Effect of Foot Orthoses on Tibialis Posterior Activation in Persons with Pes Planus KORNELIA KULIG1, JUDITH M. BURNFIELD1,3, STEPHEN REISCHL1, SUSAN MAIS REQUEJO1, CESAR E. BLANCO4, and DAVID B. THORDARSON2 Department of Biokinesiology and Physical Therapy and 2Keck School of Medicine, Department of Orthopedic Surgery, University of Southern California, Los Angeles, CA; 3Pathokinesiology Laboratory, Rancho Los Amigos National Rehabilitation Center, Downey, CA; and 4A. E. Mann Institute for Biomedical Engineering at the University of Southern California, Los Angeles, CA
1
ABSTRACT KULIG, K., J. M. BURNFIELD, S. REISCHL, S. MAIS REQUEJO, C. E. BLANCO, and D. B. THORDARSON. Effect of Foot Orthoses on Tibialis Posterior Activation in Persons with Pes Planus. Med. Sci. Sports Exerc., Vol. 37, No. 1, pp. 24 –29, 2005. Purpose: To examine the influence of footwear on tibialis posterior (TP) activation in persons with pes planus. Methods: Six asymptomatic adults with pes planus (arch index of ⱕ0.16) participated. Subjects performed a resisted foot adduction with plantar flexion exercise (3 sets of 30 repetitions). The exercise was performed barefoot and shod with foot orthoses. The two testing conditions were separated by a week. Magnetic resonance image signal intensity of the tibialis posterior, tibialis anterior, soleus, medial gastrocnemius, and peroneus longus was measured immediately before and after each exercise. Multivariate analyses of variance followed by paired Student’s t-test were performed for the signal intensity of each muscle assessed to determine whether TP was selectively activated during the barefoot and shod exercises. Results: When barefoot, five of the six subjects activated other lower-leg muscles in addition to TP. When wearing the foot orthoses and shoes, all five participants activated only TP. Additionally, activation of TP was higher when exercises were performed in shoes with orthoses than when barefoot (P ⫽ 0.019). Conclusion: Wearing the foot orthoses and shoes improved selective activation of the TP in persons with flat feet. In cases where selective activation of TP is desirable, such as persons with flat feet or TP tendon dysfunction, use of shoes and an arch supporting foot orthoses may enhance selective activation of the muscle. Key Words: EXERCISE, MAGNETIC RESONANCE IMAGING, PHYSICAL THERAPY, ORTHOTICS, LOWER EXTREMITY
T
the tibialis posterior, is lowered. This position increases the distance between the origin and insertion of the tibialis posterior and lengthens its musculotendinous unit. Adaptive changes in the muscle, such as increasing the number or size of the sarcomeres, could enable the muscle to continue functioning efficiently. Despite these adaptations, continued malalignment may necessitate activation of other muscles of the foot-ankle complex to assist in performing tasks normally performed primarily by the tibialis posterior. When tendinopathy is present, exercises to strengthen the involved musculotendinous unit have been strongly recommended in order to prevent further degeneration (3,30). Only limited literature exists, however, to guide clinicians in designing an effective strengthening program for the tibialis posterior (3,18,22,30). Kulig and colleagues (18) compared how effective three exercises were at selectively and effectively activating the tibialis posterior in healthy adults with a normal arch index. The three exercises studied were: 1) closed chain resisted foot adduction, 2) unilateral heel raise, and 3) open chain resisted foot supination. The resisted foot adduction exercise was the only exercise assessed that selectively activated the tibialis posterior and was also the exercise that most effectively recruited the muscle. The influence of variations in foot morphology (e.g., pes planus) on selective activation of posterior tibialis during the same exercise was not studied.
ibialis posterior tendinopathy may develop into posterior tibialis tendon dysfunction (PTTD) and can lead to disabling weightbearing symptoms associated with acquired flat foot deformity in adults (12). Rehabilitative interventions often emphasize the importance of selective and specific exercise (3,30). In our recent study of individuals with normal foot morphology, tibialis posterior was recruited selectively and most effectively during resisted foot adduction and plantar flexion (18). It is not known, however, whether persons with a low foot arch (pes planus) recruit the tibialis posterior similarly during the same motion. A pes planus foot frequently has a hindfoot valgus, midfoot varus, and forefoot abductus posture (28,29). In this position, the navicular, the primary tarsal insertion site of
Address for correspondence: Kornelia Kulig, Ph.D., PT, Department of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, CA, 1540 E. Alcazar Street, CHP-155, Los Angeles, CA 90033; E-mail kulig@hsc.usc.edu. Submitted for publication January 2004. Accepted for publication August 2004. 0195-9131/05/3701-0024 MEDICINE & SCIENCE IN SPORTS & EXERCISE® Copyright © 2005 by the American College of Sports Medicine DOI: 10.1249/01.MSS.0000150073.30017.46
24
Foot orthoses are frequently prescribed in an effort to more properly align the symptomatic pes planus foot (2,4,9,25,28,30). The extent to which an orthosis may alter the recruitment pattern of the tibialis posterior, however, has not been studied. The purpose of this study was to examine the influence of shoes with foot orthoses on tibialis posterior activation in persons with pes planus. We elected to examine asymptomatic persons with pes planus in order to avoid the potential confounding effects of pain on muscle activation. We hypothesized that the tibialis posterior activation level would differ in persons with pes planus compared with previous reports in persons with normal feet and that use of an arch-correcting foot orthosis would promote a more normal activation level for the tibialis posterior.
