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Active and passive mechanisms in the control of heel supination
Foot and Ankle Surgery 2001
7: 131±136
Active and passive mechanisms in the control of heel supination PA TRI C K A . TA NSE Y A ND PE TE R J. B RIG GS Department of Orthopaedic Surgery, Freeman Hospital, Heaton, Newcastle upon Tyne, UK
Summary
Heel supination when standing tiptoe may be related to the function of tibialis posterior, the plantar aponeurosis or the skeletal structure of the foot. We attempted to determine the relative importance of these different mechanisms in 20 individuals. Heel supination was measured in response to toe extension, tiptoe standing and using a specially designed platform that enabled tiptoe standing without toe extension, thus eliminating the windlass mechanism. Intra-observer and intersubject variability were assessed. Passive toe extension produced 5.0 3.2° heel supination and normal tiptoe standing (active and passive mechanisms) 7.8 3.5°. Standing tiptoe on the platform without toe extension (active mechanism alone) caused heel supination to 3.3 2.7°. This simple study demonstrates that heel supination on standing tiptoe is caused by a number of mechanisms. The windlass mechanism of the plantar aponeurosis would appear to contribute, on average, 50% of this movement in most individuals, although there was signi®cant intersubject variation. Complete loss of heel supination associated with tibialis posterior rupture implies failure of other mechanisms also. Keywords: plantar aponeurosis; tibialis posterior; biomechanics; ¯atfoot
Introduction Heel supination is an important movement occurring during heel elevation in the stance phase of gait and in standing tiptoe. A number of mechanisms may account for this movement. In 1954 Hicks [1, 2] described the function of the plantar aponeurosis and, in particular, the windlass mechanism. With hallux extension the plantar aponeurosis is pulled around the metatarsal head, causing arch elevation, heel supination and tibial external rotation. Hicks stated that this same Correspondence: P.J. Briggs, 34 Chollerford Close, Gosforth, Newcastle upon Tyne NE3 4RN, UK (e-mail: [email protected]). Ó 2001 Blackwell Science Ltd
effect would occur during heel elevation, and this has become known as the windlass mechanism. Tibialis posterior, attached at the tuberosity of the navicular, is an important supinator of the hindfoot. Loss of this function leads to progressive ¯atfoot deformity, and loss of heel supination is a feature of tibialis posterior insuf®ciency [3]. Other muscles can contribute to this active heel supination, such as the gastrocnemius±soleus complex and the long digital ¯exors, but they are not in such a mechanically advantageous position as tibialis posterior to produce this movement. The obliquity of the alignment of the lateral metatarsals may be a factor in supination of the heel. In normal gait, loading in the forefoot progresses to the medial metatarsals and hallux. However, 131
132
P.A. TANSEY AND P.J. BRIGGS
on tiptoe stance, the patient occasionally rolls out onto the lateral four metatarsals with an observable increase in the heel supination. The observation of movement of the heel during gait, tiptoe stance and toe extension is important in our evaluation of the structure and function of the foot, but the relative importance of the underlying active (i.e. muscular) and passive (i.e. windlass) mechanisms is unknown. The aim of this study was to determine the relative importance of these mechanisms in the normal foot, so as to better understand its structure and function, and the signi®cance of loss of heel supination.
