the least reliable during slow treadmill running. Speed

The results of this study indicate that running speed influences plantar loading measurements. As running speed increased from 2.24 meterslsec to 3.13 meters1 sec, the loading measurements for MF and PP were significantly higher at 3.13 meterslsec. As one progresses from walking to running, plantar forces and pressures increase compared to Kernozek et all. Few researchers have examined plantar loading during running using the same instrumentation making compar-

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Fable 2. T h e x e c t s of running speed (~2TmEj and 3.13 mls) on plantar loading variables in each ~ g i o n .Means and standard deviations fsdf are preiented at each running speed. '* indicates significant differences fp c 0.05).

,I

I i

Region

Variable 2.24

HL MF(%BW) MFF CFF LFF

58.95 32.91 42.69 26.08

HL PP(KPa) MFF CFF LFF

174.91 243.80 235.40 190.68

HL CA(cm2) MFF

33.57 13.56 17.79 15.99

(4.93) (4.39) (4.30) (3.42)

HL PTI (KPa*s) 15.09 MFF 31.09 CFF 30.30 LFF 24.74

(4.82) (9.10) (7.99) (8.17)

CFF LFF

HL FTI (%%W*s)4.46 MFF 3.89 CFF 5.35 LFF 3.15

(15.1) (9.78) (6.78) (4.80) (30.64) (48.15) (44.12) (39.51)

(1.64) (1.87) (1.16) (0.94)

1

Speed (m/s)' 3.13 65.90 (18.37)**) 35.88 (10.99)** 48.1 5 (7.26)** 29.14 (5.62)** 193.66 269.13 265.61 209.81

I

(41.01)**/ f56.90)** (53.72)** (46.55)**

33.49 13.53 17.79 16.06

(4.9211 (4.42) (4.30) (3.95)

13.04 28.14 27.57 22.59

(4.89)** (8.67jx* (7.66)** (7.83)**,

3.89 3.46 4.86 2.94

(1.62)** (1.74)**1 (1.04)**! (0.94)**1

isons difficult due to differences in sensor size, sampling rate, plantar regions analyzed, footwear differences and running speed. Rozema et al.' reported that plantar pressure was highest under the first metatarsal head and lowest under the midfoot at a slow running speed of 3.0 m/s while wearing an oxford. The authors reported their 50 Hz sampling rate was likely inappropriate resulting in an underestimation of the peak pressures reported. Peak pressures were reported as a mean of 294 KPa for the HL and 304KPa for the MFF. Other anatomical regions anatyzed were different than used in the present study, The PP data in the Rozema et a/.' study was likely higher due to the differences in footwear types (oxford vs. running shoe). Willson and Kernozek8 examined plantar loading and cadence changes in recreational runners during a rested and fatigued states at a higher self selected running speed using the same instrumentation and performing a similar analysis. PF and PP were more similar to the fatigued running condition rather than the rested, how-

ever, values were higher by approximately 10% compared to the present study. As an individual increases their running speed, the time spent in stance decreases due to less trme in contact with the ground. The PTI and FTl may decrease as running speed increases since there is less time the foot is actually in contact with the ground. However, at increased running speeds, the pressures and forces are also higher off-setting the time that the plantar regions are loaded. Compared to the Willson and Kernozek" study of rested and fatigued running at a faster self selected speed, PTI and FTI variables were lower than in the present investigation, The results of this study revealed that an increase in speed significantly decreased the PTI and FTI. Significant differences in plantar loading measurements at the different speeds indicates a need to control running speed when performing repeated trials with the Pedar in-shoe measurement system. CONCLUSIONS

The findings of this study led to the following conclusions: 1. The Pedar in-shoe measurement system showed good to excellent test-retest reliability during slow running at 2.24 and 3.13 m/s on a treadmill for MF, FTI, PP, PTI and CA in four plantar regions analyzed. ICC's ranged from 0.84-0.99. 2. Running speed significantly affects plantar loading variables in the HL, MFF, CFF and LFF regions of the plantar surface. Therefore, it is likely that speed should be controlled during in-shoe loading experiments at similar running speeds. REFERENCES 1. Baumgartner, T.A.: Norm-referenced measurements: Reliability. In M. Safrit and T. Waods (Eds.). Measurement Concepts in Physical Education and Exercise Science. Champaign, IL: Human Kinetics, 1989. pp. 42-72. 2. Kernozek, T-W,, LaMotl, E.E., and Dancisak, M.J.: Reliability of an in-shoe pressure measurement system during treadmill waiking. Foot Ankle., 17: 204-209, 1996. 3. McPoil, T., Cornwall, M.W., and Yamada, W.: A comparison of Wo in-shoe pressure measurement systems. Lower Extremity., 2: 1-9, 1995. 4. Cavanaugh, P.R., Hewitt, F.G. and Perry J.E.: in-shoe plantar pressure measurements: a review. Foot., 2: 185-194, 1992. 5, Partney, L.G. and Watkins, M.P.: Statisticai Measures of Reliability. Foundations of Clinical Research: Applications to Practice. Norwalk, Cf: Appleton and Lange, pp. 505-528, 1993. 6. Shrout, P.E. and Ftceiss, J.L.: lntraclass Correlations: Uses in Assessing Rarer Reliability. Psycho Bull., 85: 420-428, 1979. 7. Rozema, A., Ufbrecht, J.S., Pammef, S.E., and Cavanaugh, P.R.: In-shoe plantar pressures during activities of daily living: implications for therapeutic footwear design. Foot Ankle., 1 7 : 352-359, 1996. 8. Willson, J.D. and Kernozek, T.W.: P!antar loading and cadence alterations with fatigue. Med Sci Sports Exerc. 31: 1828-7833.