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The influence of shoe sole's varying thickness on lower limb muscle activity
Foot and Ankle Surgery 17 (2011) 218–223
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Foot and Ankle Surgery journal homepage: www.elsevier.com/locate/fas
The influence of shoe sole’s varying thickness on lower limb muscle activity A.K. Ramanathan Mch (Orth), E.J. Parish BMSc (Hons), MBChB, G.P. Arnold PhD, T.S. Drew PhD, W. Wang PhD, R.J. Abboud PhD* Institute of Motion Analysis & Research (IMAR), Department of Orthopaedic & Trauma Surgery, TORT Centre, Ninewells Hospital & Medical School, University of Dundee, Dundee, DD1 9SY, Scotland, UK
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 March 2010 Received in revised form 27 May 2010 Accepted 27 July 2010
Background: The lateral ligament injury of the ankle is acknowledged to be the most common ankle injury sustained in sport. Increased peroneus longus muscle contraction in the shod population has already been documented. This study aimed to quantify the effect of shoe sole’s varying thickness on peroneus longus muscle activity. Methods: Electromyographic recordings of the peroneus longus muscle activity following unanticipated inversion of the foot from 08 to 208 in a two-footplate tilting platform were collected from 38 healthy participants. The four test conditions were: barefoot, standard shoe, and shoes with 2.5 cm and 5 cm sole adaptation respectively. Results: Compared to the barefoot condition, there is an increase in the magnitude of muscle contraction on wearing shoes, which further increases with thickening shoe soles. The peroneus longus was responding earlier in the shod conditions when compared to the barefoot, although the results were variable within the three shod conditions. Conclusion: Footwear with increasing shoe sole thickness evokes a correspondingly stronger protective eversion response from the peroneus longus to counter the increasing moment at the ankle-subtalar joint complex following sudden foot inversion. Hence, fashion footwear with thicker sole is likely to increase the risk of lateral ligament injury of the ankle when such protective response is overwhelmed. Similarly, the clinicians need to be cautious regarding the amount of shoe raise that they could provide for patients with limb length discrepancy without any detrimental untoward side effects. ß 2010 Published by Elsevier Ltd on behalf of European Foot and Ankle Society.
Keywords: Lateral ligament injury Shoe sole thickness Electromyography Peroneus longus Muscle activity
1. Introduction The foot is the single structure of the body, which interacts with an external surface in normal bipedal walking [1]. Its main role is to provide support to the body and provide feedback about the terrain through which an individual is treading. The joints which are primarily involved in inversion and eversion movements are the subtalar and transverse tarsal joints [2]. The body of the talus fits into the ‘‘mortise’’ created by the tibia, fibula and the inferior transverse ligament to form the ankle joint. Morphologically, this arrangement is stable in the dorsiflexed position of the ankle, but not so much so in plantarflexion. This means that substantial demands of loading are placed onto the surrounding ligaments in plantarflexion. Immediately after heel strike before proceeding to midstance in the gait cycle, the foot is plantarflexed which along with internal rotation of the talus within the ankle mortise put these surrounding ligaments under stress, to a point where there is * Corresponding author. Tel.: +44 1382 496276; fax: +44 1382 496200. E-mail address: [email protected] (R.J. Abboud).
potential for them to being overwhelmed [3]. The ankle-subtalar joint complex naturally adopts a position of inversion [4]. Such inversion, if progresses will predominantly stress the lateral ligaments of this complex. Ankle sprains are acknowledged to be the most common ankle injuries sustained in sport [4], with injury to the lateral ligament complex occurring more frequently. Several studies have assessed the perceived risk factors for this potentially recurrent injury [5–7]. Intrinsic risk factors [5,6] such as an individual’s age, weight, height, foot width and muscle imbalance and extrinsic risk factors [7] such as playing surface, competition level and shoe type are all influential in this injury. Lateral ankle injury occurs most often when the foot is loaded in a supinated position [8]. During heel strike, the hindfoot is within a normal degree of inversion but could be forced into abnormal inversion depending on the contour of the surface with which it comes in contact [9]. The foot evertors and postural stiffening guard against such excessive movement. The peroneus longus being the evertor of the foot stabilises the ankle-subtalar joint complex [10]. The foot at swing-through phase of the gait cycle is naturally at an angle of 108 inversion [11]. This ‘‘carrying
1268-7731/$ – see front matter ß 2010 Published by Elsevier Ltd on behalf of European Foot and Ankle Society. doi:10.1016/j.fas.2010.07.003
A.K. Ramanathan et al. / Foot and Ankle Surgery 17 (2011) 218–223
angle’’ of the foot is important, not only for the foot to clear the ground, but also for making adjustments for the next step [12]. A study of surface electromyography (EMG) in patients with functional ankle instability showed the role of peroneus longus muscle in intrinsic protection of the ankle-subtalar joint complex [13]. The first muscle to show EMG activity following sudden foot inversion is the peroneus [14]. Foot positional awareness is affected by foot-terrain influences [15]. The level to which footwear affects proprioceptive feedback has been the subject of several studies [16,17]. Bates et al. [18] suggested that in barefoot running the tendency of the foot to invert is reduced compared to shod conditions, and subsequently the occurrence of injury would decrease if people ran barefoot. Abboud [1] suggested that shoes may alter the protective mechanisms of injury prevention at the ankle. Recently, Kerr et al. [19] showed that the force of peroneus longus muscle contraction was increased when wearing shoes. These findings reinforce the belief that wearing shoes places higher demand on the lower limb muscles particularly the peroneus longus to counter the physiological ankle inversion during normal gait. Sekizawa et al. [16] suggested that wearing a thicker