Methods We studied 20 subjects aged between 16 and 65 years who appeared to have normal feet. They had no history of surgery or signi®cant trauma and no symptoms relating to the foot and ankle. They also had no systemic conditions, such as rheumatoid arthritis or neurological abnormalities, which might affect the foot or ankle. They were considered to have normal mobility in the ankle, subtalar, midtarsal and toe joints, and normal strength in all muscle groups of the calf. Subjects were asked to undertake a number of tests, and the amount of heel supination in the coronal plane was recorded. Heel supination as a result of combined active and passive mechanisms was recorded after tiptoe stance. The toe extension test was used to measure supination caused by passive mechanisms. A specially designed tilting platform was then used that enabled active tiptoe stance but eliminated toe extension and, therefore, the passive effect of the plantar aponeurosis. Heel supination caused by active mechanisms alone could be recorded. The platform test was repeated with the hallux extended and the plantar aponeurosis under tension. Finally, the platform was positioned obliquely to the coronal plane, parallel to the lateral metatarsal heads to simulate the effect of tiptoe stance on the outer metatarsals without toe extension and the effect of the plantar aponeurosis. Each test followed a standardised protocol and was repeated three times. Analysis of variance was performed to compare reproducibility, intersubject variation and differences between tests. The
signi®cance of differences between tests was determined using Student's t-tests.
Test protocols In the resting position, a thin vertical calcaneal bisecting line was drawn on the posterior aspect of each heel.
Tiptoe test
In resting stance, the subjects were asked to stand on the ¯oor with their feet facing forwards, their heels on the ground approximately 10±15 cm apart and their weight distributed evenly between heels and forefeet. The starting position of the calcaneal bisecting line was then measured in the coronal plane using an electrogoniometer (Rx Laboratories) (Figure 1). They were then asked to rise up on to tiptoe so that the sole of the foot was at 45° to the ¯oor, determined by reference to a wooden block pre-cut at 45°. The new position of the line in the coronal plane was measured. This test measured heel supination resulting from active and passive mechanisms.
Toe extension test
In resting stance, the position of the calcaneal bisecting line was measured again. While the subject remained standing, the great toe was passively extended at the metatarsophalangeal joint and the
Figure 1 Measurement of the position of the calcaneal bisecting line using a hand-held electrogoniometer. Ó 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
ACTIVE AND PASSIVE CONTROL OF HEEL SUPINATION
133
Figure 2 The toe extension test using a 45° wooden block.
45° wooden block was placed underneath it (Figure 2). With weight evenly distributed between heels and forefeet, the new position of the line was measured. This test measures heel supination occurring as a result of the windlass mechanism alone.
Platform tests
Subjects were then examined on the specially designed tilting platform (Figure 3). The platform could be tilted on rollers whose centre of rotation was 1 cm above the surface of the platform, at the approximate height of the centre of rotation of the ®rst metatarsophalangeal joint. Tiptoe stance could be simulated, without toe extension, thus eliminating the effect of the windlass mechanism. (a) Platform test with hallux neutral. The subjects were asked to stand on the platform with their toes up to but not over the edge, and with their heels unsupported at the rear. Again their feet were facing forwards 10±15 cm apart. The starting position of the calcaneal bisecting line was measured. They were then asked to stand on tiptoe so that the platform tilted to 45° and the new position of the line was measured. This test measured heel supination resulting from the action of the various muscles acting around the hindfoot. (b) Platform test with hallux extended 45°. In the resting position, standing on the platform, a 45° block was placed under the toe as in the toe extension test and the position of the line measured. The subject was again asked to stand tiptoe and the Ó 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
Figure 3 Subject standing tiptoe on the tilting platform. (a) Medial view (subject standing on one foot for the purposes of illustration). (b) Lateral view. (c) Posterior view showing the heel position.
new position of the line recorded. This test measured the effect of the muscles on heel supination while the plantar aponeurosis was under tension. (c) Platform oblique tests. For this test the platform was separated into two halves. The subjects were asked to stand on the platforms with their feet pointing forward, but the platform under each foot was positioned so that its axis of rotation was
134
P.A. TANSEY AND P.J. BRIGGS
Table 1 Heel supination observed during each of the tests. Measurements in degrees Heel supination Test
Mean
Standard deviation
Tiptoe Toe extension Platform with hallux neutral Platform with hallux extended Oblique platform with hallux neutral Oblique platform with hallux extended
7.8 5.0 3.3 1.8 4.2 3.6
3.5 3.2 2.7 2.6 2.6 2.8
), Pronation; +, supination. Figure 4 Platform positioned obliquely with subject standing prior to heel elevation.
parallel to a line between the ®rst and ®fth metatarsal heads (Figure 4). In the resting position, the orientation of the calcaneal line was measured, and re-measured after asking them to stand tiptoe to 45°. The test was repeated with the 45° wooden block under the great toe as in the previous test. These tests measured the combined action of the muscles and the obliquity of the metatarsal heads while eliminating the function of the plantar aponeurosis, ®rst, with the plantar aponeurosis in neutral and, second, under tension.
