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1964;46:469-481. J Bone Joint Surg Am.ROGER MANN and VERNE T. INMAN    

Phasic Activity of Intrinsic Muscles of the Foot

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www.jbjs.org20 Pickering Street, Needham, MA 02492-3157The Journal of Bone and Joint Surgery

Fto. 1

41i9

The Journal of

Bone and Joint Surgery

American Volume

VOLUME 46-A, No. 3 APRIL 1964

Phasic Activity of Intrinsic Muscles of the Foot �t

BY 1IO(ER MANN, M.I)4, AND VERNE T. INMAN, MI)., PH.D4,

SAN FRANCISCO, CALIFORNIA

l”roiuu the Bionuechanic� Laboratory, University of California, San I’ranci.s’co and Her/c/eq

An electi-omyogi-aphic study was done of the phasic activity of six intrinsic

muscles of the foot: the extensor digitorum brevis, abductor hallucis, flexot- hallucis

bi-evis, flexot- digitoi-um bi-evis, abductor digiti minimi, and the dorsal intei’osseus

muscle between the third and fourth toe. Experimental conditions were: walking on

level gi-ound, up and down a 10-degree slope, and up and down stairs; standing on

toes; and quiet standing.

Sites of eliot 1o(l(’ I)Ia(-enwnt : .1, aI nlw-tor digiti minirm B, eXt (11501 (ligit oritns 1 irevis : ( ‘, iiiursa 1

mit (‘ti)55(’tis : 1), tI )(Iti(tOI Irailtu-is ; E, flexor digitorum brevis ; I”, flexor lIallu(-is 1 revis.

Methods

Twelve subjects whose ages i-anged fi-om fifteen to twenty-five yeats pat-tici-

pated. None had aisy gioss abisoi-malities which could pi-oduce aix altet-ed walking

pattet-it. 1’acls foot was cai’efully examined and the clsaractei-istics of the at-eli wererecoi’ded. The i-ight foot was studied in all cases. ‘I’hie skiii over the site fot elect i-odeplacement was cleaned and a one-centimeter subcutaneous lidocaine wheal was

pi-oduced through which the stet-ilized electrodes were placed (1”ig. 1).

F;lecti-odes coissisted of 36-gauge, enamel-coated, stainless-steel wire. The

electi-ode was pi’epared by cleaning its tip of enamel, sti-inging it tlii-oiigls a 26-gauge

* ( )nn’ of tIl(’ four \ernon P. ‘Fltompsun Resident-framnitsg Program awarul-wintnimnig l)aI)ers.

I�eL(l at the Annual Meeting of the Western ( )rthopedie Assouiatioti, Seattlu, Washington, Septem-

ber 25, ltlG3.t This Sttl(IV was supported b \eterans Adniinist rations Contra(-t \-l($)oM-20�3).� Biomeelsanies Lahoratorv, School of Medi(-ine, Iniversity of California, Sari Franu-iw-o 22,

(‘alifornia.

470 ROGER MANN AND V. T. INMAN

disposable hypodermic needle, and then bending the wire to produce a one-milli-

meter hai-b. The needle, with the electrode, was placed into each muscle ; the needle

was then withdrawn, leaving the electrode in place. Two such electrodes were placed

into each muscle studied.

Placerneiit of the electrodes was verified as follows: Each muscle was stimulated

through each individual electrode with a square wave current from a Grass S-4E

stimulator. If adequate muscle contraction was not obtained, the electrode was

removed and a new one inserted. In addition, the signal produced by a voluntary

contraction of the muscle was monitored on an oscilloscope and loudspeaker before

and after each experiment.

rfhe electrode sites were covered with a bandage and the wires wei-e brought up

behind the malleoli to a junction box, which was taped to the lateral aspect of the

right leg. The subject wore a sock and a rubber boot on each foot. The boot was the

type used foi skin diving ; it was soft, distensible, had no heel or arch, and readily

conformed to the foot. The right boot was equipped with a metal strip on its heel

and toe.

