J. Phys. Ther. Sci. 24: 1103–1106, 2012 The Relationship between Activity of Abductor Hallucis and Navicular Drop in the One-leg Standing Position

Ki Seok Nam, PT, MS1), Jung Won Kwon, PT, MS2), Oh-Yun Kwon, PT, PhD3)

1) Department of Physical Therapy, Yeungnam College of Science and Technology 2) Department of Rehabilitation Science, Graduate School, Daegu University 3) Department of Physical Therapy, Kinetic Ergocise Based on Movement Analysis Laboratory, College of Health Science, Yonsei University: 234 Maiji-ri, Heungup-myon, Wonju-si, Kangwon-do 220-710, Republic of Korea. TEL: +82 33-760-2721, FAX: +82 33-760-2496, E-mail: [email protected]

Abstract. [Purpose] This study examined whether abductor hallucis (AH) activity in the one-leg-standing posi- tion can be used to predict the navicular drop (ND) when it is or is not actively supporting the medial longitudinal arch. [Subjects] Forty healthy subjects without foot or ankle problems were recruited. [Methods] For all subjects, the ND was measured as the difference in navicular height between the subtalar joint resting (STJR) and subtalar joint neutral (STJN) positions while standing. AH activity was measured in both positions during one-leg standing. [Results] AH muscle activity in the STJR position was significantly negatively correlated with the ND, while AH muscle activity in the STJN position was not. Using a simple regression model, AH activity predicted the ND based on the STJR position. [Conclusion] The results suggest that AH activity is a good predictor of the ND. Therefore, intervention that activates the AH in the one-leg standing position could be used to correct the ND. Key words: Abductor hallucis, Navicular drop, Foot pronation (This article was submitted May 23, 2012, and was accepted Jun. 13, 2012)

INTRODUCTION elevation of the MLA21–23). Moreover, blocking the tibial nerve, which controls the activity of the AH, produces a Foot pronation is a complicated but critical triplanar significant increase in pronation as assessed by theND movement that absorbs the shock from ground reaction test21). Therefore, the literature suggests that strengthening forces1, 2). However, excessive pronation (pes planus) is a the AH would help to elevate the MLA. chronic condition caused by insufficient support of the medial Although previous studies have reported that the AH longitudinal arch (MLA) of the foot3–5). An excessively helps support the MLA22–25), no study has examined the pronated foot is associated with several clinical pathologies relationship between AH activity and the ND during one-leg of the lower extremity, including plantar fasciitis, tibialis standing. Therefore, this study investigated the correlation posterior tendinitis, Achilles bursitis or tendinitis, and patel- between the AH and ND and the ability to predict the ND lofemoral pain syndrome6–9). from AH activity, while actively supporting the MLA during While weight-bearing, the bony structures, ligaments, and static one-leg standing. extrinsic/intrinsic foot muscles contribute to maintaining the MLA of the foot to control excessive pronation10–13). Of SUBJECTS AND METHODS the midfoot bones, the navicular supports the structures of the MLA14, 15). Consequently, direct measurement of the Forty-nine subjects were recruited and the ND of each navicular displacement in a weight-bearing position is one was measured. Mueller suggested that an ND > 10 mm in of the simplest methods for providing clinically quantifiable the pronated foot is pathological14, 17). The exclusion criteria information regarding pronation of the foot16–18). Clinically, were: a history of lower extremity surgery; foot and ankle navicular displacement in a weight-bearing position is injury within 6 months of participation; previous or current measured using the navicular drop (ND) test. inflammatory arthritis; diabetes; severe foot deformities The intrinsic foot muscles, including the abductor hallucis such as hallux valgus, hammer toe, and claw toe; rigid foot (AH), flexor hallucis brevis, and interosseous, contribute to with an ND < 3 mm; and an inability to actively support the stabilization of the foot arch during locomotion19, 20). Of MLA. After excluding nine subjects, the remaining forty these, the AH is the medial-most muscle in the first layer participated in this experiment. All subjects provided their of intrinsic muscles at the plantar surface of the foot. Many written informed consent before participating, and the study studies have reported that the AH plays an important role was approved by the Institutional Review Board of Yonsei in supporting the MLA and works in conjunction with University Wonju Campus. 1104 J. Phys. Ther. Sci. Vol. 24, No. 11, 2012