Results Analysis of variance showed signi®cant variation between different tests (P < 0.005) and between different subjects (P < 0.005). There was a small but signi®cant difference between left and right feet (P < 0.005). There was no signi®cant intra-observer variation. The results of the various tests are shown in Table 1. The tiptoe test, combining active and passive mechanisms, resulted in 7.8 3.5° of heel supination, compared with 5.0 3.2° in the toe extension test, which eliminated active mechanisms. This difference was statistically signi®cant (P < 0.05). Standing tiptoe on the platform, eliminating toe extension and passive mechanisms, caused 3.3 2.7° heel supination. This again was signi®cantly less than the normal tiptoe test (P < 0.005) and also appeared less than the toe extension test; however, this difference did not reach statistical signi®cance (P < 0.1).
When the platform was placed obliquely in line with the ®rst and ®fth metatarsal heads 4.2 2.6°, heel supination occurred after heel elevation. Although, on average, this was greater than with the platform transverse, the difference was not statistically signi®cant. The magnitude of heel supination on the toe extension test was expressed as a proportion of that seen on the tiptoe test for each individual. Similar calculation was made for the platform test. Marked variation was seen between individuals, but the majority showed heel supination on the toe extension test between 40% and 60% of that seen on the tiptoe test (Figure 5). To further con®rm the validity of our ®ndings, the position of the heel in different tests were compared. With the foot and toes ¯at at the beginning of the tiptoe, toe extension and platform tests, the heel positions were not signi®cantly different (Table 2).
Figure 5 Percentage contribution of active and passive mechanisms to total heel supination. Ó 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
ACTIVE AND PASSIVE CONTROL OF HEEL SUPINATION
Table 2 The heel position observed when the foot is ¯at with the hallux in neutral, or when the foot is ¯at with the hallux extended during the various tests. Measurements in degrees Heel position Foot position Test (start/end)
Mean
Standard deviation
Foot ¯at/hallux neutral Tiptoe (start) Platform, hallux neutral (start) Oblique platform, hallux neutral (start)
)3.8 )3.8 )3.8
5.4 5.1 4.2
Foot ¯at/hallux extended Toe extension (end) Platform, hallux extended (start) Oblique platform, hallux extended (start)
1.2 0.5 )0.3
5.6 4.4 5.7
), Pronation; +, supination.
When the foot was ¯at but with the toe extended, as in the toe extension test and at the beginning of the platform tests with the 45° block under the hallux, there was again no difference in the position of the heel. With the foot elevated and the toe extended as in the tiptoe test and the platform test with the block, no difference in heel position was seen.