midsoled shoe could place the body in an unstable position due to the body’s centre of gravity being raised. Footwear with thick sole components is said to decrease proprioceptive feedback from the sole of the foot [20]. This is said to impair foot positional awareness and stability [15]. The literature suggests that shoes with thick midsole and greater heel height provide less stability in the elderly population [21,22]. While many studies have surmised the influence footwear has on foot stability and injury [17,23,24], no study appears to have taken each element of a shoe, considered variations in such element and assessed its influence on the lower limb muscle activity, as a standard shoe has four components; upper, insert, midsole and outsole [25]. The outsole is the element of the shoe which comes into contact with the ground surface. Its role is to provide grip and is usually designed to outlast the midsole [26]. The aim of this study is to quantify the effect of shoe sole’s varying thickness on peroneus longus muscle activity following unanticipated foot inversion with the hypothesis being the more the shoe sole thickness, the greater the peroneus longus muscle activity. 2. Materials and methods 2.1. Study population Thirty-eight healthy volunteers [28 males and 10 females (mean age 26.42 7.28 years, mean BMI 24.14 3.23)] with no previous history of lower limb or foot and ankle injury consented to participate in the study. Subjects were recruited after obtaining ethical approval from the University Research Ethics Committee. 2.2. Data collection The three main elements of the apparatus used in this study were a portable EMG system, a two-footplate tilting platform and a personal computer. Participants were requested to stand relaxed on the two-footplate tilting platform with each foot in the centre of each plate, by which each foot corresponds to the axis of footplate rotation. On computer command, the footplate inverted the foot from 08 to 208 rotating at an angular velocity of 100 degrees per second by the action of two chambers of compressed air, which are contained within two Rotary Vane Actuators [27]. As suggested by Kerr et al. [19], 08 to 208 of inversion was only considered. Signals of muscle activity were delivered to amplifiers, which were attached by strips of Velcro1 to the thighs of the participant. The signals then travel to a band-pass filter box which filtered the extremes of frequency signals to allow for a better range of EMG. Although the
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documented highest EMG signal frequency of the lower limb muscles in the literature is 116 Hz [28], Kerr et al.’s [19] investigation on analysing the power frequency spectrum of the peroneus longus using a sampling rate of 2000 Hz showed that the highest 95% power frequency was 87 Hz. The EMG system used in the current study was the same that was used by Kerr et al. [19] and as opined by them after investigation, the sampling rate of 200 Hz was used as this was found to be more than adequate to avoid aliasing. Self-developed computer software programme synchronised this data with the movement of the two-footplate tilting platform. 2.3. Participant setup Silver/silver chloride (Ag/AgCl) surface EMG electrodes were attached to the participant’s lower limbs to study the peroneus longus muscle activity. Two electrodes were positioned per muscle (i.e per leg), one on the muscle belly and the other at the point of bony origin of that muscle (i.e fibular head). The muscle belly was located by palpation following voluntary contraction of the muscle by the participant, and this was confirmed by the use of a muscle stimulator. The skin surface was prepared by removing dead skin cells and natural oils with an alcohol wipe and abrasive tape. The leg hairs, if any, were removed for the placement of the electrodes so as to avoid any interference with the signals obtained. 2.4. Test shoes The soles of the test shoes were altered for this study. The rubber outsole (5 mm thick) was removed but retained. The loop component of Velcro1 was attached to the entire underside of the midsole, while the hook component was moulded to the cut surface of the rubber outsole. Thus, the sole could be reattached to the remaining shoe using Velcro1. Artificial soles were made using the medium density Ethylene Vinyl Acetate (EVA) and these could be attached to the under surface of the test shoe’s midsole and the cut surface of the original rubber outsole using Velcro1. After personal communication with the orthotists in our centre, two artificial soles of 2.5 cm thickness each were adapted. This meant that a shoe sole height can be increased by 2.5 cm when one adaptation was used and by 5 cm with both adaptations. Adaptation up to 5 cm was tested as this was generally the maximum raise provided by the majority of orthotists for patients with limb length discrepancy with the assumption of no undue side effects. 2.5. Test sequences Four sequences were considered; left foot inversion, right foot inversion and two other random foot inversion sequences. In order to avoid participants becoming familiar with the test sequences, two random sequences were performed but the data from which were not saved. In all the sequences the inversion was from 08 to 208. Fig. 1 shows 208 left foot inversion in the two-footplate tilting platform on wearing a shoe with 5 cm sole adaptation. Each test sequence ran for two cycles and data were collected throughout both cycles. The order of study was randomised for test sequences and shoe sequences using the Latin square design. 2.6. Data extraction A custom designed computer program extracted the necessary data from the EMG signal. Upon sudden inversion of the foot, there is a related peak in the amplitude of the recorded EMG signal from the ipsilateral peroneus longus muscle. This initial peak is referred to as the ‘Latency Maximum Peak’ (Max) and the time taken for this
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2.7. Data analysis The chosen significance value was 5% with minimum power set to 80%. Repeated measures analysis of variance was applied to investigate differences between peroneus longus EMG activities for the four test conditions in 0–208 of inversion. Sole height, sides (right and left) and cycles (1 and 2) were also included as withinsubjects’ factors. 3. Results
Fig. 1. Shod left foot inversion to 208 in the two-footplate tilting platform with 5 cm sole adaptation.