In all cases the subject walked on a carbon-particle or copper mat through

which a small direct current (four to eight volts) was passed. It was thus possible

to record heel and toe strike from the metal strips on the boot. In order to ensure

the gi-eatest possible accuracy in relating the activity of each foot muscle studied

to the whole walking cycle, electrodes were also placed in the tibialis anterior and

gastrodnemius muscles.

Each subject was instructed to walk at his natural speed. Every run was carefully

supervised for splinting of the experimental foot, abnormal arm or leg movements,

and gait peculiarities. No difficulties arose because of pain in the experimental foot.

Level walking was performed on a forty-foot mat on which five to seven corn-

plete steps could be taken; the 10-degree slope was twelve feet long, pei-mitting

two or three complete steps; the stairs consisted of six standard steps (six-inch

rise and nine-inch tread) with a platform on top. Quiet standing was performed by

having the subject place the feet a comfortable distance apart (approximately eight

to twelve inches) and bear equal weight on each foot. Electromyographic recording

was cairied out for approximately fifteen seconds.

The electrical activity from each muscle was relayed from the junction box

on the subject’s right leg through an overhead boom into the amplifier. The am-

plifier had a band width of twenty to thirty thousand cycles per second and an

equivalent peak-to-peak noise level of five microvolts.

The signal was recorded on an eight-channel Offner Dynagraph. Simultaneous

recordings were made each time from the tibialis anterior and gastrocnemius mus-

cles, as well as from the heel and toe contacts.

Data were reduced as follows: For each experimental condition, the avei-age

step length for the subject was computed, as well as the extent of variation from this

mean for each individual step. The electromyographic record for each step was then

enlarged or reduced proportionately by means of a pantograph. For each experi-

mental condition and for each muscle, the records of the individual steps were

superimposed, and a composite record of the phasic activity of each muscle was

thus obtained. (It is obvious that the amplitudes of the records were altered by this

method; however, only the phasic activity was considered in this study.)

The composite was then mathematically expanded to a teix-centimeter scale

so that all the results could be compared and expressed as per cent of the full walking

cycle.

1”rom this scale was obtained the ideal or average activity which is reported

herein.

THE JOURNAL OF B0NF: AN1) JOINT SURGERY

PHASIC ACTIViTY OF INTRINSIC MUSCLES OF THE FOOT 471

Results

All or parts of the records obtained from eight of twelve subjects were accurate

enough to warrant reduction. The discarded records were inadequate because of the

many technical problems that arose. In several cases, although electrode placement

was accii tate and voluntary contractions produced normal electromyograms, phasic

activity could not be recorded. Sometimes the heel-toe device did not record prop-

erly ; in othei’ cases, an electrical artefact was produced, probably by movement of

the electiodes, either within or outside the muscle.

Thee of the eight subjects had asymptornatic bilateral flat-foot. Theii- patterns

for level walking differed from the patterns of subjects with normal feet. There

was essentially no difference in electrical activity between normal and flat-footed

subjects (lxiling stair climbing or descending, walking down and up slopes, or

staiiding on the toes.

L(P(’l lVaII1’iiig -

The electrical activity of the six intrinsic muscles studied occuired only during

the stance phase (l’�igs. 2 and 3).

11w abductor digiti minimi and extensor digitorum brevis muscles became

active at approximately 20 per cent of the cycle. In normal feet, the abductoi-

hallucis, flexor digitorum brevis, and flexor hallucis brevis muscles showed activity

at �3S, 40, aisd 28 per cent of the cycle, respectively. In subjects with pronated feet,

these muscles showed activity at 0, 26, and 14 per cent of the cycle. The activity in

both the noi-mal aisd pronated feet ceased ixear toe-off.

