O&P Library > POI > 1985, Vol 9, Num 3 > pp. 129 - 136


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Bouncy knee: a stance phase flex-extend knee unit

L. D. Fisher*
G. W. Judge*


A Bouncy Knee is a knee control device for use in above-knee prostheses, designed to give a natural flex-extend action during the stance phase of the walking cycle. Tests with an adjustable device fitted to an amputee identified optimum angles of bounce (peak initial knee flexion after foot contact). Subsequently a clinical trial was conducted with six patients. The patients' gait was assessed using an instrumented walkway and polarized light goniometry. Symmetry of gait was improved in all cases and the patients reported a marked improvement in comfort and increased control in downhill walking. After removal of the units, cyclic testing was carried out without collapse of any Bouncy Knee unit. The clinical fitting procedure has been established and a simple peak-reading goniometer designed to enable the prosthetist fitting the unit to assess its performance. A production batch, suitable for Blatchford Modular Assembly Prostheses, Stabilised Knee users has been made.


During the stance phase of normal level walking the natural knee flexes and re-extends before starting to flex again as the toe lifts off at the end of the stride. The amount of stance-phase knee flexion in the natural leg can be between ten and twenty degrees, being generally greater at higher speeds of walking (Murray et al, 1954; Andriacchi et al, 1982). This flex/re-extend action, or knee bounce, permits the pelvis to travel forward over the foot at approximately the same horizontal level, because of the effective leg shortening caused by knee flexion at mid stance. Conventional prostheses lack such a facility, and in consequence vaulting occurs over the 'stiff' limb. This results in increased energy expenditure (Bresler et al, 1957) and an asymmetry in gait.

Various research projects have been carried out which have investigated this area, and have provided prosthetic substitutes for the stance phase flex-extend action of the natural knee.

The early American research approach was aimed at reducing the "fierceness" of knee-locking mechanisms. The muscle-bulge operated devices with which Wagner and Catranis (1954) experimented were able to delay knee-lock actuation until the quadriceps bulge force had reached a certain level, by which time the knee may have gone into an initial flexion. Catranis added this initial flexion control to a modified Adel cross-linkage leg prosthesis (Wagner and Catranis, 1954) in which an hydraulic link is locked by the muscle-bulge signal: in this case the linkage itself forces the knee to re-extend as the shank rotates over the foot. Catranis also reported a knee lock in which a spring at the upper end of an hydraulic cylinder allowed, upon knee lock, up to five degrees of "shock-absorbing knee flexion". Collins (1973) included, in his hydraulic knee lock patent, the option of introducing into the hydraulic fluid "a quantity of air to serve as a resilient cushion".

Several hydraulic devices have been developed which permit the knee of a prosthesis to flex slowly under weightbearing; however, these do not re-extend. Such controlled-yield units have been made by Northrop, Catranis, Hosmer and Stuart-Vickers (Wagner and Catranis, 1954), Regnell (Staros, 1965) and Henschke-Mauch (Mauch, 1968).

Linear shortening of the prosthetic leg is inherent in some prosthetic equipment. Shortening by about 13 millimetres was favourably received by some wearers of a telescopic knee less prosthesis developed by Seliktar and Kenedi (1976): as weight is borne on the prosthesis the shortening stores energy for later release during swing phase.

Grimes et al, (1977) have shown that two young active above-knee amputee test subjects preferred a flex/re-extend function put into an electrohydraulic knee-control simulator, compared to their own prostheses. In this simulator, at heel contact a step position input is applied to a servo which sends the knee of the prosthesis into a damped oscillation: this oscillation is so timed as to fit into a normal gait pattern. The test subjects not only adapted easily to walking with this bounce function, but also preferred it when set to give angles of bounce flexion at the knee as large as 16 to 20 degrees.

The Bouncy Knee described in the following sections is the first device specifically aimed at providing stance phase flexion and re-extension in a unit lending itself to subsequent commercial application. In philosophy, the unit solely provides the bounce function, and the essential stability of the knee is separately ensured by a stabilised knee device.

The Bouncy Knee was patented on behalf of the Department of Health and Social Security (Judge, 1978).

Theory of the Bouncy Knee

There are two knee flexions of a natural leg during the stance phase, one initiated at heel strike and a second prior to toe-off (Elftman 1951; Rose, 1982). Fig. 1 shows these two conditions, and also the relevant ground-foot force vectors, from which it can be seen that an external flexion moment is acting upon the knee from just after heel strike until mid stance. The Bouncy Knee mechanism is designed to use this to provide the initial flexion part of the bounce action. In mid-stance the external moment changes to extension before returning to flexion prior to toe-off: the Bouncy Knee similarly uses these changes to give the re-extension and final flexion of the knee which characterise the bounce action.

