energy storage and release of prosthetic feet part 1- biomechanical analysis related to user...

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DOI: 10.3109/03093649709164526

1997 21: 17Prosthet Orthot IntK. Postema, H. J. Hermens, J. De Vries, H. F.J.M. Koopman and W. H. Eisma

ergy storage and release of prosthetic feet Part 1: Biomechanical analysis related to user benefits

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Prosiheiics and Orihotics Intemaiional, 1997, 21, 17-27

Energy storage and release of prosthetic feetPart 1:biomechanical analysis related to user benefits

K. POSTEMA*, H. J. H E M E N S * * , J. D E V R E S *** , H.F.J.M. KOOPMAN***an d W. H. EISMA*****Research and Development, St. M aartenskh iek, Nijmegen, The Netherlands**Roessingh R esearch and Development,Enschede, he Netherlands***Bwmed ical Engineering Groupofthe Universityof Twente,Emcheak, Th eNether&****Department of Rehabiliiaiion Academic Hospital of Groningen, Th e Neiherlands NL

AbstractThe energy storing and releasing behaviour of2 energy storing feet (ESF) and 2 conventionalprosthetic feet (CF) were compared (ESF: OttoBock Dynamic Pro and Hanger Quantum; CF:Otto Bock Multi Axial and Otto Bock Lager).Ten trans-tibial amputees were selected. Thestudy was designed as a double-blind,randomised trial. For gait analysis a VICONmotion analysis system was used with 2 AMTIforce platforms. A special measuring devicewas used for measuring energy storage andrelease of the foot during a simu lated step.The impulses of the anteroposteriorcomponent of the ground force showed small,statistically non-significant differences (de-celeration phase: 22.7-23.4 N s; accelerationphase: 17.0-18.4 Ns). The power storage andrelease phases as well as the net results alsoshowed sm all differences (maximum differencein net result is 0.03 J kg ). It was estimated thatthese differences lead to a maximum saving of3% of metabolic energy during walking. It wasconsidered unlikely that the subjects wouldnotice this difference. It was concluded thatduring walking differences in mechanicalenergy expenditure of this magnitude areprobably not of c linical relevance.Ankle power, as an indicator for energystorage and release gave different results to theAll correspondence to be addressed toK. Postema, Research and Development. St .Maartenskliniek, Postbus 901 1, 6500 GM Nijmegen,The Netherlands.

energy storage and release as measured with thespecial test device, especially during landingresponse. In the biomechanical model (based oninverse dynamics) used in the gait analysis thedeformation of the material is not taken intoconsideration and hence this method of gaitanalysis is probably not suitable for calculationof shock absorption.IntroductionThe general concept of energy storage andrelease of prosthetic feet is that they storeenergy during mid-stance and release the energywhen it is desired, i.e. during push-off. Theseevents are based on two major phases (Winterand Sienko, 1988) consistently seen in anklepower graphs in normal subjects. A long energydissipation phase, A l , is thought to be a resultof eccentric contraction of plantar flexors as theleg rotates forward over the flat foot, which isfollowed by a large energy generation phase,A2 (see Figure 4, sound side). This phase is dueto concentric activity of the plantar flexorsbefore toe-off. These parameters are calculatedwith the use of a biomechanical model, basedon inverse dynamics.In conventional prosthetic feet most of thestored energy is dissipated in the m aterial. In socalled energy storing feet most of the energy issaid not to be dissipated in the material, butstored in the spring mechanism that shouldrelease it during push-off. Quantities of energystorage and release, as calculated from gaitanalysis, are not only dependen t on the material

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18 K. Postema, H . J . Hemens , J de Vries. H. F. J. M.Koopman and W. . Eismaand construction of the prosthetic foot, but alsoon many variables concerning the user, such aswalking speed and body weight. Besides,footwear has a m ajor influeylce on the propertiesof prosthetic feet (Jaarsveld et al., 1990).One of the main contributors in thecalculations of energy storage and release is theground reaction force. Th e pattern of the groundreaction force may be a valuable indicator,because different authors suggest that a largerenergy release of the prosthetic foot, duringpush-off, results in an increase of the secondmaximum of the anteroposterior groundreaction force (Blumentritt et al., 1994; Wagneret al., 1987). This second maximum of theground reaction force is a direct measuredparameter without being influenced byassumptions of a biomechanical model.However the literature is not unanimous anddifferent authors were not able to confirm anincrease in this parameter for energy storingfeet (Arya et al., 1995; Barr and Siegel, 1992;