Discussion This simple study demonstrates that a number of mechanisms contribute to the observed heel supination when standing tiptoe in the normal foot. The action of the plantar aponeurosis and the active muscular mechanisms appeared to be the most important. The accurate assessment of heel position without invasive tests or the use of radiography is notoriously dif®cult. We found a small but signi®cant difference between right and left feet, which we can only presume is related to a small but consistent measurement error. In the clinical situation, however, we have little choice but to use non-invasive methods. We sought to reduce the inaccuracies inherent in this by using a thin black line bisecting the heel, analysing only movement of the line rather than actual position, and having all observations made by one observer using a standardised technique. This was justi®ed by ®nding no signi®cant intra-observer variation on analysis of variance. Checking the position of the heel at various stages in the various tests further validated the methodoÓ 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
135
logy. When the foot and toes were in comparable positions in the tests the heel position would be expected to be similar, and this was con®rmed. In the toe extension test the plantar aponeurosis function is being tested. Subjects try and actively assist the arch elevation as the hallux is extended, but use of a 45° block allowed the foot to relax before measurements were taken. Inactivity of the long digital tendons can be determined by noting free passive mobility of the interphalangeal joints of the toes. It is also known from electromyographic (EMG) studies that the intrinsic muscles of the foot are quiet in static stance [4, 5]. Therefore, we assume that the heel movement seen with toe extension re¯ects the windlass function of the plantar aponeurosis. Standing tiptoe requires active muscular effort. All ¯exors and the peroneal muscles are involved in active heel elevation and stabilisation of the hindfoot. Our study did not allow us to separate the function of different muscles, but anatomical and cadaveric studies indicate that tibialis posterior is most important in heel supination [6] assisted by the gastrocnemius±soleus and digital ¯exors. Standing tiptoe on the rolling platform eliminates toe extension and, therefore, the function of the plantar aponeurosis. When the platform is transverse to the line of progression of the foot, the observed heel supination on heel elevation can be attributed to active muscular effort. If the platform is placed in line with the obliquity of the metatarsals the additional effect of the metatarsal obliquity can be observed. The standard tiptoe test produced on average 7.8° of heel supination, which we consider to be the result of the combined effect of the plantar aponeurosis and the active muscle action. The toe extension test (passive mechanisms only) caused 5.0° of heel supination, and standing tiptoe on the platform (active mechanisms only) resulted in 3.3°. This would appear to indicate that neither mechanism alone causes all of the heel supination seen when standing tiptoe. Therefore, it is inferred that both mechanisms contribute to this movement. Analysis of individual subjects indicates that there is considerable intersubject variation in the relative contribution of these mechanisms. In the majority of individuals the plantar aponeurosis would appear to account for between 40% and 60% of the heel supination seen in tiptoe stance.
136
P.A. TANSEY AND P.J. BRIGGS
A small additional increase in heel supination may occur when standing tiptoe on the lateral metatarsals. Altering the position of the platform under the foot in an attempt to mimic this showed a small but statistically insigni®cant increase in heel supination when compared with transverse alignment of the platform. In conclusion, we have shown that heel supination when standing tiptoe is the result of a combined effect of the plantar aponeurosis and active muscular action. Failure of heel supination in the presence of a ¯exible foot would imply that both mechanisms have failed. The lack of heel supination seen in tibialis posterior insuf®ciency suggests that there is also failure of function of the plantar aponeurosis. This may be from attenuation of the plantar aponeurosis or from a change in the shape of the foot such that its biomechanical function is changed. Cadaver studies have shown only a small, although signi®cant, change in the shape of the foot with
release of tibialis posterior [6]. This would support our opinion that the development of ¯atfoot and loss of heel supination associated with tibialis posterior tendon rupture must be the result of failure of the function of other structures.
References 1 Hicks JH. The mechanics of the foot. I. The joints. J Anat 1953; 87: 345±357. 2 Hicks JH. The mechanics of the foot. II. The plantar aponeurosis and the arch. J Anat 1954; 88: 25±31. 3 Johnson KA. Tibialis posterior tendon rupture. Clin Orthop Rel Res 1983; 177: 140±147. 4 Mann RA, Inman VT. Phasic activity of intrinsic muscles of the foot. J Bone Jt Surg [Am].: 46±A: 469±481, 1964. 5 Gray EG, Basmajian JV. Electromyography and cinematography of leg and foot (`normal' and ¯at) during walking. Anat Rec 1968; 161: 1±16. 6 Kitaoka HB, Luo Z-P, An K-A. Effect of the posterior tibial tendon on the arch of the foot during simulated weight bearing: biomechanical analysis. Foot Ankle Int 1997; 18: 43±46.