In order to assess the suitability of a Bouncy Knee for a reasonably active amputee an evaluation knee mechanism was manufactured and a feasibility study carried out (Judge and Fisher 1981). This established the suitability of the Bouncy Knee and defined the amount of bounce required

A Bouncy Knee design for the Blatchford Stabilised Knee

Following the successful preliminary trial it was decided to carry out a more extensive assessment of the Bouncy Knee principle. For this purpose it was desirable to design a unit which when fitted to an existing prosthesis would provide a bounce, but not otherwise modify its alignment, weight or action. A simple way of achieving this was to modify a Blatchford Stabilised Knee (B.S.K.) unit by replacing the main pivot shaft with a specially designed rubber torsional bush.

The B.S.K. is fitted to the prosthesis of many reasonably active above-knee amputees. The main pivot shaft is fixed securely to the shank of the prosthesis and carries an integral brake drum on a radial web. The thigh section is mounted on the main pivot shaft and can normally rotate about it. However, when the prosthesis is under load, a brake band anchored to the thigh section is made to tighten firmly around the drum. This prevents rotation of the knee joint and gives security to the wearer. As the prosthesis is lifted clear of the ground, the brake band automatically frees itself, allowing a normal swing-through action to occur.

Fig. 2 shows a standard B.S.K. unit together with its main pivot shaft and the definitive Bouncy Knee B.S.K. together with the specially designed rubber torsional bush,

When the B.S.K main pivot shaft was replaced by the rubber torsional bush the alignment and action of the prosthesis were unaltered, with the exception of the added bounce, whilst the weight was only marginally increased by 1Og (less than 2% of the weight of a standard B.S.K and negligible in comparison to the complete prosthesis).

In the B.SK., the brake drum's rigid connection to the main pivot shaft keeps the leg stiff throughout the weight-bearing phase of walking. By replacing its integral metal web by a bonded rubber bush, the knee becomes able to flex under load without affecting the security established by the brake band around the drum: it becomes a bouncy knee. Originally the rubber bush could also deflect radially, but the amputee's gait was found to be affected adversely if this were permitted. For this reason radial deflections in the bouncy knee are now blocked by two acetal homopolymer (Delrin) annular discs, one at each end of the brake drum, between it and the main pivot shaft. The bush is thus constrained to operate as a purely torsional coupling between shaft and drum, the torsional stiffness depending upon the grade of rubber used. Four separate grades of rubber were used giving a wide range of torsional stiffness.

Testing of the rubber bonding

The type of amputee likely to be most suitable for a Bouncy Knee could be taking well over a million steps per year on his prosthesis (Day, 1978). At each step, his security depends upon the integrity of its mechanical structure, of which the Bouncy Knee is a part.

In order to examine this integrity, Bouncy Knee units of differing stiffnesses were subjected in a machine to repeated cycles of loading, chosen to represent the conditions of normal level walking.

The Bouncy Knee rubber torsional bush was tested by applying a 1350 Newton vertical load (ISPO, 1978), whilst continually flexing the bush from 2 degrees to +8 degrees. Each of the four grades of bonded rubber torsional bush completed one million cycles of this test without exhibiting any signs of failure. The one using the hardest grade of rubber (seventy degrees Shore hardness) was tested for a further two and a half million cycles; at the end of this test its stiffness had reduced to that of the next softest rubber (sixty degrees Shore hardness). It is not clear to what extent the continuous nature of the tests influenced this result.

They were also loaded once to an angular deflection of up to 30 degrees to see what would happen if they were stressed beyond normal operating limits, as could occur if too soft a Bouncy Knee were fitted to a patient's prosthesis. All samples survived the 30 degree deflection test, excepting the hardest grade which started to yield at 25 degrees. In none of the tests did the rubber become detached from the metal components.

The test results therefore show that the Bouncy Knee will 'fail safe' in cyclic loading; and that providing the stiffness of the unit is carefuly prescribed (according to body weight and limb alignment, both of which can be measured), it will also remain structurally sound under the occasional unexpectedly high loading. With the grades of rubber currently in use amputees who weigh more than a certain amount may have to be excluded from consideration for this type of Bouncy Knee.

Patient trial

Six young healthy adults normally wearing B.S.K. limbs, all by chance fitted with suction sockets, had a Bouncy Knee fitted to their prosthesis. Their reaction to its use over a period was assessed. Table 1 gives details of test subjects.

Every effort was made to obtain good alignment. The knee offset from the load line of the subject was determined using the Force Vector Visualisation Unit (Cook et al, 1980). Together with the height and weight of the subject, this offset enabled the torsional stiffness of the Bouncy Knee coupling to be ascertained, and the required Bouncy Knee torsional bush issued to Chas. A Blatchford & Co. Ltd. for incorporation into a B.S.K. unit (Table 1.).