Otto Bock Dynamic Proa: synthetic springb: adaptor

b0

Menard et al., 1992; Torburn et al. , 1990 and1995).Kinematics and stride characteristics are oftenused to prove differences between conventionaland energy storing prosthetic feet. Barr andSiegel (1992), Lehmann et al. (1993), Perry andShanfield (1993) and Wagner et al. (1987) re-ported for the energy storing feet a statisticallysignificant increase in the ankle range ofmotion, especially in late stance dorsiflexion.The increase in dorsiflexion range is probablydependent on the construction of the prostheticfoot. It is not clear however whether there is arelation between properties of energy storageand release and late stance dorsiflexion.Aim of this part of the studyThe aim of the study was to obtain a betterunderstanding related to user benefits of energystoring and release behaviour of som e prostheticfeet that are used regularly in patient care. Itwas clear that there were some subsidiary

Hangar Quantuma: cosmetic coverb: glass fibre springc: attachment place modulefor adaptor

C Cb ba a

Otto Bock Multi Axial Otto Bock LagerFig 1. Basic constituents of 4 prosthetic feet



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Prosthetic feet: biomechanicsand benefits 19questions which needed to be answered, suchas: does gait analysis show differences in

kinematic data and mechanical energystorage and release between so calledenergy storing feet and conventional feetwhen the feet are selected from the sameprice range? If differen ces do exist, are theyclinically relevant?does energy storage and release asmeasured in a special test device(Biomedical Engineering Group of theUniversity of Twente) with a standardizedstance phase differ from the energystorage and release as calculated from thegait analysis?In other words what is theinfluence of the subject, in the laboratorysituation, on the storage and release ofenergy of the prosthetic feet?

Materials and methodsProsthetic feetThe following 4 designs of prosthetic feetwere chosen, Otto Bock Multi Axial (CFl),Otto Bock Lager (CF2). Otto Bock DynamicPro (ESFl) and Hanger Quantum (ESF2). Thefirst 2 are of a conventional variety and theother 2 are known to have energy storingproperties. Some experienced orthopaedictechnicians described the subjectivecharacteristics of these 4 as follows:Otto Bock Multi Axial: stiff, mobile insagittal and frontal plane.Otto Bock Lager: supple, mobile inplantar flexion direction (with ankleaxis), stiff in dorsiflexion direction.Otto Bock Dynamic Pro: stiff in alldirections.

34346352586650435042

Hanger Quantum: very supple in allFigure 1 shows the basic constituents of the

feet. The energy storing feet both show a springmechan ism wh ile the others do not.Since it is generally known that the propertiesof the shoe (e.g. stiff or supple) influence theproperties of the prosthetic foot during walkingall subjects were provided with the same brandof supple shoes.

directions.

SubjectsTen trans-tibial amputees were selected. Theywere all active walkers who were able to walkat least 1 kilometre without any problem. Noneof the subjects had any stump problem. Allsubjects were informed in detail about the studyand signed an informed consent form. Table 1summ arises the descriptions of the subjects.

Table 1. Description of subjects

87859564.5828482801685

Study design and data analysisThe study was designed as a double blind,randomised trial. Neither the investigator northe subjects know which variety of foot wasmounted on the prosthesis. A co-worker carriedout the randomisation with the aid of a dice.The code was broken after the entire trial wascompleted. Every time a foot was supplied therewas a habituation period of 2 weeks. Themeasurements were then canied out. After themeasurements the foot was replaced with adifferent kind. Correct alignment of theprosthesis is very important because itinfluences the walking properties of the footand consequently the energy absorption andrelease. The feet were therefore always fittedand an alignment carried out by the sameorthopedic technician who was not otherwise

Body-weight &g) Years sinceamputation16(reamputation)282623462023231

34-24

Reason foramputation Own prostheticfoottraumaticcongenitaltraumatictraumatictraumatictraumatictraumatictraumaticvasculartraumatic

QuantumSeattleQuantumSeattle1H32EndoliteEndoli eID10ID101H32



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20 K. Po s t e r n , H . J . Hermens, J d e Vries, H. F. J . M.Koopman and W.H. Eismainvolved in the trial.Gait analysis