Ó 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
7: 131±136
Active and passive mechanisms in the control of heel supination PA TRI C K A . TA NSE Y A ND PE TE R J. B RIG GS Department of Orthopaedic Surgery, Freeman Hospital, Heaton, Newcastle upon Tyne, UK
Summary
Heel supination when standing tiptoe may be related to the function of tibialis posterior, the plantar aponeurosis or the skeletal structure of the foot. We attempted to determine the relative importance of these different mechanisms in 20 individuals. Heel supination was measured in response to toe extension, tiptoe standing and using a specially designed platform that enabled tiptoe standing without toe extension, thus eliminating the windlass mechanism. Intra-observer and intersubject variability were assessed. Passive toe extension produced 5.0 3.2° heel supination and normal tiptoe standing (active and passive mechanisms) 7.8 3.5°. Standing tiptoe on the platform without toe extension (active mechanism alone) caused heel supination to 3.3 2.7°. This simple study demonstrates that heel supination on standing tiptoe is caused by a number of mechanisms. The windlass mechanism of the plantar aponeurosis would appear to contribute, on average, 50% of this movement in most individuals, although there was signi®cant intersubject variation. Complete loss of heel supination associated with tibialis posterior rupture implies failure of other mechanisms also. Keywords: plantar aponeurosis; tibialis posterior; biomechanics; ¯atfoot
Introduction Heel supination is an important movement occurring during heel elevation in the stance phase of gait and in standing tiptoe. A number of mechanisms may account for this movement. In 1954 Hicks [1, 2] described the function of the plantar aponeurosis and, in particular, the windlass mechanism. With hallux extension the plantar aponeurosis is pulled around the metatarsal head, causing arch elevation, heel supination and tibial external rotation. Hicks stated that this same Correspondence: P.J. Briggs, 34 Chollerford Close, Gosforth, Newcastle upon Tyne NE3 4RN, UK (e-mail: [email protected]). Ó 2001 Blackwell Science Ltd
effect would occur during heel elevation, and this has become known as the windlass mechanism. Tibialis posterior, attached at the tuberosity of the navicular, is an important supinator of the hindfoot. Loss of this function leads to progressive ¯atfoot deformity, and loss of heel supination is a feature of tibialis posterior insuf®ciency [3]. Other muscles can contribute to this active heel supination, such as the gastrocnemius±soleus complex and the long digital ¯exors, but they are not in such a mechanically advantageous position as tibialis posterior to produce this movement. The obliquity of the alignment of the lateral metatarsals may be a factor in supination of the heel. In normal gait, loading in the forefoot progresses to the medial metatarsals and hallux. However, 131
132
P.A. TANSEY AND P.J. BRIGGS
on tiptoe stance, the patient occasionally rolls out onto the lateral four metatarsals with an observable increase in the heel supination. The observation of movement of the heel during gait, tiptoe stance and toe extension is important in our evaluation of the structure and function of the foot, but the relative importance of the underlying active (i.e. muscular) and passive (i.e. windlass) mechanisms is unknown. The aim of this study was to determine the relative importance of these mechanisms in the normal foot, so as to better understand its structure and function, and the signi®cance of loss of heel supination.
Methods We studied 20 subjects aged between 16 and 65 years who appeared to have normal feet. They had no history of surgery or signi®cant trauma and no symptoms relating to the foot and ankle. They also had no systemic conditions, such as rheumatoid arthritis or neurological abnormalities, which might affect the foot or ankle. They were considered to have normal mobility in the ankle, subtalar, midtarsal and toe joints, and normal strength in all muscle groups of the calf. Subjects were asked to undertake a number of tests, and the amount of heel supination in the coronal plane was recorded. Heel supination as a result of combined active and passive mechanisms was recorded after tiptoe stance. The toe extension test was used to measure supination caused by passive mechanisms. A specially designed tilting platform was then used that enabled active tiptoe stance but eliminated toe extension and, therefore, the passive effect of the plantar aponeurosis. Heel supination caused by active mechanisms alone could be recorded. The platform test was repeated with the hallux extended and the plantar aponeurosis under tension. Finally, the platform was positioned obliquely to the coronal plane, parallel to the lateral metatarsal heads to simulate the effect of tiptoe stance on the outer metatarsals without toe extension and the effect of the plantar aponeurosis. Each test followed a standardised protocol and was repeated three times. Analysis of variance was performed to compare reproducibility, intersubject variation and differences between tests. The
signi®cance of differences between tests was determined using Student's t-tests.