peak to occur following the movement of the tilting platform was referred to as ‘Latency’ (Response time). As few peaks do occur after the initial peak, the ‘Overall Maximum Peak’ (Emax) was considered. The maximum peak in the EMG waveform within 500 ms following computer initiation of the tilting platform movement was referred to as the ‘Overall Maximum Peak’, as it was proved by Kerr et al. [19] that the peak contractions did not occur beyond this time period unless there was another stimulus. After the initial movement, the footplate remained stationary for 5 s. There was sustained muscle contraction during this stationary period. The variable ‘Average amplitude (Av)’ was acquired by rectifying the EMG signal and taking an average of the readings over a predefined time period of 2.5 s from the end. Hence, for each EMG signal, five variables were extracted; Latency 0–208 (ms), Overall Maximum Peak 0–208 (V), Latency Maximum Peak 0–208 (V), Average amplitude 08 (V), and Average amplitude 0–208 (V).
Analysis of the EMG activity of the peroneus longus muscle (Table 1) showed that the muscle was responding earlier in the shod conditions as against the barefoot. Within the three shod conditions, the pattern of response was variable and did not follow any trend (Fig. 2a). The ‘Overall Maximum Peak’ (Fig. 2b) and ‘Latency Maximum Peak’ (Fig. 2c) were demonstrating the same trend for the various test conditions. The peak amplitude of the EMG signal increased following sudden inversion and showed statistically significant changes between the shod and barefoot conditions, with a continuing increase in the peak amplitude on increasing thickness of the shoe sole. Significant difference was noted in the ‘Average amplitude’ between the standard shoe and the shoe with 5 cm sole adaptation even when no movement occurred in the tilting platform (Fig. 2d). On inversion, there was also an increase in the ‘Average amplitude’ phase of EMG activity from barefoot to wearing shoes, with a further significant increase with thickening shoe sole (Fig. 2e). 4. Discussion Basmajian and De Luca [29] ascertain that the peroneus longus has no great function in the static positioning of the foot in relaxed standing. Therefore it is acceptable to assume that any activity seen in this muscle during the study procedure can be equated to their intrinsic involvement in maintaining the ankle-subtalar joint complex stability, thereby protecting the lateral ligament complex of the ankle.
Table 1 Peroneus longus muscle activity. Parameter
Test condition
Mean
Standard error
95% Confidence interval
Pairwise comparisons
Lower bound
Upper bound
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
197.397 178.151 187.466 179.384
6.373 4.523 6.426 3.732
184.692 169.134 174.655 171.944
210.103 187.167 200.276 186.823
Emax 0–20 (V)
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
1.733 2.134 2.206 2.245
.147 .147 .145 .142
1.440 1.841 1.918 1.963
2.025 2.427 2.495 2.527
**Sa, **Sb, **Sc **S0 **S0 **S0
Max 0–20 (V)
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
1.718 2.113 2.171 2.221
.149 .148 .147 .144
1.422 1.819 1.878 1.935
2.014 2.408 2.464 2.507
**Sa, **Sb, **Sc **S0 **S0 **S0
Av 0 (V)
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
.085 .071 .073 .087
.010 .007 .006 .007
.065 .058 .061 .072
.106 .085 .084 .102
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
.098 .103 .123 .148
.008 .009 .010 .011
.083 .086 .102 .126
.114 .121 .144 .170
Latency 0–20 (ms)
Av 0–20 (V)
Emax – Overall Maximum Peak; Max – Latency Maximum Peak; Av – Average amplitude; *.05 > P .01; **P < .01.
*Sa, **Sc *S0 **S0
*Sc *Sa **Sb, **Sc *Sb, **Sc **S0, *Sa, *Sc **S0, **Sa, *Sb
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Fig. 2. Graphs showing comparisons of the various test conditions on peroneus longus muscle activity.
Unlike Kerr et al. [19], the peroneus longus muscle was clearly responding earlier in all the shod conditions compared to the barefoot. The reason for this could be the predominance of central (brain) control in maintaining overall stability rather than the regional influence especially with the impairment in proprioception, which the shoes are documented to provide the user with. The current study also showed that the latency was variable among the shod conditions. There is no clear trend in its response time with respect to increasing shoe sole thickness. This parameter needs further investigation. The peak amplitude findings of this study support the work of Kerr et al. [19] and provide some further knowledge as to which component of a shoe contribute to its effect on lower limb muscle activity, especially the peroneus longus. A thicker shoe sole increases contraction of this foot evertor and causes a higher level of contraction to be sustained after the initial response. Therefore, the increase in peak amplitude of the EMG signal must be due to the additional contraction of the muscle fibres when wearing shoes, with these contractions increasing further when the shoe sole height was increased. Fig. 3 shows a free body diagram of an inverted foot in three conditions: barefoot, standard shoe and thickened shoe sole. Two effects can be seen from the diagram. Firstly, as highlighted
by Kerr et al. [19], the lever arm of the ground reaction force is increased when wearing shoes. The lever arm is increased further when shoes with thicker soles are worn. This can be noticed when considering the moment of eversion (ME), which is a product of distance y and muscle force from the evertor (Fm). The distance of the lever arm y remains the same for the conditions of barefoot, standard shoe and a thickened shoe sole. However, this is not true for the moment of inversion (MI) which is a product of distance x and the ground reaction force (Fr). When compared to the barefoot condition (xb), it is evident that the lever arm x increases in standard shoe condition (xs) which increases further with a thickened shoe sole (xt) (i.e xt > xs > xb). To maintain equilibrium during inversion, the moment of external inversion (x.Fr) must equal the moment of internal eversion (y.Fm). Considering the moment of internal eversion, as the length of the lever arm (y) remains unchanged for all the different conditions, it has to be the increasing force of evertor muscle contraction which would increase the moment of internal eversion thereby equalling the external inversion, restoring equilibrium. This increasing force of muscle contraction corresponds to increasing amplitude of the EMG signal [30]. Albeit the experimental angle of inversion was the same (i.e 0–208), the escalating evertor muscle response to increasing height of the shoe sole is probably the body’s protective