All the measurements were carried out at theRoessingh Research and DevelopmentLaboratory based at Het Roessingh, Centre forRehabilitation. Th e gait analysis was carried outusing a VICON m otion analysis system (OxfordMetrics Ltd, Botley, Oxford, UK). The systemconsisted of 5 standard ccd cameras fitted withinfrared filters linked to an Etherbox dataacquisition system and a host computer (MicroVAX 3100). AMA SS software for three-dimensional data collection was used forcapturing kinematic data. The marker detectionrate was 50 Hz. Two AMTI force platforms,operating at a sampling rate of 200 Hz, wereused in conjunction to determine the groundreaction forces. VICON Clinical Managersoftware as used to compute walking speed,kinematic and kinetic data.During the gait analysis 13 reflective m arkerswere taped to both sides of the body atdesignated anatomical landmarks such as thesacrum, hips, upper legs, knees, lower legs,ankles and at the dorsal aspect of the metatarsalphalangeal joint 11.The subject was positioned at the end of thewalkway and was asked to walk at acomfortable speed (free velocity). The distancebetween the force platforms was adapted, basedon estimated step length of the subject, toaccomplish a clean foot strike on each forceplatform. The walkway was 10 metres long butonly a length of 4metres in the middle of it wasdesignated for data collection. The subjectswere not informed about the use of the forceplatforms and they were not asked to hit them.At each session 10 trials were selected in whichboth feet hit the force platforms cleanly. Usingthe VICON Clinical Manager an average of the10 trials was calculated. Then the kinematicsand kinetic parameters were ascertained.Walking speed and cadence were measured inorder to determine if differences exist which arelikely to influence the ground reaction force.The following parameters were calculatedfrom the gait analysis:walking speed.cadence (stepm in'l).range of movement at hips, knees andankles, with early stance plantar flexionand late stance dorsiflexion.

impulse of deceleration and accelerationphase of the anteroposterior component ofthe ground reaction force. The impulse isthe time-integral between the zerocrossings.energy storage (A1 phase), release (A2phase) and final net values are calculatedfrom the total ankle power.

HysteresisHysteresis (internal friction) of the material ofa prosthetic foot results in loss of energy whenvariable loading on the foot is applied. This lossof energy for the 4 test feet was measured usinga special test device of the biomedicalEngineering Group of the University of Twente,as described by Van Jaarsveld' el al. (1990), bu twith the difference that the foot is loadedcontinuously while rotating from heel toforefoot (artificial roll-off movement). All ofthe feet were of the same size and weremounted on the test device with the same shoe.Energy is calculated as the integral of forcewith respect to displacement. The forcegenerated by the test device was equal to themeasured vertical ground reaction force as afunction of the shank floor angle of a subject(amputee, go od walker) of m ass 80 kg. Thedisplacement was taken as the deformation ofthe foot as a result of this applied force. Thehorizontal ground reaction force was notapplied. In this study the angle between theshank and the floor was 32" at initial floorcontact and 40' at toe-off.Figure 2 shows the graph of energy storageand release as measured with the test device. Itshows the amount of energy (J , Y-axis) againstshank-floor angle (X-axis). Initially there isstorage of energy by the hindfoot duringlanding (shock absorption) as indicated by thedownward direction of the graph during earlystance (A). The amount of energy stored (A') isa measure of the stiffness of the hindfoot. Theless the energy stored the stiffer is the hindfoot.From foot-flat to mid-stance (B ) some energy isreturned (B'). The amount of energy which isnot returned (B") is mainly dissipated althoughpart might be transferred to the forefoot andreturned during push-off. However, it isassumed that the amount of energy transferredto the forefoot in this way is negligible. Aftermid-stance until push-off (C ) the origin of theground reaction force shifts to the forefoot. Th e



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Prosthetic feet: biomechanics an d benefits 21

0-1

-2-34

-5-6-7-8-9

loading on hindfoot loadingon orefoot w

shank-floor angle e>B

4 0 -30 -20 -10 0 10 20 30 . - 40Fig. 2. Energy storage and release o f a prosthetic foot, as measured with the test devi ce (for explanation see text).

forefoot is deformed and therefore storesenergy. The amou nt of energy stored (C') duringthe latter half of the stance phase (from mid-stance until push-off) is a measure of thestiffness of the forefoot. During push-off (D)part of that energy is returned (D). Th edifference between the value at mid-stance andthe value at toe-off (D') is the energy loss as aresult of absorption of energy at the forefoot.B " + D gives the amount of the total loss ofenergy, due to absorption.StatisticsFor statistical calculations multivariateanalysis of variance was used with repeatedmeasures' design with difference contrast. Thusthe results for the different feet of every subjectare compared within this subject, the onlywithin subject factor was the type of prostheticfoot.ResultsOne subject did not show up for themeasurements with one of the feet. Thisresulted in a missing value. All statistical