‘[lie onset of activity in the interosseus and gastrocuemius muscles was at 35

and 1.� per cent of the cycle, respectively. Activity ceased at (iS per cent and 56

pci- cent , and thei-e was rio difference between normal and pronated feet. The

tibiahis anterior became active just before toe-off, and activity continued throughout

the swing phase until early stance.

The amplitude was nearly constant for all the muscles except foi’ the tihialis

antei-ioi, which displayed decreased activity just before heel-strike.

Lp�sIope lie/king

The onset of electrical activity of four of the intrinsic muscles (Fig. 4) occurred

at about :30 per cent of the cycle, while the abductor digiti minimi and extensor

digitorum brevis began to show activity at 18 and 10 per cent. Gastrocnemius

activity began at 25 pei cent of the cycle; the tibialis anterior was the only one of

the muscles under study that was active during swing phase (from 60 per cent of

the previous cycle until 34 per cent of the new cycle). All the intrinsic muscles and

the gasti’ocnemius remained active until just before toe-off, at 70 per cent.

The amplitude of the electromyogram was nearly constant, except foi’ that of

the tibiahis anterior, which had two peaks of activity.

Down slope II a/king

l”our of the intriiisic muscles became active in the first 7 per cent of the cycle

(I”ig. 5). The extensor digitorum brevis and the interosseus muscles did not become

active until 42 and 16 per cent of the cycle, respectively. None of the inti-insic mus-

cles ceased activity until just l)efore toe-off, at 67 per cent. The gastrocnemius

showed activity by 7 per cent of the cycle and ceased at 57 per cent. The tihialis

anterior was active from 52 per cent of the previous cycle until 18 pci- cent of the

new cycle. The amplitude was neai-ly constant throughout stance phase foi the

inti-insic muscles and the gasti-ocnemius; the til)iahs anterior again showed two

peaks of activity.

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472 ROGER MANN AND V. T. INMAN

THE JOURNAL OF BONE AND JOINT SURGERY

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PHASIC ACTIVITY OF INTRINSIC MUSCLES OF THE FOOT 473

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I’HASIC ACTIVITY OF INTRINSIC MUSCLES OF TIlE FOOT 473

All the insti-insic muscles were active when the subject was standing on his toes.

lie/king (p Stairs

Fout- of the intrinsic muscles showed activity in the fit-st 4 pet- cent of the cycle

and stopped just before toe-off at 66 per cent (Fig. 6). The dorsal interosseus muscle

became active at 33 per cent of the cycle and the gastrocnemius at 21 pet- cent. Both

ceased activity at toe-off.

Under this experimental condition, the extensor digitorum brevis became active

during swiIsg phase, along with the tibialis anterior. Activity for each stat-ted at

61 and 36 per cent of the cycle and ceased at 3 and 7 per cent, respectively.

The amplitude of the electromyographic record for each individual muscle

was constant in all cases.

11 a/king 1)own �Stairs

The abductor hallucis, flexot- digitorum brevis, flexot- hallucis bi-evis, aiid in-

tet’osseus muscles became active in the first 9 per cent of the cycle, whereas the

abductot- digiti minimi became active in late swing phase, at 90 per cent of the

pi-evious cycle (Fig. 7). All continued their activity until 60 to 70 per cent of the

cycle, except for the abductor digiti minimi muscle, whose activity declined at 46

pet- cent of the cycle.

Under this condition, the extensor digitorum brevis and the gastt-ocnetnius

muscles both were active during swing phase; their activity began at 39 and 63

per cent, respectively, and ceased at 38 and 55 per cent.

The til)iahs anterior muscle did not evidence any consistent phasic activity.

There was no peak in the record, except in that of the gastrocnemius muscle, near

heel-sti-ike.

�S�ta nding on Toes

Arrows represent resultant axes of rotation of the calcaneocuboid and talotsavicular joints: A,

pronated foot. When the resultant axes are parallel to each other, free motion of the talotsavicularand calcaneocuhoid joints is possible. B, supinated foot. When the resultant axes are divergent,motion is restricted in the transverse tarsal articulation since each individual joint has a different

axis of rotation.