Test protocols In the resting position, a thin vertical calcaneal bisecting line was drawn on the posterior aspect of each heel.
Tiptoe test
In resting stance, the subjects were asked to stand on the ¯oor with their feet facing forwards, their heels on the ground approximately 10±15 cm apart and their weight distributed evenly between heels and forefeet. The starting position of the calcaneal bisecting line was then measured in the coronal plane using an electrogoniometer (Rx Laboratories) (Figure 1). They were then asked to rise up on to tiptoe so that the sole of the foot was at 45° to the ¯oor, determined by reference to a wooden block pre-cut at 45°. The new position of the line in the coronal plane was measured. This test measured heel supination resulting from active and passive mechanisms.
Toe extension test
In resting stance, the position of the calcaneal bisecting line was measured again. While the subject remained standing, the great toe was passively extended at the metatarsophalangeal joint and the
Figure 1 Measurement of the position of the calcaneal bisecting line using a hand-held electrogoniometer. Ó 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
ACTIVE AND PASSIVE CONTROL OF HEEL SUPINATION
133
Figure 2 The toe extension test using a 45° wooden block.
45° wooden block was placed underneath it (Figure 2). With weight evenly distributed between heels and forefeet, the new position of the line was measured. This test measures heel supination occurring as a result of the windlass mechanism alone.
Platform tests
Subjects were then examined on the specially designed tilting platform (Figure 3). The platform could be tilted on rollers whose centre of rotation was 1 cm above the surface of the platform, at the approximate height of the centre of rotation of the ®rst metatarsophalangeal joint. Tiptoe stance could be simulated, without toe extension, thus eliminating the effect of the windlass mechanism. (a) Platform test with hallux neutral. The subjects were asked to stand on the platform with their toes up to but not over the edge, and with their heels unsupported at the rear. Again their feet were facing forwards 10±15 cm apart. The starting position of the calcaneal bisecting line was measured. They were then asked to stand on tiptoe so that the platform tilted to 45° and the new position of the line was measured. This test measured heel supination resulting from the action of the various muscles acting around the hindfoot. (b) Platform test with hallux extended 45°. In the resting position, standing on the platform, a 45° block was placed under the toe as in the toe extension test and the position of the line measured. The subject was again asked to stand tiptoe and the Ó 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
Figure 3 Subject standing tiptoe on the tilting platform. (a) Medial view (subject standing on one foot for the purposes of illustration). (b) Lateral view. (c) Posterior view showing the heel position.
new position of the line recorded. This test measured the effect of the muscles on heel supination while the plantar aponeurosis was under tension. (c) Platform oblique tests. For this test the platform was separated into two halves. The subjects were asked to stand on the platforms with their feet pointing forward, but the platform under each foot was positioned so that its axis of rotation was
134
P.A. TANSEY AND P.J. BRIGGS
Table 1 Heel supination observed during each of the tests. Measurements in degrees Heel supination Test
Mean
Standard deviation
Tiptoe Toe extension Platform with hallux neutral Platform with hallux extended Oblique platform with hallux neutral Oblique platform with hallux extended
7.8 5.0 3.3 1.8 4.2 3.6
3.5 3.2 2.7 2.6 2.6 2.8
), Pronation; +, supination. Figure 4 Platform positioned obliquely with subject standing prior to heel elevation.
parallel to a line between the ®rst and ®fth metatarsal heads (Figure 4). In the resting position, the orientation of the calcaneal line was measured, and re-measured after asking them to stand tiptoe to 45°. The test was repeated with the 45° wooden block under the great toe as in the previous test. These tests measured the combined action of the muscles and the obliquity of the metatarsal heads while eliminating the function of the plantar aponeurosis, ®rst, with the plantar aponeurosis in neutral and, second, under tension.