procedures, concerning the gait anaIysis, wereperformed on the resu lts of 9 subjects.Walking speed a nd kinem atic dataWalking speed and kinematic data are shownin Table 2. During every session the subjectswere asked to walk at a comfortable walkingspeed.The mean w alking speed was almost the samefor the 4 feet (1.3-1.36 ms'). The meandifference between the 4 feet was 0.02m s'.The mean cadence was almost the same forthe 4 feet (0.87-0.88 steps min'). The meandiffyrence between the 4 feet was 0.01 stepsmin .The range of motion of the hip with the CF2was statistically significantly (p=0.04) largerthan with the ESFl and the CF1. The differencewas less than 2".The range of motion of the knee showed nostatistically significant differences between the4 feet ( ~ 4 . 1 1 7 ) . he range of motion at theankle with the CF2 was greater than that of theother 3 feet. The difference was statisticallysignificant (p=0.003). The larger range of the



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22 K. Postema. H . J . Hermens, J de Vries,H. F. J . M. oopman and W . H. EismaTable 2. Walking speed and kinematic data of amputated side; mean values and standard deviations.

Kinematic data

cadence (steps min I)hip ROM (O)knee ROM (9ankle ROM e)late stance dorsiflexion (9early stance plantar flexion (O)

walking speed (m s I)~ ~ESFl (s.d.)

1.35 (0.20)0.88 (0.07)46.6 (5.0)65.2 (7.7)16.8 (3.6)13.4 (2.9)3.4 (1.6)

*Statistically significant.motion with the CF2 was probably due to theankle mechanism, which allows the foot tomake a plantar flexion movement directly afterheel contact. The early stance plantar flexionwith the CF2 was 9.4", while for the other feetthis motion was 4 to 6" smaller (p=O.OOl).There was no statistically significant differencein late stance dorsiflexion for the 4 prostheticfeet (p=O.145).

Energystorage (A l)release (A2)absorption (net result A 1 and A 2)

Anteroposterior component of the groundreaction forc eTable 3 gives the averages of the impulses ofthe anteroposterior component of the groundreaction force in the deceleration phase and inthe acceleration phase. The measurements withthe 4 prosthetic feet did not show anystatistically significant difference in theimpulses of the deceleration (p=0.307) oracceleration (p=O. 179) phase.

ESFl (s.d.) ESF2 (s.d.) CFl (s.d.) CF2 (s.d.)0.125 (0.03) 0.17 (0.02) 0.13 (0.06) 0.15 (0.10)0.05 (0.02) 0.04 (0.02) 0.04 (0.01) 0.03 (0.02)0.10 (0.02) 0.12 (0.02) 0.09(0.05) 0.12 (0.09)

Energy storage and release as calculated fro mthe total ankle po we rTable 4 gives the mean values of energystorage during phase A1 and energy releaseduring phase A2 with all prosthetic feet,

ESF2 (s.d.)1.34 (0.21)0.88 (0.06)46.7 (4.8)62.4 (13.5)17.2 (2.8)13.7 (2.5)3.2 (1.5)

0.88 (0.05)45.6 (4.4)58.1 (12.6)17.2 (3.8)12.4 (3.0)5. 3 (2.1)

CF2 (s.d.)1.34 (0.20)0.87 (0.07)41.3 (4.7)*60.0(11.9)21.2 (2.6)*11.9 (2.2)9.4 (3.6)*

calculated from the total ankle power. The meanstorage of the ESF2 (0.17) was more than thatof thhe; other feet (whic h varied from 0.13 to 0.15J kg). The differences however were notstatistically significant (pd.675). Thedifferences in the release of energy, ,between the4 feet, were smaller (0.03-0.05 J kg .However these differences were statisticallysignificant. The release of energy of the ESF2was greater than that of the CF2 (p=0.026) andthe release of energy of the ESFl was greaterthan that of the other feet (pd.025). The netresult of the energy storage and release givesthe absorption during the A1 and A2 phases.There were only small differences in the netresults of the energy storing and releasingphases and these differences were notstatistically significant (p= 0.549).Energy storage and release as calculated withthe special test deviceFigure 3 gives the curves of the energystorage and release of 4 prosthetic feet (samesize, same shoe). It shows the amount of energy(J , Y-axis) needed to reach a specified shankangle (X-axis). The values are given in Table 5.