At the commencement of the trial each subject's gait was monitored, first with the prosthesis incorporating the standard B.S.K., and secondly when it was removed and replaced by the Bouncy Knee B.S.K. unit. The subject was re-assessed at weeks 2,6 and 34. At week 34 the Bouncy Knee B.S.K. unit was removed and replaced by the standard B.S.K. unit. The subject was then re-assessed at weeks 34, 36 and 40, at which point the evaluation trial was ended. Thermograms of the stump were taken at weeks 1, 6, 34 and 40.

Gait analysis consisted of monitoring the subjects' cadence by means of a foot switch, and the thigh and shank angles by means of polarised light goniometry.

During each trial three different self-adopted walking speeds were monitored (slow, normal and fast) as well as a normal walk in which records were made of foot contact with a force plate. All walks were along a level surface.

For comparison the gait patterns of five non-amputees were recorded.


When a conventional above-knee prosthesis is fitted to an active unilateral amputee, it is frequently made a little shorter than its natural counterpart. This reduces the chance of the amputee catching the foot on the ground as it swings through. When fitting Bouncy Knee limbs this was found in every case to be unnecessary.

Examination of the Bouncy Knee torsional rubber bushes removed from the subjects' prostheses at the end of their part in the trial showed only superficial evidence of use (slight discolouration), with no change in torsional stiffness.

Analysis of foot contact timing showed no significant difference in Temporal Asymmetry Index (Dewar and Judge, 1980) between walking with the Bouncy Knee and without. In contrast, plots of thigh angle against shank angle (Fig. 3) for each subject show how the kinematic, and therefore visual, symmetry of gait has been improved.

The bounce action showed up clearly in continuous recordings of knee angle from each of the six subjects. Most were happy with around ten degrees during walking on the level at normal speed, but one who preferred a torsionally softer unit reached 16 degrees.

In the B.S.K. Bouncy Knee, the knee action includes a significant period at zero flexion (shown at 'A' in Fig. 4, left) which is not present in the action of the natural knee (Fig. 4, right). This results from the ground reaction force passing in front of the Bouncy Knee axis. There is a similar but briefer dwell at zero flexion immediately following heel strike which is better observed at 'B' in Fig. 3.

Changes in stump condition, assessed thermographically over the trial period could not be linked to Bouncy Knee use.

Examination of the New York University, Prosthetic and Orthotic Studies, "Prosthetic Evaluation Scale (January 1966)" which each subject completed at the commencement of the trial generally revealed similar subjective comments about their respective prostheses for all subjects. However, the occurrence of stump sores varied between subjects. Standing time and walking time were estimated to be some three to four hours per day. Both limb and suspension were deemed to be reasonably comfortable despite moderate jarring on heel-strike and sometimes a painful crotch.

Prosthesis weight was generally felt to be about right but there was a tendency for the stump to pespire in warm weather. Subjects reported that other persons noticed differences between each limb when walking or sitting but not when standing. No subject reported ever experiencing backache.

The most marked and consistent advantages of using the Bouncy Knee were subjective: examination of the questionnaires completed by each subject at 2, 34 and 36 weeks revealed that all six amputees found walking more comfortable and no disadvantages were reported. Some typical comments were:

very comfortable like walking on air'
'feels more like a natural leg'

After some months of regular use, further comments were volunteered:

'gives more confidence when walking down slopes'
'easier to walk over rough ground'
'easier to balance when lifting heavy loads' (ambulance man)
'allows a better dancing action'
'vast improvement'
'easier walking less effort'

All users found it difficult to adjust back to using a conventional limb after the trial, and each subsequently asked to have the Bouncy Knee re-fitted permanently. All these requests have been met.


Although the initial concept of the Bouncy Knee was to improve the symmetry of the gait, the trial also revealed three other benefits, due entirely to knee flexion during stance phase:

  1. Shock absorption.

  2. Greater control when walking down slopes.

  3. Better balance when lifting loads.

A further trial with subjects using other means of suspension is also indicated. Suspensions of the rigid pelvic band type would need the articulated joint carefully sited at the hip, since a slightly posterior or anterior position would tend to constrain the action of the Bouncy Knee.

The fitting of a Bouncy Knee to subjects wearing a semi-automatic knee lock prosthesis is being investigated. The initial use of the prototype Bouncy Knee by one subject wearing such a prosthesis showed promise, and work is proceeding to produce a design for a suitable definitive Bouncy Knee, for this type of prosthesis.