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varying masses may require a differing degree of contraction to overcome the lateral shift in weight. Another interesting observation is that there is a significant difference in the ‘Average amplitude’ muscle activity between the standard shoe and the shoe + 5 cm sole adaptation conditions at 08 inversion. This explicitly explains the effect of increasing shoe sole thickness on the lower limb muscle activity even without any movements. As this significance is not noticeable with 2.5 cm sole raise, further investigations are underway with sole adaptations of varying thickness to identify the cut-off point to signal the significant change as up to 5 cm sole raise are routinely used in clinical practice to combat the limb length discrepancies with the assumption of no detrimental side effects. The increase in the ‘Average amplitude’ EMG activity could be argued to be an indicator of proprioceptive deficit along with the other previously cited possible explanations. Bloem et al. [32], in their study of a patient with a dorsal root ganglionopathy found that if the individual had no proprioceptive input, then they would adopt a strategy to maintain postural balance by increasing background muscle activity. If it is considered that the participants of this study were also adopting this strategy to create a stiffer body posture in order to maintain their balance then it may be the case that footwear with increasing sole thickness proportionately impairs proprioceptive input resulting in a corresponding increase in background muscle activity. This study explains the effect of shoe sole’s varying thickness on peroneus longus muscle function upon unanticipated foot inversion. Future research can be geared to evaluate the effect of other components of a shoe, various types of shoe and the role of shoe sole material or the lacing system to assess their influence in the stability of the foot and ankle. Considering the effect of contact surface on balance control mechanisms [33], it would seem beneficial to look beyond the shoes and also consider the treading terrain. 5. Conclusion
Fig. 3. Free body diagram of the inverted foot in the frontal plane (barefoot, standard shoe and thickened shoe sole).
intrinsic response to counter the enhanced tendency of foot inversion in shoes with thickened sole, and hence lateral ligament complex injury. Secondly the centre of gravity is offset on inversion movement. The weight of the individual acting downwards through the ankle and subtalar joint is shifted laterally on inversion. As the foot is elevated from the ground by a thickened shoe sole, at a certain height the weight would be shifted so far laterally on inversion that it produces increased leverage to tip over the lateral edge of the shoe. At this time an increased pull from the evertor muscles (Fm) is also required to stabilise the foot. If this cannot be achieved then the moment of external inversion will greatly exceed the moment of internal eversion. This imbalance may predispose to lateral ligament injuries of the ankle [31]. The response from the peroneus longus to increase and sustain contraction when shoe sole adaptations are worn supports this theory. As the ankle-subtalar joint complex is kept at a constant inversion of 208 after the initial movement from 08, the peroneus longus exerts persistent contraction to counteract this inversion. As mentioned earlier, the footplates were inverted to 208 at an angular velocity of 100 degrees per second. This angular velocity has been the same for all the test conditions. Although the speed of tilt is crucial, since this is kept constant across all conditions, it is quite evident that the increasing force of muscle contraction corresponds to increasing shoe sole thickness. Individuals of
The peroneus longus muscle activity increases from barefoot to shod conditions, with increasing shoe sole thickness adding to this effect. Hence, a thicker soled shoe imposes a detrimental effect on the maintenance of ankle-subtalar joint complex stability and also poses a threat to the lateral ligament complex of the ankle. Conflict of interest statement The authors have no personal, professional or financial affiliations with other people or organisations that may be perceived to have biased the presentation. Acknowledgments The study was internally funded by the Institute of Motion Analysis and Research. The authors would like to thank Mr Ian Christie and Mr Guthrie Boath for their technical assistance. References [1] Abboud RJ. Relevant foot biomechanics. Curr Orthop 2002;16:165–79. [2] Moore KL, Agur AMR. Lower limb. Essential clinical anatomy, 2nd ed., Lippincott: Williams and Wilkins; 1995, ISBN: 0-7817-2830-4. [3] Cailliet R. Foot and ankle pain, 3rd ed., Philadelphia: F.A. Davis Company; 1997, ISBN: 0-8036-0216-2. [4] Garrick JG. The frequency of injury, mechanism of injury and epidemiology of ankle sprains. Am J Sports Med 1977;5(6):241–2. [5] Baumhauer JF, Alosa DM, Renstrom AF, Trevino S, Beynnon BD. A prospective study of ankle injury risk factors. Am J Sports Med 1995;23(5):564–70. [6] Milgrom C, Shlamkovitch N, Finestone A, Eldad A, Laor A, Danon YL, et al. Risk factors for lateral ankle sprain: a prospective study among military recruits. Foot Ankle 1991;12(1):26–30.