Table 3. Impulses (N ) of the deceleration and acceleration phases of the anteroposterior ground reaction forces withstandard deviation shown in brackets.I Phase I ESFl (s.d.) I ESF2 (s.d.) I CF1 (s.d.) I CF2 (s.d.) 1

DecelerationAcceleration

22.7 (4.5) 23.4 (4.2) 23.1 (3.9)I 18.4(3.9) I ::::,:::::18.3 (4.1) 17.6 (4.1)



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Prosthetic feet: biomechanics and benefits

FootESFlESF2C F lCF2

23

A B' c D E F G"5.7 (0.071) 3.5 (0.044) 2.2 (0.028) 5.0 (0.063) 4.0 (0.050) 1.0 (0.013) 3.2 (0.040)5.8 (0.073) 3.1 (0.039) 2.7 (0.034) 4.5 (0.056) 3.7 (0.046) 0.8 (0.010) 3.5 (0.044)6.8 (0.085) 4.0 (0.050) 2.8 (0.035) 5.3 0.066) 3.7 (0.046) 1.6(0.020) 4.4 (0,055)6.3 (0.079) 3.0 (0.038) 3.3 (0.041) 3.9 (0.049) 3.4 (0.043) 0.5 (0.006) 3.8 (0.048)

ESF2

shank-floor angle (O)-9 . . . I4 0 -30 -20 -10 0 10 20 30 40Fig. 3. Energy sto rage and release of 4 prosthetic feet: ESF l (Otto Bock Dynamic Pro),ESF2 (Hanger Quantum), CFI(Otto Bock Multi Axial), CF2 (Otto Bock Lager) as measured with the test device (J).

The results can be summarised as:hindfoot, energy storage (A): the hindfootof both non-energy storing feet store moreenergy.hindfoot, energy release (B'): most energywas released by the CFl .hindfoot, energy dissipation: the largestdissipation of the hindfoot, was found inthe CM. oth energy storing feet showedthe lowest dissipation.

forefoot, energy storage (C):he forefootof the CF1 stored the most energy, whilethe CF2 stored the least energy.forefoot, energy release (D): most energywas returned by the ESF1.forefoot, energy dissipation (D'): lowestdissipation was in the CM and highest inthe CF1.total dissipation (B"+D): his was lowestfor the ESFl and highest for the CF1.



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24 K. Posrema, H . J. Hermens, J d e Vries, H . F. J. M. oopman and W. . EismaDiscussionWalking speed and kinematic dataWalking speed and ca dence: in the laboratorysituation the mean of the differences of walkingspeed and cadence were so small that it isassumed that it is unlikely these differencesinfluenced the parameters which are based uponthe ground reaction force. This is in agreementwith most reports in literature.Hip and knee range of motion: in the resultsreported here the CF2 showed a very small, butstatistically significant (p=0.040) larger hiprange of motion than the ESFl and the CFI.The difference however was so small that ithardly could be of clinical importance. Thedifferences of the knee range of motion weremuch bigger, but these differences werestatistically not significant (p=0.117).Ankle range of motion: most studiesdemonstrate an increase in the range of motionof the ankle of energy storing feet (Barr andSiege], 1992; Lehmann et al., 1993; Perry andShanfield 1993; Wagner et al., 1987).The results reported here were different fromthese findings. It was found that the range ofmotion at the ankle of the CF2 was clearly andstatistically significantly (p=0.003) greater thanthe range of motion at the ankle of the otherfeet. This was probably due to the anklemechanism in this foot.Looking a t the ankle motion, the total range isthe sum of motions during early stance in theplantar flexion direction, and then in late stancein dorsiflexion. The early stance plantar flexionof the CF2 was clearly and statisticallysignificantly greater than that of the other feet(p=O.OOl).The early stance plantar flexion ofthe CF1 was greater than that of the ESFl andthe ESF2 (p=0.050). Both conventional feet(with ankle axis) showed greater early stanceplantar flexion motion. The late stancedorsiflexion motion in the ankle showed nostatistically significant difference (p=O. 145).Various authors have reported a greater latestance dorsiflexion for energy storing feet.However, mostly the SACH foot was used asthe conventional foot, while probably otherconventional feet, especially those with anankle joint, would have shown a greater rangeof motion at the ankle. Besides an ankle axis,the stiffness of the foot also will have aninfluence on the range of m otion at the ankle.Not only foot-related factors play a role in the