The height of the navicular tuberosity was measured joint and holding the contraction for 5 s. The RMS values using Vernier height calipers (Mitutoyo Korea Corpo- of the EMG data were calculated and the maximal EMG ration, Mitutoyo, Japan). The ND was determined as the signals for normalization were acquired during MVIC. The displacement of the navicular in the sagittal plane between values of the first and last second were discarded and the the neutral sitting and resting stance positions14). The neutral mean RMS of the middle 3 s was calculated. The %MVIC sitting or subtalar joint neutral (STJN) position was deter- was measured three times with a 60-s rest between each trial. mined by palpating the talar head congruency when the After measuring the AH MVIC, the AH EMG data were participant was seated in an adjustable-height chair. The collected in both the STJR and STJN positions for 5 s in subtalar joint resting stance (STJR) position was when the the one-leg standing position with full weight-bearing. To subject relaxed the foot in full-weight bearing17). The ND prevent postural sway, the subject lightly held a chair placed was calculated as the difference in navicular height between to the side of his leg. The initial and last second of the EMG the STJN and STJR positions. The reliability and validity of values were discarded, and the remaining 3 s of data were measuring the ND are well established18, 26, 27). used as the %MVIC. The activity of the AH was measured using a surface Demographic data, including sex, age, height, and EMG system (MP100; BIOPAC Systems Inc., Goleta, CA, weight, were analyzed using descriptive statistics. The ND USA). EMG was performed using a preamplified electrode and AH EMG activity were assessed in both the STJR and (BIOPAC Systems Inc.). The skin surface at the electrode STJN positions. The correlations between the %MVIC of site was prepared by sanding and cleansing by rubbing with AH EMG activity and ND in the STJR and STJN positions alcohol. The electrodes were placed over the middle of the were determined using Pearson’s correlation coefficient. A AH, parallel to the muscle fibers. A reference electrode simple regression analysis was performed to predict the ND was placed on the lateral malleolus. Data were sampled at from the measured AH muscle activation, with ND (mm) as 1000 Hz and band-pass filtered between 10 and 500 Hz. the dependent variable and AH EMG activity (%MVIC) as Root-mean-square (RMS) values were calculated using the independent variable. All data analyses were performed a moving 50-ms window. The EMG data were converted using SPSS, ver. 12.0, and are reported as the average ± to a digital signal that was recorded using AcqKnowledge standard deviation. Values of p < 0.05 were considered software (BIOPAC Systems Inc.) for processing. statistically significant. The ND and the EMG activity of AH were measured for all subjects, with the subject seated on a chair with the hips, RESULTS knees, and ankles at approximately 90°, and the feet placed on the floor with the tibia perpendicular to the floor. In order The demographic data of the subjects are summarized to measure ND in the STJN position, the examiner palpated in Table 1. AH EMG activity in the STJR position had a the medial and lateral aspects of the talus with the thumb and significant negative correlation with ND. To better under- index finger while the subject slowly inverted and everted stand the relationship between AH EMG activity and the his hindfoot until neither the medial nor the lateral sides of ND, a linear regression model was constructed (Table 2). the talar head were protruding or depressed. The examiner This was expressed by the equation y = −0.290x + 15.951 repositioned the foot until the talus was located centrally (y = ND, x = %MVIC of AH EMG activity). For the model, and could be felt equally under each point of palpation. The R2 = 0.505. AH EMG activity in the STJN position and ND examiner palpated the navicular tuberosity and marked the were not correlated. site with a washable pen in the STJN position. The calipers were positioned, and the movement arm adjusted so that it DISCUSSION was aligned with the navicular tuberosity. The examiner measured the height of the mark on the foot from the floor in This study investigated the correlation between AH the STJN position. Next, the subject was instructed to stand muscle activity in the one-leg standing position and the with full weight-bearing, keeping the tibia perpendicular to ND and predicted the ND from the AH EMG activity in the floor. The examiner again measured the height ofthe the one-leg standing position. AH EMG activity in the mark on the foot from the floor and recorded it as the ND STJR position was significantly correlated with the ND. In in the STJR position. The difference in navicular height addition, the ND could be estimated from AH EMG activity between the seated and standing positions was calculated as using a linear regression model. These results imply that the ND. The ND was measured three times and the mean an assessment of AH muscle activity in the STJR position was calculated. is a good predictor of the ND. AH EMG activity in the For EMG measurement each subject was seated with STJN position, while actively supporting the MLA, was not the foot placed on a foot plate to record the EMG and the significantly correlated with the ND. belly of the AH on the test foot was located by palpation. ND is defined as the difference between the height of The foot was fixed to the foot plate using a strap to prevent the navicular in the STJN position, while sitting, and in movement of the ankle joint. The subject was instructed to the STJR position, while weight-bearing17, 28). Movement make sure that his foot was always in this same position of the subtalar joint can be detected by measuring the before performing maximal voluntary isometric contractions extent of movement of the navicular. Regarding the ND (MVICs). The subject was instructed to maximally contract test, a difference of 10 mm is considered normal, and one the AH by plantar flexion of the first metatarsophalangeal > 10 mm abnormal14, 29). Many studies have reported that 1105