A.K. Ramanathan et al. / Foot and Ankle Surgery 17 (2011) 218–223 [7] Murphy DF, Connolly DA, Beynnon BD. Risk factors for lower extremity injury: a review of the literature. Br J Sports Med 2003;37(1):13–29. [8] Safran MR, Benedeff RS, Bartolozzi R, Arthur R, Mandelbaum BR. Lateral ankle sprains: a comprehensive review part 1: etiology, pathoanatomy, histopathogenesis and diagnosis. Med Sci Sports Exerc 1999;31(7):S429–37. [9] Konradsen L, Ravn JB, Sorensen AI. Proprioception at the ankle: the effect of an anaesthetic blockade of ligament receptors. J Bone Joint Surgery 1993;73B:433–6. [10] Neptune RR, Wright IC, van den Bogert, Anton J. Muscle coordination and function during cutting movements. Med Sci Sports Exerc 1999;31(2):294– 302. [11] Winter DA. The biomechanics and motor control in human gait: normal, elderly and pathological, 2nd ed., University of Waterloo Press; 1991 088898-105-8. [12] Waddington G, Adams R. Football boot insoles and sensitivity to extent of ankle inversion movement. Br J Sports Med 2003;37:170–5. [13] Santilli V, Frascarelli MA, Paoloni M, Frascarelli F, Camerota F, De Natale L, et al. Peroneus longus muscle activation pattern during gait cycle in athletes affected by functional ankle instability: a surface electromyographic study. Am J Sports Med 2005;33(8):1183–7. [14] Konradsen L. Sensori-motor control of the uninjured and injured human ankle. J Electromyogr Kinesiol 2002;12:199–203. [15] Robbins S, Waked E, McClaran J. Proprioception and stability: foot position awareness as a function of age and footwear. Age Ageing 1995;24:67–72. [16] Sekizawa K, Sandrey MA, Ingersoll CD, Cordova ML. Effects of shoe sole thickness on joint position sense. Gait Posture 2001;13:221–8. [17] Waddington G, Adams R. Textured insole effects on ankle movement discrimination while wearing athletic shoes. Phys Ther Sport 2000;1:119–28. [18] Bates BT, James SL, Osternig LR, Mason BR. Effect of running shoes on selected aspects of lower extremity function. J Biomech 1979;12(8):630. [19] Kerr R, Arnold GP, Drew TS, Cochrane L, Abboud RJ. Shoes influence lower limb muscle activity and may predispose the wearer to lateral ankle ligament injury. J Orthop Res 2009;27(3):318–24.
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Contents lists available at ScienceDirect
Foot and Ankle Surgery journal homepage: www.elsevier.com/locate/fas
The influence of shoe sole’s varying thickness on lower limb muscle activity A.K. Ramanathan Mch (Orth), E.J. Parish BMSc (Hons), MBChB, G.P. Arnold PhD, T.S. Drew PhD, W. Wang PhD, R.J. Abboud PhD* Institute of Motion Analysis & Research (IMAR), Department of Orthopaedic & Trauma Surgery, TORT Centre, Ninewells Hospital & Medical School, University of Dundee, Dundee, DD1 9SY, Scotland, UK
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 March 2010 Received in revised form 27 May 2010 Accepted 27 July 2010
Background: The lateral ligament injury of the ankle is acknowledged to be the most common ankle injury sustained in sport. Increased peroneus longus muscle contraction in the shod population has already been documented. This study aimed to quantify the effect of shoe sole’s varying thickness on peroneus longus muscle activity. Methods: Electromyographic recordings of the peroneus longus muscle activity following unanticipated inversion of the foot from 08 to 208 in a two-footplate tilting platform were collected from 38 healthy participants. The four test conditions were: barefoot, standard shoe, and shoes with 2.5 cm and 5 cm sole adaptation respectively. Results: Compared to the barefoot condition, there is an increase in the magnitude of muscle contraction on wearing shoes, which further increases with thickening shoe soles. The peroneus longus was responding earlier in the shod conditions when compared to the barefoot, although the results were variable within the three shod conditions. Conclusion: Footwear with increasing shoe sole thickness evokes a correspondingly stronger protective eversion response from the peroneus longus to counter the increasing moment at the ankle-subtalar joint complex following sudden foot inversion. Hence, fashion footwear with thicker sole is likely to increase the risk of lateral ligament injury of the ankle when such protective response is overwhelmed. Similarly, the clinicians need to be cautious regarding the amount of shoe raise that they could provide for patients with limb length discrepancy without any detrimental untoward side effects. ß 2010 Published by Elsevier Ltd on behalf of European Foot and Ankle Society.
Keywords: Lateral ligament injury Shoe sole thickness Electromyography Peroneus longus Muscle activity
1. Introduction The foot is the single structure of the body, which interacts with an external surface in normal bipedal walking [1]. Its main role is to provide support to the body and provide feedback about the terrain through which an individual is treading. The joints which are primarily involved in inversion and eversion movements are the subtalar and transverse tarsal joints [2]. The body of the talus fits into the ‘‘mortise’’ created by the tibia, fibula and the inferior transverse ligament to form the ankle joint. Morphologically, this arrangement is stable in the dorsiflexed position of the ankle, but not so much so in plantarflexion. This means that substantial demands of loading are placed onto the surrounding ligaments in plantarflexion. Immediately after heel strike before proceeding to midstance in the gait cycle, the foot is plantarflexed which along with internal rotation of the talus within the ankle mortise put these surrounding ligaments under stress, to a point where there is * Corresponding author. Tel.: +44 1382 496276; fax: +44 1382 496200. E-mail address: [email protected] (R.J. Abboud).