A small number of Bouncy Knee B.S.K. units has been provided by D.H.S.S. for fitting to prostheses of selected patients. The appendix outlines a suitable method of fitting these.


This paper has described the successful application of the Bouncy Knee principle to the B.S.K. A six patient trial, restricted to young active A/K amputees possessing a healthy stump of reasonable length, and wearing a prosthesis fitted with a suction socket and a stabilised knee, showed no adverse reactions or affects on gait.

The Bouncy Knee brought advantages of balance and control in addition to the anticipated benefits of shock-absorption and symmetry.

Static and cyclic testing of the torsional rubber bush has shown this component to be safe. The eventual deterioration of the rubber is evidenced to the amputee by increased flexion during walking.

Although the design of the Bouncy Knee described took the form of a torsional rubber bush fitted to the B.S.K., the concept may be applied to other knee mechanisms. This is under investigation.


The Department of Health and Social Security has arranged for the supply of 150 BSK Bouncy Knee units, as used on the trial. These will be fitted to the prosthesis of selected patients at Artificial Limb Centres. During the work described in this paper, selection of the most suitable Bouncy Knee for a chosen subject involved the use of equipment not normally available. However, in order to enable a B.S.K. Bouncy Knee unit to be fitted by a prosthetist in the field, an easily fitted peak-reading goniometer was specially designed to register the maximum bounce (knee flexion) during a slow level walk.

Beginning with a medium grade of rubber, successively stiffer or softer Bouncy Knee units are fitted until the bounce deflection during slow walking on the level registers between five and ten degrees. A unit with greater or less stiffness could be fitted after a period of initial experience on the Bouncy Knee limb, according to the wishes of the patient and the judgement of the limb-fitting prosthetist.


  1. Andriacchi, T. P., Calante, J. O., Fermier, R. W. (1982). The influence of total knee replacement design on walking and stair climbing. J. Bone Joint Surg. 64A 1328-1335.
  2. Bresler, B., Radcliffe, C. W., Berry, F. R. (1957). Energy and power in the legs of above-knee amputees during normal level walking. Berkeley C. A., University of California, Lower-extremity Research Project. (Series II, Issue 31).
  3. Collins, D. W. (1973). British Patent 1311462.
  4. Cook, T. M., Kenosian, H. (1980). The use of force visualisation in gait. In: Biological Engineering Society. International Conference on Recent Advances in Biomedical Engineering-London: the Society, 21-23.
  5. Day, H. J. B., Mann, A., Neal, A. J. (1978) Step Counting In: Department of Health and Social Security, Biomechanical Research and Development Unit. Report, 1978 - London: the Dept., 253-257.
  6. Dewar, M. E., Judge, G. (1980). Temporal asymmetry as a gait quality indicator. Med. Biol. Eng. Comput. 18, 689-693.
  7. Elftman, H., (1951). The basic pattern of human locomotion. Ann. N. Y. Acad. Sci. 51, 1207-1212.
  8. Grimes, D. L., Flowers, W. C.,Donath,M. (1977). Feasibility of an active control scheme for above-knee prosthesis. Trans. A.S.M.E. J. Biomec. Eng. 99, 215-221.
  9. International Society for Prosthetics and Orthotics, (1978) Standards for lower limb prostheses: report of a conference Philadelphia P.A.: I.S.P.O.
  10. Judge, G. W. (1978) Patent GB 2014855 Priority Data 5677178.
  11. Judge, G. W., Fisher, L. D. (1981). A bouncy knee for above-knee amputees. Eng. Med. 10, 27-32.
  12. Mauch, H. A. (1968). Stance control for above-knee artificial legs - design considerations in the S-N-S knee. Bull. Prosthet. Res. 10-10, 61-72.
  13. Murray, M. P., Drought, A. B., Kory, R.C. (1954). Walking patterns of normal man. J. Bone Joint Surg. 46A, 335-360.
  14. Rose, G. K., Butler, P. B., Stallard, J. (1982). Gait: principles, biomechanics and assessment-Oswestry: ORLAU Publishing.
  15. Seliktar, R., Kenedi, R. M. (1976). A kneeless leg prosthesis for the elderly amputee, advanced version. Bull. Prosthet. Res. 10-25, 97-119.
  16. Staross, A. (1965). Semiannual Report of the Veterans Administration Prosthetics Center. Bull. Prosthet. Res. 10-3, 122-123.
  17. Wagner, E. M., Catranis, J. G., (1954. New developments in lower extremity prostheses. In: Klopsteg, P. E., Wilson, P. D. (eds) Human limbs and their substitutes-New York: Hafner Pub., Co.

O&P Library > POI > 1985, Vol 9, Num 3 > pp. 129 - 136

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