range of motion at the ankle. Different authorshag reported that the physical condition of theamputee, traumatic amputees versus vascularamputees, is an important influence on gaitparameters such as the late stance dorsiflexionand walking speed (Barth et al . , 1992; Casillaset al., 1995; Hermodsson et al., 1994; Torburnet al., 1990). This means that the more activethe subject, the more late dorsiflexion heneedstmakes. In the authors study the grou p ofsubjects was too small and too homogeneous(all active walkers) to enable this to beconfirmed.Importance of late stance dorsiflexionIt can be argued that the late stancedorsiflexion of the prosthetic foot is related tobalance control. Balance control is, among otherthings, dependent on proprioceptive feedback.Froprioceptive feedback is impaired in patientssuffering from polyneuropathy (i.e. due todiabetes).An increase in late stance dorsiflexion resultsin an increase of knee flexion moment andthereby decreases knee stability. With poorbalance control this leads to unsafe situationsand therefore these amputees prefer prostheticfeet which allow only limited late stancedorsiflexion. This is in agreement with theconclusion of Casillas et al. (1995) that thevascular patient seeks easy proprioceptivesupport control on the amputated side - in otherwords, maximum safety. Late stancedorsiflexion permits a supple roll-offmovement. With limited dorsiflexion the roll-off movement will be impaired. Most activewalkers do not prefer this, they prefer a suppleroll-off. High level active subjects, withoutimpairment of balance control, therefore shouldbe provided with a prosthetic foot which allowsa wide extent of dorsiflexion (Barth et al . ,1992).Energy storage and releaseIn the literature different methods aredescribed to assess energy storage and releaseof prosthetic feet. Some authors calculated anefficiency parameter from energy storage andrelease (Barr and Siege], 1992; Schneider et al.,1992). The energy release is expressed as apercentage of that stored. h i s is a relativeparameter which gives information about theproperties of the foot materials. However it



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Prosthetic feet: biomechanics and benefits 25gives no information about the absolutequantities of energy storage and release andthus it gives no information about the am ount ofenergy dissipation which in the authors' opinionis essential with respect to ener gy expenditure.Others consider the sum of the absolutevalues of the A1 and A2-power bursts as ameasure of efficiency (Ehara er al., 1983; Gohet al., 1994). With resp ect to energy expend iturehowever this gives no clear information,because the 'same energy' is appraised twice(first during the storing phase and secondlyduring the releasing phase).Sacchetti et al. (1994) looked at both bursts(A1 and A2) sepa rately and also at the net valueof both bursts as a measure of energy storingand releasing capacities of the prosthetic foot.In this way information about energy storageand release, as well as total amount ofdissipation is obtained. The authors considerthis is preferable and therefore used this methodof describing and discussing energy storage,release and dissipation.This study showed only little differences inenergy storage, release and dissipation, despitethe differences in construction of theconventional and energy storing feet. Thestorage during the A1 phase showed nostatistically significant differences between the4 feet. The differences in energy release duringthe A2 phase were even smaller, however,statistically significant. The net results also didnot show a statistically significant difference. Itis possible that with a greater number ofsubjects the.differences in energ y storage and inthe final net results also become statisticallysignificant. Even if this were true, aredifferences of this s ize of clinical relevance? Alower leg amputee needs f 10% more energythan a normal subject. A normal subject needsabout 3.1 to 3.3 J kg.lrn-' and a lower legamputee needs about 3.4 till 3.6 J kglm-' o!metabolic energy to walk at a speed of 5 km h-(Corcoran 1971; DOM and Roberts, 1992). +stride, about 1.5 m long, needs 5.1 till 5.4 J k g .The biggest difference in the net absorptionbetween the 4 feet was 0.03 J kg' per stride(mechanical energy). This means that thesubject needs 0.03 J kgl less for walking withthe foot with the least net absorption. Thepower, necessary for walking, is the result ofmuscle activity. By a rough estimation, theefficiency of muscles is 20 to 25%. The refore to

supply 0.03 J kg' ffectively for wafking, themuscles have to raise 0.12 to 0.15 J k g ,which isless than 2.5 to 3% of the energy needed for 1stride. There does not appear to be data in theliterature identifying the difference in theexpenditure of metabolic energy which can benoticed by subjects, while walking at comfortablespeed. It is however unlikely that subjects noticesuch small differences during normal walking, orthat a gain of this size is of clinical importance.Only at the top level of sport could differences ofthis size be of importance.Gait analysis versus rest deviceA comparison of the energy storage and releaseof the hindfoot, as shown in the graph of the totalankle power and as shown in the grap h of the testdevice showed some remarkable differences.Figure 3 shows the pattern of mechanical energystorage of the hindfoot, as measured with the testdevice. The storage takes place at heel loading. Itis directly followed by a release of a part of thestored energy, until mid-stance. Although thestorage of energy in the hindfoot during loadingwas a little larger for both conventional feet, thesubjects did not experience a clear differenceduring heel loading. The storage and release ofenergy during the first part of the stanc e phase, asmeasure d with the test device, was not seen in thegraph of the ankle power, as is shown in Figure 4.The reason for the observed difference ofenergy absorption during loading seems to betwofold. Firstly: the special test device uses forcalculation forces and displacement., caused bydeformation of the heel. Th e gait analysis uses forcalculation an inverse dynamic model, based onrigid bodies approximation. This model does nottake into consideration the deformation, andtherefore it maybe not accurate enough tomeasure absorption during loading. Secondly: theaccuracy of the gait analysis measuring systemmay be insufficient to cope with this problem.