476 ROGER MANN AND V. T. INMAN

THE JOURNAL OF BONE AND JOINT SURGERY

The onset of activity was the same for all the muscles. The activity of the gastrocne-

mius coincided with that of the intrinsic muscles. The tibialis anterior muscle was

electrically silent.

Quiet Standing

There was no electrical activity recorded from the intrinsic muscles, except

for spot-adic bursts of activity at intervals of five to ten seconds in some subjects.

Discussion

Before discussing the different electromyographic patterns obtained in each

expet-imental condition, it is appropriate to summarize the investigations of others

regarding the axes of motion of the foot and the muscles that control rotation about

them.

The axis of the subtalar joint has been shown by Manter and Hicks to pass ft-orn

medial to lateral at an angle of 16 degrees to the longitudinal axis of the foot (Fig. 8) and

from superior to inferior at 42 degrees to the vertical. The motion at the subtalar

joint was studied by Close and Inman and shown to be progressively gi-eater

when subjects with normal feet, pronated feet, and flat-foot wei-e compared.

Wright and associates studied the motion at the subtalar joint during the stance

phase of walking; they found an increase in motion at the joint in subjects walking

downslope and in subjects with pronated feet.

The axes of the transverse tarsal or mid-tarsal joint as described by Elftman �

consist of two axes for the calcaneocuboid joint and two axes for the talonavicular

joint. From the two calcaneocuboid axes, Elftman calculated the resultant axis

about which the combined movements of the transverse tarsal joint must occur

(Fig. 9). In the pronated foot, the resultants of the axes of the transverse tarsal

joint are parallel to each other, allowing free motion to occur about a single axis.

In the supinated foot, the resultants are divergent, and motion in the transverse

tarsal joint is restricted, since the component joints (talonavicular and calcaneocu-

PULL OFTRICEPS SURAE

Fm. 10-A FIG. 10-B

Approximate direction of force exerted by the triceps surae (Fig. 10-A) and the intrinsic musclesof the foot (Fig. 10-B).

PHASIC ACTIVITY OF INTRINSIC MUSCLES OF THE FOOT 477

VOl.. 46-A. NO 3. APRIL 1964

boid) are attempting to rotate through their own axes which no longer coincide.

it is obvious that the latter system is more stable mechanically.

Since the transvei-se tarsal joint has the head of the talus in common with the

subtalar joint, supination of the subtalar joint produces increased stability of the

tt-ansvet-se tat-sal joint �. This foot stability is necessary when excessive forces are

applied to the foot just before toe-off.

In Figure 10-A, it can be seen that the triceps surae muscle is a powerful

supinator at the subtalar joint when the fore part of the foot is fixed on the floor.

The pull of the main intrinsic muscles, that is, the abductor hallucis, flexor

hallucis brevis, flexor digitorum brevis, and abductor digiti minimi, lies essentially

in the long axis of the foot and perpendicular to the transverse tarsal joints. Thus,

it can be stated that the intrinsic muscles exert considerable flexion force on the

fore part of the foot and play the principal role in the muscle stabilization of the

transverse tarsal joint; they are, thet-efore, the main contributors to the muscle

support of the arch (Fig. 10-B).

It should be kept in mind that control of the transverse tarsal joint is provided

by muscle, ligament, and hone. Exactly to what extent each of these participates

at the joint during locomotion is not known. It should be noted that external rota-

tion of the tibia causes movement of the talus, which in turn transmits sufficient

foi-ce to the transvet-se tarsal joint to raise the arch without direct use of muscle

power �.

With these facts in mind t-egarding the various axes of the foot, we cati now

consider the electromyographic pattettss obtained in the present study.