Table 1. Characteristics of subjects strength of the AH and neuromuscular control based on Demographic values intact sensorimotor function are needed to actively support the MLA. Accordingly, we speculate that continuous Gender (Male/Female) 16/26 intrinsic muscle strengthening is required to maintain the Age 21.9 ± 2.1 MLA structure in subjects with pronated feet. Height (cm) 167.4 ± 7.2 This study had a number of limitations, including the fact Weight (kg) 59.9 ± 12.1 that other intrinsic/extrinsic muscles supporting the MLA Mean ± SD were not considered. This may be why the predictability of the ND using AH activity was 50.5% in the regression analysis. Another limitation of this study was that all the subjects were in their twenties. Further study is needed to Table 2. Linear regression model of AH EMG activity with determine the therapeutic effect of AH exercises and to ND in the STJR position ascertain the correlation between AH activity and the ND Measurement Correlation Coefficient during a dynamic activity (e.g., gait). ND 10.29 ± 3.91 –0.710* AH 19.55 ± 9.58 REFERENCES Simple regression analysis 1) Nack JD, Phillips RD: Shock absorption. Clin Podiatr Med Surg, 1990, 7: ND Simple R 0.710 391–397. [Medline] Simple R2 0.505 2) Sherman KP: The foot in sport. Br J Sports Med, 1999, 33: 6–13. [Medline] [CrossRef] 2 Adjusted R 0.492 3) Arangio GA, Reinert KL, Salathe EP: A biomechanical model of the F 38.709* effect of subtalar arthroereisis on the adult flexible flat foot. Clin Biomech (Bristol, Avon), 2004, 19: 847–852. [Medline] [CrossRef] Mean ± SD STJR: subtalar joint resting position. ND: navicular 4) Giza E, Cush G, Schon LC: The flexible flatfoot in the adult. Foot Ankle drop. AH: abductor hallucis. *p<0.05 Clin, 2007, 12: 251–271. [Medline] [CrossRef] 5) Snook AG: The relationship between excessive pronation as measured by navicular drop and isokinetic strength of the ankle musculature. Foot Ankle Int, 2001, 22: 234–240. [Medline] the ND is correlated with foot pronation, which is linked 6) Pohl MB, Hamill J, Davis IS: Biomechanical and anatomic factors to the stability of the MLA11, 13, 30). Moreover, the intrinsic associated with a history of plantar fasciitis in female runners. Clin J Sport plantar musculature clearly plays a significant role in Med, 2009, 19: 372–376. [Medline] [CrossRef] 21, 31) 7) Powers CM, Maffucci R, Hampton S: Rearfoot posture in subjects with supporting this structure . Consequently, the ND is a patellofemoral pain. J Orthop Sports Phys Ther, 1995, 22: 155–160. useful measurement for understanding the biomechanical [Medline] relationship between the MLA structures and intrinsic foot 8) Ryan M, Grau S, Krauss I, et al.: Kinematic analysis of runners with achilles mid-portion tendinopathy. Foot Ankle Int, 2009, 30: 1190–1195. muscles. In this study, to predict the ND from AH activity [Medline] [CrossRef] using regression analysis, subjects with excessively low (≤ 9) Tome J, Nawoczenski DA, Flemister A, et al.: Comparison of foot 3.00) or high (≥ 18.00) ND were excluded, as were subjects kinematics between subjects with posterior tibialis tendon dysfunction and with an excessively rigid foot (pes cavus), and those who healthy controls. J Orthop Sports Phys Ther, 2006, 36: 635–644. [Medline] 10) Franco AH: Pes cavus and pes planus. Analyses and treatment. Phys Ther, could not actively support the MLA in a short foot exercise. 1987, 67: 688–694. [Medline] Our finding of a correlation between AH activity and 11) Huang CK, Kitaoka HB, An KN, et al.: Biomechanical evaluation of the ND is in agreement with studies showing increased longitudinal arch stability. Foot Ankle, 1993, 14: 353–357. [Medline] contractive effort of the AH in maintaining a well-supported 12) Nakamura H, Kakurai S: Relationship between the medial longitudinal arch movement and the pattern of rearfoot motion during the stance phase 21, 22, 32) MLA as the ND increases . In other words, if the of walking. J Phys Ther Sci, 2003, 15: 13–18. [CrossRef] ND increases significantly, AH activity decreases signifi- 13) Thordarson DB, Schmotzer H, Chon J, et al.: Dynamic support of the cantly. Many studies have revealed that decreased AH human longitudinal arch. a biomechanical evaluation. Clin Orthop Relat Res, 1995, 165–172. [Medline] activity contributes to flattening of the MLA, which induces 14) Brody DM: Techniques in the evaluation and treatment of the injured excessive pronation while sitting or in a one-leg standing runner. Orthop Clin North Am, 1982, 13: 541–558. [Medline] position16, 21, 22). In addition, the strength of the AH in 15) Menz HB: Alternative techniques for the clinical assessment of foot subjects with excessive foot pronation is less than that in pronation. J Am Podiatr Med Assoc, 1998, 88: 119–129. [Medline] 16) McPoil TG, Cornwall MW, Medoff L, et al.: Arch height change during subjects with normal foot pronation. Therefore, short foot sit-to-stand: an alternative for the navicular drop test. J Foot Ankle Res, exercises are recommended for activating the AH32, 33). Jung 2009, 2: 17. [Medline] [CrossRef] et al. reported that the AH plays a role in plantarflexion, 17) Mueller MJ, Host JV, Norton BJ: Navicular drop as a composite measure of excessive pronation. J Am Podiatr Med Assoc, 1993, 83: 198–202. which stabilizes the midfoot against the ground during the [Medline] 31) stance phase of gait . Therefore, in the MLA supporting 18) Picciano AM, Rowlands MS, Worrell T: Reliability of open and closed complex, the AH function may increase ND, inducing kinetic chain subtalar joint neutral positions and navicular drop test. J excessive pronation during static one-leg standing. Orthop Sports Phys Ther, 1993, 18: 553–558. [Medline] 19) Gray EG, Basmajian JV: Electromyography and cinematography of leg The main finding of this study is that the lower the activity and foot (“normal” and flat) during walking. Anat Rec, 1968, 161: 1–15. of the AH in the STJR position, the greater the ND. This was [Medline] [CrossRef] expressed as a regression equation with a predictive ability 20) Mann R, Inman VT: Phasic activity of intrinsic muscles of the foot. J Bone of 50.5%. However, AH activity in the STJN position was Joint Surg Am, 1964, 46: 469–481. [Medline] 21) Fiolkowski P, Brunt D, Bishop M, et al.: Intrinsic pedal musculature not correlated with the ND. These results imply that normal support of the medial longitudinal arch: an electromyography study. J Foot 1106 J. Phys. Ther. Sci. Vol. 24, No. 11, 2012