It is concluded that the Al-power burst, asdescribed by Winter and Sienko (1988), seems tocontain only limited information about theamount of stored energy during shock absorption.Therefore it is not appropriate to consider the A l-power burst as an adequate indicator of energystorage caused by shock absorption.The shock absorption however seems to beimportant in relation to comfortable walking sinceit has been proven that trans-tibial amputeesprefer prosthetic feet which develop greater



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K. Postern, H. J . Hemens, J de Vries. H. . J . M. Koopman and W. H. Eism64

3

2

1

0

-1

-2

*1 .. Ankle Power (VCM)

II

I

8IIIIII

II

I

II -mputated sideII I - - sound side.1IIII..

-""""0 90 100% of gait cycleA 1A 1 ; A 2_ _ . . , . . . . . . . . _ . . _ _ _ _ . . . . . ..........)-( . . . . . . . *,

Al=ene rgy storageA2=energy releaseFig.4.Total ankle powerX-axis: % gait cycleY-axis: Watts kg', pos=release; neg=storage)

damping at loading, in other words more energydissipation (Wirta et al., 1991). This means thata lower net energy storage and release is no tautomatically related to a 'better' foot.Both measurement systems were alsocompared in relation to the data of the energystorage and release of the forefoot.The absolute data calculated from the A1 andA2 phases of the ankle power and the testdevice showed some striking differences. Theamount of storage as measured with the testdevice is 2 to 3 times smaller than is calculatedfrom the A1 phase, while the amount of energyrelease is about equal for both measurementsystems. The net results, storage minus release,of the test device were about 10 times smallerthan those of the A1 and A 2 phases.These differences cannot be explained onlyby influences on the subjects such as differentwalking pattern, speed, non-linear weightinfluences etc., which are measuredautomatically in the gait analysis, but not withthe test device. It is likely that differences inmethod of calculation of the 2 measurementsystems and possibly differences in accuracy,

are mainly responsible.ConclusionWhen comparing the 2 energy storing and the2 conventional feet, there are no cleardifferences in kinematic data or in kinetic data.The range of motion at the ankle of the CF2 isbigger than that of the other feet, but this is dueto the ankle mechanism and not to energystoring features.The differences in mechanical energy storageand release (net results),as calculated from theankle power, are small and not significant; themean net absorption of the ESFl is smallest. Itis unlikely that differences in net dissipation ofenergy of the m agnitude found in this study canbe noticed during normal walking and thereforethese differences are probably not clinicallyrelevant.The data of the test device (simulated stancephase) are in favour of the 2 energy storing feet.The total dissipation of energy for both ESF isless than for the CF . Th e size of the differenceshowever, are ag ain small and are unlikely to beclinically relevant.



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Prosrhetic feet: biomechan ics and benefits 27During loading, energy is absorbed by thedeformation of the foot material. This ismeasured with the test device (integral of force

with respect to displacement) but not with thegait analysis system which uses f or calculationsan inverse dynamic model, based on rigidbodies approximation. Therefore this kind ofgait analysis may not be suitable for calculationof energy storage due to shock absorption.With respect to energy expenditure, in normalwalking, energy storage and release of theprosthetic foot, seem only to be important whenthe gain in net absorption is much larger thanfor the energy storing feet in this study. Awooden foot would give the 'best' results(almost no energy storage, nor energy release)but would be also very uncomfortable, amongother things, due to lack of energy absorptionduring loading and the fixed roll-off.AcknowledgementsThe project group wishes to thank the DutchHealth Research Promotion Programme, whichmade this study possible.