potential for them to being overwhelmed [3]. The ankle-subtalar joint complex naturally adopts a position of inversion [4]. Such inversion, if progresses will predominantly stress the lateral ligaments of this complex. Ankle sprains are acknowledged to be the most common ankle injuries sustained in sport [4], with injury to the lateral ligament complex occurring more frequently. Several studies have assessed the perceived risk factors for this potentially recurrent injury [5–7]. Intrinsic risk factors [5,6] such as an individual’s age, weight, height, foot width and muscle imbalance and extrinsic risk factors [7] such as playing surface, competition level and shoe type are all influential in this injury. Lateral ankle injury occurs most often when the foot is loaded in a supinated position [8]. During heel strike, the hindfoot is within a normal degree of inversion but could be forced into abnormal inversion depending on the contour of the surface with which it comes in contact [9]. The foot evertors and postural stiffening guard against such excessive movement. The peroneus longus being the evertor of the foot stabilises the ankle-subtalar joint complex [10]. The foot at swing-through phase of the gait cycle is naturally at an angle of 108 inversion [11]. This ‘‘carrying
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angle’’ of the foot is important, not only for the foot to clear the ground, but also for making adjustments for the next step [12]. A study of surface electromyography (EMG) in patients with functional ankle instability showed the role of peroneus longus muscle in intrinsic protection of the ankle-subtalar joint complex [13]. The first muscle to show EMG activity following sudden foot inversion is the peroneus [14]. Foot positional awareness is affected by foot-terrain influences [15]. The level to which footwear affects proprioceptive feedback has been the subject of several studies [16,17]. Bates et al. [18] suggested that in barefoot running the tendency of the foot to invert is reduced compared to shod conditions, and subsequently the occurrence of injury would decrease if people ran barefoot. Abboud [1] suggested that shoes may alter the protective mechanisms of injury prevention at the ankle. Recently, Kerr et al. [19] showed that the force of peroneus longus muscle contraction was increased when wearing shoes. These findings reinforce the belief that wearing shoes places higher demand on the lower limb muscles particularly the peroneus longus to counter the physiological ankle inversion during normal gait. Sekizawa et al. [16] suggested that wearing a thicker midsoled shoe could place the body in an unstable position due to the body’s centre of gravity being raised. Footwear with thick sole components is said to decrease proprioceptive feedback from the sole of the foot [20]. This is said to impair foot positional awareness and stability [15]. The literature suggests that shoes with thick midsole and greater heel height provide less stability in the elderly population [21,22]. While many studies have surmised the influence footwear has on foot stability and injury [17,23,24], no study appears to have taken each element of a shoe, considered variations in such element and assessed its influence on the lower limb muscle activity, as a standard shoe has four components; upper, insert, midsole and outsole [25]. The outsole is the element of the shoe which comes into contact with the ground surface. Its role is to provide grip and is usually designed to outlast the midsole [26]. The aim of this study is to quantify the effect of shoe sole’s varying thickness on peroneus longus muscle activity following unanticipated foot inversion with the hypothesis being the more the shoe sole thickness, the greater the peroneus longus muscle activity. 2. Materials and methods 2.1. Study population Thirty-eight healthy volunteers [28 males and 10 females (mean age 26.42 7.28 years, mean BMI 24.14 3.23)] with no previous history of lower limb or foot and ankle injury consented to participate in the study. Subjects were recruited after obtaining ethical approval from the University Research Ethics Committee. 2.2. Data collection The three main elements of the apparatus used in this study were a portable EMG system, a two-footplate tilting platform and a personal computer. Participants were requested to stand relaxed on the two-footplate tilting platform with each foot in the centre of each plate, by which each foot corresponds to the axis of footplate rotation. On computer command, the footplate inverted the foot from 08 to 208 rotating at an angular velocity of 100 degrees per second by the action of two chambers of compressed air, which are contained within two Rotary Vane Actuators [27]. As suggested by Kerr et al. [19], 08 to 208 of inversion was only considered. Signals of muscle activity were delivered to amplifiers, which were attached by strips of Velcro1 to the thighs of the participant. The signals then travel to a band-pass filter box which filtered the extremes of frequency signals to allow for a better range of EMG. Although the
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documented highest EMG signal frequency of the lower limb muscles in the literature is 116 Hz [28], Kerr et al.’s [19] investigation on analysing the power frequency spectrum of the peroneus longus using a sampling rate of 2000 Hz showed that the highest 95% power frequency was 87 Hz. The EMG system used in the current study was the same that was used by Kerr et al. [19] and as opined by them after investigation, the sampling rate of 200 Hz was used as this was found to be more than adequate to avoid aliasing. Self-developed computer software programme synchronised this data with the movement of the two-footplate tilting platform. 2.3. Participant setup Silver/silver chloride (Ag/AgCl) surface EMG electrodes were attached to the participant’s lower limbs to study the peroneus longus muscle activity. Two electrodes were positioned per muscle (i.e per leg), one on the muscle belly and the other at the point of bony origin of that muscle (i.e fibular head). The muscle belly was located by palpation following voluntary contraction of the muscle by the participant, and this was confirmed by the use of a muscle stimulator. The skin surface was prepared by removing dead skin cells and natural oils with an alcohol wipe and abrasive tape. The leg hairs, if any, were removed for the placement of the electrodes so as to avoid any interference with the signals obtained. 