Results Analysis of variance showed signi®cant variation between different tests (P < 0.005) and between different subjects (P < 0.005). There was a small but signi®cant difference between left and right feet (P < 0.005). There was no signi®cant intra-observer variation. The results of the various tests are shown in Table 1. The tiptoe test, combining active and passive mechanisms, resulted in 7.8 3.5° of heel supination, compared with 5.0 3.2° in the toe extension test, which eliminated active mechanisms. This difference was statistically signi®cant (P < 0.05). Standing tiptoe on the platform, eliminating toe extension and passive mechanisms, caused 3.3 2.7° heel supination. This again was signi®cantly less than the normal tiptoe test (P < 0.005) and also appeared less than the toe extension test; however, this difference did not reach statistical signi®cance (P < 0.1).
When the platform was placed obliquely in line with the ®rst and ®fth metatarsal heads 4.2 2.6°, heel supination occurred after heel elevation. Although, on average, this was greater than with the platform transverse, the difference was not statistically signi®cant. The magnitude of heel supination on the toe extension test was expressed as a proportion of that seen on the tiptoe test for each individual. Similar calculation was made for the platform test. Marked variation was seen between individuals, but the majority showed heel supination on the toe extension test between 40% and 60% of that seen on the tiptoe test (Figure 5). To further con®rm the validity of our ®ndings, the position of the heel in different tests were compared. With the foot and toes ¯at at the beginning of the tiptoe, toe extension and platform tests, the heel positions were not signi®cantly different (Table 2).
Figure 5 Percentage contribution of active and passive mechanisms to total heel supination. Ó 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
ACTIVE AND PASSIVE CONTROL OF HEEL SUPINATION
Table 2 The heel position observed when the foot is ¯at with the hallux in neutral, or when the foot is ¯at with the hallux extended during the various tests. Measurements in degrees Heel position Foot position Test (start/end)
Mean
Standard deviation
Foot ¯at/hallux neutral Tiptoe (start) Platform, hallux neutral (start) Oblique platform, hallux neutral (start)
)3.8 )3.8 )3.8
5.4 5.1 4.2
Foot ¯at/hallux extended Toe extension (end) Platform, hallux extended (start) Oblique platform, hallux extended (start)
1.2 0.5 )0.3
5.6 4.4 5.7
), Pronation; +, supination.
When the foot was ¯at but with the toe extended, as in the toe extension test and at the beginning of the platform tests with the 45° block under the hallux, there was again no difference in the position of the heel. With the foot elevated and the toe extended as in the tiptoe test and the platform test with the block, no difference in heel position was seen.