Ankle Surg, 2003, 42: 327–333. [Medline] [CrossRef] supinated foot postures on static and dynamic postural stability. J Athl 22) Headlee DL, Leonard JL, Hart JM, et al.: Fatigue of the plantar intrinsic Train, 2005, 40: 41–46. [Medline] foot muscles increases navicular drop. J Electromyogr Kinesiol, 2008, 18: 29) Loudon JK, Jenkins W, Loudon KL: The relationship between static 420–425. [Medline] [CrossRef] posture and ACL injury in female athletes. J Orthop Sports Phys Ther, 23) Wong YS: Influence of the abductor hallucis muscle on the medial arch of 1996, 24: 91–97. [Medline] the foot: a kinematic and anatomical cadaver study. Foot Ankle Int, 2007, 30) Williams DS, McClay IS: Measurements used to characterize the foot and 28: 617–620. [Medline] [CrossRef] the medial longitudinal arch: reliability and validity. Phys Ther, 2000, 80: 24) Kelly LA, Kuitunen S, Racinais S, et al.: Recruitment of the plantar 864–871. [Medline] intrinsic foot muscles with increasing postural demand. Clin Biomech 31) Jung DY, Kim MH, Koh EK, et al.: A comparison in the muscle activity (Bristol, Avon), 2012, 27: 46–51. [Medline] [CrossRef] of the abductor hallucis and the medial longitudinal arch angle during toe 25) Pinto RZ, Souza TR, Maher CG: External devices (including orthotics) curl and short foot exercises. Phys Ther Sport, 2011, 12: 30–35. [Medline] to control excessive foot pronation. Br J Sports Med, 2012, 46: 110–111. [CrossRef] [Medline] [CrossRef] 32) Jung DY, Koh EK, Kwon OY: Effect of foot orthoses and short-foot exercise 26) Shrader JA, Popovich JM Jr, Gracey GC, et al.: Navicular drop on the cross-sectional area of the abductor hallucis muscle in subjects with measurement in people with rheumatoid arthritis: interrater and intrarater pes planus: a randomized controlled trial. J Back Musculoskelet Rehabil, reliability. Phys Ther, 2005, 85: 656–664. [Medline] 2011, 24: 225–231. [Medline] 27) Vinicombe A, Raspovic A, Menz HB: Reliability of navicular displacement 33) Heo HJ, Koo YM, Yoo WG: Comparison of selective activation of the measurement as a clinical indicator of foot posture. J Am Podiatr Med abductor hallucis during various exercises. J Phys Ther Sci, 2011, 23: Assoc, 2001, 91: 262–268. [Medline] 915–918. [CrossRef] 28) Cote KP, Brunet ME, Gansneder BM, et al.: Effects of pronated and