In each experimental condition, it can he seen that, for the most part, the in-

trinsic muscles acted as a gi-oup. This is especially true with regard to the abductor

digiti minimi, flexor hallucis brevis, abductor hallucis, and flexor digitorum brevis.

In level walking, the ititi-insic muscles in subjects with normal feet showed the

oiiset of activity in the main intrinsic mass at approximately :35 per cent of the cycle,

as contrasted with 0 to 26 per cent for the flat-footed subjects. In all cases, activity

ceased just before toe-off. During downslope walking, however, activity occurred

fi-om 0 to 7 pet’ cent of the cycle, again with cessation just before toe-off.

The electromyographic findings in this study are in accord with the observa-

tiotis of Wright and associates of onset and degree of subtalar rotation during

statice in a normal and a flat-footed subject (Fig. 11).

In level walking, pronation occui-t-ed early and, in the tiormal subject, supina-

478 ROGER MANN ANI) V. T. IXMAN

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THE JOURNAL OF BONE ANI) JOINT SURGERY

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Cl

PHASIC ACTIVITY OF INTRINSIC MUSCLES OF THE FOOT 479

tion l)egan at 33 to 40 per cent of the cycle, whet-eas it began at appi-oximately

10 pet’ cent of the cycle in the sul)ject with pronated feet. The t-otatioii at the

suhtalai’ joint w-as one of ptogressive supination, reaching a peak at 57 pet’ cent

of the cycle fot- the sul)ject with not-mal feet and 67 pet- cent for the one with flat-

foot �. This pt’ogiesSive supination can be linked to the pet-iod of activity in the in-

tt-insic nmscles recorded in this study.

1)ui-iiig upslope �valking, thete ��-as delayed i-otation in the subtalar joint of the

flat-footed subject, but the normal subject showed neat-ly the same pattern as he

did in level walking. Once again, the electt-ical activity in this study cati be t-elated

to the pattetiis of subtalat- t-otation.

PER CENT OF WALKING CYCLE0 50 00

LEVEL o

UPSLOPE �

DOW NSLOPE 0 40

UPSTAIRS � � --40 -

40

DOWN STAIRS 0

40L

FIG. 13

lore-and-aft shear on the foot during walking. Positive indicates fore and negative indicates aft.

Modified from Cunningham 2

In the downslope pattern, one can ohsei-ve activity in the intrinsic muscles

from heel-strike on and, concomitantly, rapid supination in the subtalat- joint of the

Sul)ject with flat-foot and to a lesset- degt-ee in the one with normal feet.

If these changes are considered in the light of out’ knowledge about the subtalar

aisd transverse tai-sal axes, the following obset’vations can be made.

In level walking, in a subject with normal feet, there is no activity in the in-

triiLsic IflltScles from heel-strike until 40 per cent of the cycle. In a subject with

pt’onated feet, there is tio activity fot- the fit-st 10 pet- cent of the cycle. .Just before

toe-off in level walking, when the foi-ces (Fig. 12) at-c greatest, maximum supiiiation

(Fig. 11) occurs and thet’e is optimum transverse tat-sal stabilization. In a not’mal

subject, this stabilization begins at appi-oximately 40 per cent of the cycle as

demonstt-ated by the degtee aisd rapidity of subtalat- motion and confirmed by

Vol. 46-A, N( ) 3, Al’ 1(11. 1164

480 ROGER MANN AND V. T. INMAN

electrical activity in the intrinsic muscles. In a subject with pronated feet, this

stabilization starts in the first 10 per cent of the cycle and gradually progresses to

toe-off. Again, this greatest period of subtalar rotation is paralleled by activity in

the intrinsic muscles.

In upslope walking, the degree of supination and transverse tarsal stabiliza-

tion is delayed until the center of gravity passes over the relatively flexible fore part

Of the foot at approximately 30 per cent of the cycle ; rapid supination then takes

place and there is optimum transverse tarsal stabilization for the forces (Fig. 13)

created by the propulsion of toe-off. The delay in foot stabilization is paralleled by

delayed activity in the intrinsic muscles.