2.4. Test shoes The soles of the test shoes were altered for this study. The rubber outsole (5 mm thick) was removed but retained. The loop component of Velcro1 was attached to the entire underside of the midsole, while the hook component was moulded to the cut surface of the rubber outsole. Thus, the sole could be reattached to the remaining shoe using Velcro1. Artificial soles were made using the medium density Ethylene Vinyl Acetate (EVA) and these could be attached to the under surface of the test shoe’s midsole and the cut surface of the original rubber outsole using Velcro1. After personal communication with the orthotists in our centre, two artificial soles of 2.5 cm thickness each were adapted. This meant that a shoe sole height can be increased by 2.5 cm when one adaptation was used and by 5 cm with both adaptations. Adaptation up to 5 cm was tested as this was generally the maximum raise provided by the majority of orthotists for patients with limb length discrepancy with the assumption of no undue side effects. 2.5. Test sequences Four sequences were considered; left foot inversion, right foot inversion and two other random foot inversion sequences. In order to avoid participants becoming familiar with the test sequences, two random sequences were performed but the data from which were not saved. In all the sequences the inversion was from 08 to 208. Fig. 1 shows 208 left foot inversion in the two-footplate tilting platform on wearing a shoe with 5 cm sole adaptation. Each test sequence ran for two cycles and data were collected throughout both cycles. The order of study was randomised for test sequences and shoe sequences using the Latin square design. 2.6. Data extraction A custom designed computer program extracted the necessary data from the EMG signal. Upon sudden inversion of the foot, there is a related peak in the amplitude of the recorded EMG signal from the ipsilateral peroneus longus muscle. This initial peak is referred to as the ‘Latency Maximum Peak’ (Max) and the time taken for this
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2.7. Data analysis The chosen significance value was 5% with minimum power set to 80%. Repeated measures analysis of variance was applied to investigate differences between peroneus longus EMG activities for the four test conditions in 0–208 of inversion. Sole height, sides (right and left) and cycles (1 and 2) were also included as withinsubjects’ factors. 3. Results
Fig. 1. Shod left foot inversion to 208 in the two-footplate tilting platform with 5 cm sole adaptation.
peak to occur following the movement of the tilting platform was referred to as ‘Latency’ (Response time). As few peaks do occur after the initial peak, the ‘Overall Maximum Peak’ (Emax) was considered. The maximum peak in the EMG waveform within 500 ms following computer initiation of the tilting platform movement was referred to as the ‘Overall Maximum Peak’, as it was proved by Kerr et al. [19] that the peak contractions did not occur beyond this time period unless there was another stimulus. After the initial movement, the footplate remained stationary for 5 s. There was sustained muscle contraction during this stationary period. The variable ‘Average amplitude (Av)’ was acquired by rectifying the EMG signal and taking an average of the readings over a predefined time period of 2.5 s from the end. Hence, for each EMG signal, five variables were extracted; Latency 0–208 (ms), Overall Maximum Peak 0–208 (V), Latency Maximum Peak 0–208 (V), Average amplitude 08 (V), and Average amplitude 0–208 (V).
Analysis of the EMG activity of the peroneus longus muscle (Table 1) showed that the muscle was responding earlier in the shod conditions as against the barefoot. Within the three shod conditions, the pattern of response was variable and did not follow any trend (Fig. 2a). The ‘Overall Maximum Peak’ (Fig. 2b) and ‘Latency Maximum Peak’ (Fig. 2c) were demonstrating the same trend for the various test conditions. The peak amplitude of the EMG signal increased following sudden inversion and showed statistically significant changes between the shod and barefoot conditions, with a continuing increase in the peak amplitude on increasing thickness of the shoe sole. Significant difference was noted in the ‘Average amplitude’ between the standard shoe and the shoe with 5 cm sole adaptation even when no movement occurred in the tilting platform (Fig. 2d). On inversion, there was also an increase in the ‘Average amplitude’ phase of EMG activity from barefoot to wearing shoes, with a further significant increase with thickening shoe sole (Fig. 2e). 4. Discussion Basmajian and De Luca [29] ascertain that the peroneus longus has no great function in the static positioning of the foot in relaxed standing. Therefore it is acceptable to assume that any activity seen in this muscle during the study procedure can be equated to their intrinsic involvement in maintaining the ankle-subtalar joint complex stability, thereby protecting the lateral ligament complex of the ankle.
Table 1 Peroneus longus muscle activity. Parameter
Test condition
Mean
Standard error
95% Confidence interval
Pairwise comparisons
Lower bound
Upper bound
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
197.397 178.151 187.466 179.384
6.373 4.523 6.426 3.732
184.692 169.134 174.655 171.944
210.103 187.167 200.276 186.823
Emax 0–20 (V)
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
1.733 2.134 2.206 2.245
.147 .147 .145 .142
1.440 1.841 1.918 1.963
2.025 2.427 2.495 2.527
**Sa, **Sb, **Sc **S0 **S0 **S0
Max 0–20 (V)
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
1.718 2.113 2.171 2.221
.149 .148 .147 .144
1.422 1.819 1.878 1.935
2.014 2.408 2.464 2.507
**Sa, **Sb, **Sc **S0 **S0 **S0
Av 0 (V)
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
.085 .071 .073 .087
.010 .007 .006 .007
.065 .058 .061 .072
.106 .085 .084 .102
Barefoot (S0) Standard shoe (Sa) Shoe + 2.5 cm (Sb) Shoe + 5 cm (Sc)
.098 .103 .123 .148
.008 .009 .010 .011
.083 .086 .102 .126
.114 .121 .144 .170
Latency 0–20 (ms)
Av 0–20 (V)
Emax – Overall Maximum Peak; Max – Latency Maximum Peak; Av – Average amplitude; *.05 > P .01; **P < .01.