Discussion This simple study demonstrates that a number of mechanisms contribute to the observed heel supination when standing tiptoe in the normal foot. The action of the plantar aponeurosis and the active muscular mechanisms appeared to be the most important. The accurate assessment of heel position without invasive tests or the use of radiography is notoriously dif®cult. We found a small but signi®cant difference between right and left feet, which we can only presume is related to a small but consistent measurement error. In the clinical situation, however, we have little choice but to use non-invasive methods. We sought to reduce the inaccuracies inherent in this by using a thin black line bisecting the heel, analysing only movement of the line rather than actual position, and having all observations made by one observer using a standardised technique. This was justi®ed by ®nding no signi®cant intra-observer variation on analysis of variance. Checking the position of the heel at various stages in the various tests further validated the methodoÓ 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136
135
logy. When the foot and toes were in comparable positions in the tests the heel position would be expected to be similar, and this was con®rmed. In the toe extension test the plantar aponeurosis function is being tested. Subjects try and actively assist the arch elevation as the hallux is extended, but use of a 45° block allowed the foot to relax before measurements were taken. Inactivity of the long digital tendons can be determined by noting free passive mobility of the interphalangeal joints of the toes. It is also known from electromyographic (EMG) studies that the intrinsic muscles of the foot are quiet in static stance [4, 5]. Therefore, we assume that the heel movement seen with toe extension re¯ects the windlass function of the plantar aponeurosis. Standing tiptoe requires active muscular effort. All ¯exors and the peroneal muscles are involved in active heel elevation and stabilisation of the hindfoot. Our study did not allow us to separate the function of different muscles, but anatomical and cadaveric studies indicate that tibialis posterior is most important in heel supination [6] assisted by the gastrocnemius±soleus and digital ¯exors. Standing tiptoe on the rolling platform eliminates toe extension and, therefore, the function of the plantar aponeurosis. When the platform is transverse to the line of progression of the foot, the observed heel supination on heel elevation can be attributed to active muscular effort. If the platform is placed in line with the obliquity of the metatarsals the additional effect of the metatarsal obliquity can be observed. The standard tiptoe test produced on average 7.8° of heel supination, which we consider to be the result of the combined effect of the plantar aponeurosis and the active muscle action. The toe extension test (passive mechanisms only) caused 5.0° of heel supination, and standing tiptoe on the platform (active mechanisms only) resulted in 3.3°. This would appear to indicate that neither mechanism alone causes all of the heel supination seen when standing tiptoe. Therefore, it is inferred that both mechanisms contribute to this movement. Analysis of individual subjects indicates that there is considerable intersubject variation in the relative contribution of these mechanisms. In the majority of individuals the plantar aponeurosis would appear to account for between 40% and 60% of the heel supination seen in tiptoe stance.
136
P.A. TANSEY AND P.J. BRIGGS
A small additional increase in heel supination may occur when standing tiptoe on the lateral metatarsals. Altering the position of the platform under the foot in an attempt to mimic this showed a small but statistically insigni®cant increase in heel supination when compared with transverse alignment of the platform. In conclusion, we have shown that heel supination when standing tiptoe is the result of a combined effect of the plantar aponeurosis and active muscular action. Failure of heel supination in the presence of a ¯exible foot would imply that both mechanisms have failed. The lack of heel supination seen in tibialis posterior insuf®ciency suggests that there is also failure of function of the plantar aponeurosis. This may be from attenuation of the plantar aponeurosis or from a change in the shape of the foot such that its biomechanical function is changed. Cadaver studies have shown only a small, although signi®cant, change in the shape of the foot with
release of tibialis posterior [6]. This would support our opinion that the development of ¯atfoot and loss of heel supination associated with tibialis posterior tendon rupture must be the result of failure of the function of other structures.
References 1 Hicks JH. The mechanics of the foot. I. The joints. J Anat 1953; 87: 345±357. 2 Hicks JH. The mechanics of the foot. II. The plantar aponeurosis and the arch. J Anat 1954; 88: 25±31. 3 Johnson KA. Tibialis posterior tendon rupture. Clin Orthop Rel Res 1983; 177: 140±147. 4 Mann RA, Inman VT. Phasic activity of intrinsic muscles of the foot. J Bone Jt Surg [Am].: 46±A: 469±481, 1964. 5 Gray EG, Basmajian JV. Electromyography and cinematography of leg and foot (`normal' and ¯at) during walking. Anat Rec 1968; 161: 1±16. 6 Kitaoka HB, Luo Z-P, An K-A. Effect of the posterior tibial tendon on the arch of the foot during simulated weight bearing: biomechanical analysis. Foot Ankle Int 1997; 18: 43±46.
Ó 2001 Blackwell Science Ltd, Foot and Ankle Surgery 2001, 7, 131±136