R E F E R E N C E SARYA P, LEESA, NIRULAC, KLENERMAN(1995). Abiomedical comparison of the SACH, Seattle andJaipur feet using ground reaction forces. Prosther

Orrhor Inr 19 , 37-45.BARTHDG, CHUMACHER, SIENKO HOMAS (1992).Gait analysis and energy cost of below-knee amputeeswearing six d ifferent prosthetic feet. J Prosrher OrrhorBARR E, SIEGEL L (1992). Biomechanical comparisonof the energy-storing capabilities of SACH andCarbon Copy II prosthetic feet during the stanc e phaseof gait in a person with below-knee amputation. PhysTher 72(5), 344-354.BLUMENTRITIS, SCHERER HW, WELLERSHAUS1994).Biomechanisch-ganganalystische bewertung vonprothesanfiikn. Med Orrh Tech 114,287-292.CASILLAS-M, DULIEUV, COHENM. MAR- I DIDIERJP (1995). Bioen ergetic comparison of a new energy -storing foot and SACH foot in traumatic below-kneevascular amputations. Arch Phys Med Reh abil76 , 39-44.COR COR AN J (1971). Energy expenditure duringambulation. In: Physiological basis of rehabilita tionmedicine. Downey JAR, Darling RC.-Philadelphia,W.B. Saunders.DON N M, ROBER TS (1992). A review of the energyexpenditure of disabled locomotion with specialreference to lower limb amputees. PhysiotherapyTheory Practice 8.97-108.

4.63-75.

EHA RA , BEPPUM, NOMURA, KUNIMI, AKAHASHI(1983 ). Energy storing property of so-ca lled energy-storing prosthetic feet. Arch Phys M ed Reha bil74 .68-72.GOH CH, TAN PH, TOH SL, TA Y TE (1994). Gaitanalysis study of an energy-storing prosthetic foot - apreliminary report. Gait Posture 2.95-101.HERMODSON , EKD AHL, PERSON BM, ROZENDAL(1994). Gait in male trans-tibial amputees: acomparative study with healthy subjects in relation towalking speed. Prosther Orrhor Inr 18.68-77.VAN AARSVELD'WL, GROOTENBOERJ, D E VRIES ,KOOPMAN JFM (1990). Stiffness and hysteresisproperties of some prosthetic feet. Prosrher Orrhor In tVAN AARSVELD',WL, G RO O EN BO ERJ, DE VRIESJ(1990). Accelerations due to impact at heel strike

using below-knee prosthesis. Prosther Orrhor Inr 14,LEHMANNF, PRICE , BOSWELL-BESSEITE, DARALLEA (1993). Com prehensive analysis of dynam ic elasticresponse feet: Seattle AnkldLite foot Versus SACHfoot. Arch Phys Med Rehabil74,853-861.MENARD R, MCBRIDEME, SANDERSONJ, MURRAYDD (1992). Comparative biomechanical analysis ofenergy-storing prosthetic feet. Arch Phys M ed Rehab il

14, 117-125.

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73,451-458.PERRY, SHANEFIELD (1993). Efficiency of dynamicelastic response prosthetic feet. J Rehabil Res DevSACCHEI-II R. SC HM ID L , GR ON IN GE N v (1994) .

Orienterende undersuchung iiber energie-speidherungund energieriickgabe von prothesenfiikn. Med OrrhTech 114,293-295.SCHNEIDER , HAR TT. ZERNICKEF, SETOGUCHI,OPPE NHEW (1992), B elow-knee child-amputee gait:SACH foot vems FLEX foot. In: EuropeanSymposium on Clinical Gait Analysis, April 1-3,1992.-Zurich:Laboratoriumi r Biomechanik der ETHZiirich.

30(1), 137-143.

TORBURN, PERRY , AYYAPPA, SHANFIELDL (1990).Below-knee amputee gait with dynamic elasticresponse prosthetic feet: a pilot study. J Rehabil ResDev 27,369-384.TORBURN, POWERS M, GUITEREZ , PERRYJ (1995).

Energy expenditure during ambulation in dysvascularand traumatic below-knee amputees: a comparison offive prosthetic feet. J Rehabil Res Dev 32. 111-119.WA G N ER, SIENKO, SUPAN, BAR TH (1987). Motionanalysis of SACH vs. Flex-Foot in moderately activ ebelow-knee amputees. Clin Prosther Orrhor 1.55-62.WINTERA, SIENKOE (1988). Biom echanics of below-WIRTARW, MASONR, CALVOK, GOLEIRANSONL(1991). Effect on gait using various prosthetic ankle-foot devices. J Rehabil Res Dev 28(2), 13-24.

knee amputee gait. J B i o m c h 21,361-376.


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