*Sa, **Sc *S0 **S0
*Sc *Sa **Sb, **Sc *Sb, **Sc **S0, *Sa, *Sc **S0, **Sa, *Sb
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Fig. 2. Graphs showing comparisons of the various test conditions on peroneus longus muscle activity.
Unlike Kerr et al. [19], the peroneus longus muscle was clearly responding earlier in all the shod conditions compared to the barefoot. The reason for this could be the predominance of central (brain) control in maintaining overall stability rather than the regional influence especially with the impairment in proprioception, which the shoes are documented to provide the user with. The current study also showed that the latency was variable among the shod conditions. There is no clear trend in its response time with respect to increasing shoe sole thickness. This parameter needs further investigation. The peak amplitude findings of this study support the work of Kerr et al. [19] and provide some further knowledge as to which component of a shoe contribute to its effect on lower limb muscle activity, especially the peroneus longus. A thicker shoe sole increases contraction of this foot evertor and causes a higher level of contraction to be sustained after the initial response. Therefore, the increase in peak amplitude of the EMG signal must be due to the additional contraction of the muscle fibres when wearing shoes, with these contractions increasing further when the shoe sole height was increased. Fig. 3 shows a free body diagram of an inverted foot in three conditions: barefoot, standard shoe and thickened shoe sole. Two effects can be seen from the diagram. Firstly, as highlighted
by Kerr et al. [19], the lever arm of the ground reaction force is increased when wearing shoes. The lever arm is increased further when shoes with thicker soles are worn. This can be noticed when considering the moment of eversion (ME), which is a product of distance y and muscle force from the evertor (Fm). The distance of the lever arm y remains the same for the conditions of barefoot, standard shoe and a thickened shoe sole. However, this is not true for the moment of inversion (MI) which is a product of distance x and the ground reaction force (Fr). When compared to the barefoot condition (xb), it is evident that the lever arm x increases in standard shoe condition (xs) which increases further with a thickened shoe sole (xt) (i.e xt > xs > xb). To maintain equilibrium during inversion, the moment of external inversion (x.Fr) must equal the moment of internal eversion (y.Fm). Considering the moment of internal eversion, as the length of the lever arm (y) remains unchanged for all the different conditions, it has to be the increasing force of evertor muscle contraction which would increase the moment of internal eversion thereby equalling the external inversion, restoring equilibrium. This increasing force of muscle contraction corresponds to increasing amplitude of the EMG signal [30]. Albeit the experimental angle of inversion was the same (i.e 0–208), the escalating evertor muscle response to increasing height of the shoe sole is probably the body’s protective
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varying masses may require a differing degree of contraction to overcome the lateral shift in weight. Another interesting observation is that there is a significant difference in the ‘Average amplitude’ muscle activity between the standard shoe and the shoe + 5 cm sole adaptation conditions at 08 inversion. This explicitly explains the effect of increasing shoe sole thickness on the lower limb muscle activity even without any movements. As this significance is not noticeable with 2.5 cm sole raise, further investigations are underway with sole adaptations of varying thickness to identify the cut-off point to signal the significant change as up to 5 cm sole raise are routinely used in clinical practice to combat the limb length discrepancies with the assumption of no detrimental side effects. The increase in the ‘Average amplitude’ EMG activity could be argued to be an indicator of proprioceptive deficit along with the other previously cited possible explanations. Bloem et al. [32], in their study of a patient with a dorsal root ganglionopathy found that if the individual had no proprioceptive input, then they would adopt a strategy to maintain postural balance by increasing background muscle activity. If it is considered that the participants of this study were also adopting this strategy to create a stiffer body posture in order to maintain their balance then it may be the case that footwear with increasing sole thickness proportionately impairs proprioceptive input resulting in a corresponding increase in background muscle activity. This study explains the effect of shoe sole’s varying thickness on peroneus longus muscle function upon unanticipated foot inversion. Future research can be geared to evaluate the effect of other components of a shoe, various types of shoe and the role of shoe sole material or the lacing system to assess their influence in the stability of the foot and ankle. Considering the effect of contact surface on balance control mechanisms [33], it would seem beneficial to look beyond the shoes and also consider the treading terrain. 5. Conclusion
Fig. 3. Free body diagram of the inverted foot in the frontal plane (barefoot, standard shoe and thickened shoe sole).
intrinsic response to counter the enhanced tendency of foot inversion in shoes with thickened sole, and hence lateral ligament complex injury. Secondly the centre of gravity is offset on inversion movement. The weight of the individual acting downwards through the ankle and subtalar joint is shifted laterally on inversion. As the foot is elevated from the ground by a thickened shoe sole, at a certain height the weight would be shifted so far laterally on inversion that it produces increased leverage to tip over the lateral edge of the shoe. At this time an increased pull from the evertor muscles (Fm) is also required to stabilise the foot. If this cannot be achieved then the moment of external inversion will greatly exceed the moment of internal eversion. This imbalance may predispose to lateral ligament injuries of the ankle [31]. The response from the peroneus longus to increase and sustain contraction when shoe sole adaptations are worn supports this theory. As the ankle-subtalar joint complex is kept at a constant inversion of 208 after the initial movement from 08, the peroneus longus exerts persistent contraction to counteract this inversion. As mentioned earlier, the footplates were inverted to 208 at an angular velocity of 100 degrees per second. This angular velocity has been the same for all the test conditions. Although the speed of tilt is crucial, since this is kept constant across all conditions, it is quite evident that the increasing force of muscle contraction corresponds to increasing shoe sole thickness. Individuals of
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