In walking down a slope, the body requires the foot to be a rigid lever arm

early in the cycle to counteract the moment created by the accelerating body. The

magnitudes of the vertical and fore-and-aft shear floor reactions at-c greater for

downslope walking than for any of the other experimental conditions (Figs. 12 and

13). Early supination 8 and transverse tarsal stabilization occur together with eat’ly

activity in the intrinsic muscles.

The electromyographic recordings made during the ascent and descent of stairs

closely resemble those made during downslope walking. In ascent or descent of

stairs, the subject strikes each step with his toes and metatarsal heads first and the

heel does not as a rule come into contact with the tread. Thus the foot is loaded very

rapidly after initial contact. If one considers the excessive and rapidly applied load

upon the fore part of the foot (Figs. 12 and 13) while descending and, to a lesser

extent, while ascending stairs 2, it is obvious that complete stabilization of the foot

would be essential to stabilize the body by no later than 5 to 10 per cent of the cycle.

Rigidity of the foot is also required during standing on the toes. It can be seen

in the recordings made during this experimental condition that all the intrinsic

muscles were active.

When a person is standing quietly, there is no activity in the intrinsic muscles

(except for short bursts of activity, which presumably are evidence of postural ad-

justments). This electrical silence supports the concept that muscle activity is not

necessary to maintain the arch of the loaded foot when it is at rest.

Summary and Conclusions

1. The intrinsic muscles of the foot act as a functional unit.

2. The electrical activity of the intrinsic muscles of the foot closely pat-alleled

the progressive supination at the subtalar joint during level, upslope, and dowaslope

walking.

3. Since such a close relationship exists between the intrinsic muscles and the

axes of the subtalar and transverse tarsal joints, they may be considered to play

the principal active role in the stabilization of the foot during propulsion.

4. The pronated foot requires greater intrinsic muscle activity to stabilize

the transverse tarsal and subtalar joints than does the normal foot.

5. Muscle activity is not necessary to support the arches of the fully loaded

foot at rest.

References

1. CLOSE, J. R., and IN�I.kN, V. T.: The Action of the Subtalar Joint. Prosthetic l)evices ResearchProject, University of California, Berkeley. Series 11, Issue 24, May 1953.

2. CUNNINGHAM, D. M.: Components of Floor Reactions During Walking. Prosthetic l)evicesResearch Project, Institute of Engineering Research, University of California, Berkeley. Series11, Issue 14, November 1950 (reissued October 1958).

3. ELVrMAN, HERBERT: The Ankle Joints and the Foot. In Human Limbs and Their Substitutes,

edited by P. E. Klopsteg and P. D. Wilson, pp. 417-419. New York, McGraw-Hill, 1954.4. ELFTMAN, HERBERT: The Transverse Tarsal Joint and Its Control. Clin. Orthop., 16: 41-46,

1960.

THE JOURNAL OF’ BONE AND JOINT SURGERY

PHASIC ACTIVITY OF INTRINSIC MUSCLES OF THE FOOT 481

5. Hicks, J. H.: The Mechalsics of the Foot. I. The Joints. J. Anat., 87: 345-357, 1953.6. HicKs, J. H.: The Mechanics of the Foot. II. The Plantar Aponeurosis and the Arch. J. Anat.,

88: 25-30, 1954.7. MANTER, J. T.: Movements of the Subtalar and Transverse Tarsal Joints. Anat. Rec., 80:

397-410, 1941.

8. WRIGHT, 1). G.; DE5AI, S. M.; and HENDERSON, W. H.: Action of the Subtalar and Ankle-JointComplex I)uring the Stance Phase of Walking. J. Bone and Joint Surg., 46-A: 361-382, March1964.

VOl.. 46-A, NO. 3, A1’RIL 1964