METHODS Subjects. Six healthy pain-free adult subjects, with an arch index at least two standard deviations below normative values, participated in this study. In adults, an arch index is identified as normal if its value is 0.227 ⫾ 0.037 (31). The subjects’ average age, weight, and height were 25.0 ⫾ 2.0 yr, 71.7 ⫾ 7.7 kg, and 177.8 ⫾ 7.6 cm, respectively. A low longitudinal arch was the only observable deformity on all subjects who participated. Potential subjects were excluded from this investigation if they had an orthopedic or neurological disorder that precluded them from completing the exercise protocol. Also, potential participants were excluded if there were precautions or contraindications associated with having magnetic resonance imaging (MRI) (i.e., cardiac pacemaker, central aneurysm clips, cardiac valve replacement, metal pins, metal prosthesis, removable dentures, or nerve stimulators) (26). All subjects were provided and signed written informed consent to participate in this study according to a protocol approved by the Institutional Review Board and Health Research Association of the University of Southern California. Assessment of the longitudinal arch of our subject’s dominant foot indicated that each had an arch index that was at least two standard deviations below the norm (31). The arch index was calculated as the ratio of the navicular height to the truncated foot length (measured from the posterior aspect of the calcaneus to the first metatarsophalangeal joint) (31). Subjects stood during the measurement and placed approximately 90% of their body weight through the tested limb (31). The average arch index of participants in this study was 0.146 ⫾ 0.014. Exercise protocol. As previous work indicated that closed chain resisted foot adduction (foot adduction) was the exercise that most selectively and effectively activated tibialis posterior in persons with a normal arch index (18), this exercise was selected to determine the influence of shoes and foot orthoses on tibialis posterior activation in persons with pes planus. The exercise had both a concentric and eccentric phase, and was performed both barefoot and with shoes and orthoses (Fig. 1A). SELECTIVE LEG MUSCLE ACTIVATION IN PES PLANUS
FIGURE 1—A. Subjects performed a closed chain resisted foot adduction exercise while barefoot (column 1) and while wearing shoes with nonprescription foot orthoses (column 2). B. Examples of transaxial images obtained before exercise which were used to determine the baseline signal intensity. C. Examples of transaxial images recorded immediately after the resisted adduction exercises. Note that during the barefoot condition (column 1), a lighter shade of gray is present in the area posterior to the tibia (i.e., the location of the tibialis posterior), as well as the medial gastrocnemius. This reflects an increase in activity in these two muscles during the barefoot exercise condition. After exercise in the shoe and foot orthoses (column 2), however, the tibialis posterior is the only region that increases in brightness. Five of the six subjects evaluated demonstrated a similar pattern. D. Group means and standard errors of the mean of signal intensity changes associated with the two footwear conditions. * Posterior tibialis activity was significantly greater during the orthoses condition compared with barefoot (P ⴝ 0.019).
Subjects were seated with their knees maintained at a forearm’s length apart and flexed approximately 80°. Their feet were placed flat on the ground. The leg being exercised Medicine & Science in Sports & Exercise姞
25
was stabilized by having the participant place their contralateral forearm between the knees and reinforce it with their ipsilateral hand. A silver-colored elastic band (Theraband, GmbH, Mainzer Lamdstrasse19, D-65589 Hadamar, Germany; Hygenic Corporation, Akron, OH) was looped around the distal medial foot being exercised. The band was stretched laterally by the examiner to full tension, while maintaining a 45° angle of inclination with the floor. The investigator held the band with the goal of maintaining constant tension throughout the exercise. From an abducted foot position, the subject slid the forefoot into adduction and then slowly returned to the starting position. Each subject’s individual foot range of motion in the transverse plane was marked on the floor and the subject was instructed before exercise, and reminded during exercise, to always complete the full available range of motion. The foot was maintained flat on the floor during the entire exercise. Magnetic resonance imaging. The intensity of muscle activation was evaluated using MRI. This method was selected as it provided a noninvasive method for assessment. MRI signal intensity is typically depicted by varying shades of gray on an image. Exercise induced changes in concentration of cytoplasmic byproducts (e.g., lactate, phosphate, and sodium) can contribute to changes in the signal intensity within muscles (10,13,21). Areas of increased activity appear lighter. Measures of T2 relaxation times recorded immediately after exercise strongly correlate with integrated electromyographic recordings of the resistive exercise session (R ⫽ 0.99, P ⬍ 0.05) (1). MRI was performed using a 1.5-T MRI system (General Electric Medical Systems, Milwaukee, WI). Transaxial images of the lower leg were obtained using a fast inversion recovery pulse sequence at a repetition time (TR) of 2500 ms, echo time (TE) of 90 ms, and inversion time (TI) of 140 ms. The imaging parameters used are generally considered T2 weighted. Whereas the presence of an inversion pulse reduces the effect of T2 weighting in some circumstances, its principal effect in the current sequence was to eliminate the signal from lipid resonances and thus increase the relative T2 of water. In this study, greater water content mainly increased signal on T2-weighted images because of the relatively long T2 of water. In contrast, the T1 effects would be relatively minor when using a TR of 2500 at 1.5 T. Whereas these relationships may change in the presence of very high field strengths (e.g., 14.7 T), these magnets were not used, as they are not appropriate for human experimentation. The transaxial slice thickness was 1.5 cm without interval spacing with a field of view (FOV) of 20 ⫻ 20 cm and imaging matrix of 256 ⫻ 192. The total imaging time was 2 min. Procedures. Each subject participated in two imaging sessions that were separated by at least 1 wk. During one session, the subject performed the resisted foot adduction exercise while barefoot (barefoot). During the other session, the subject wore shoes and an appropriately sized nonprescription foot orthoses when performing the exercise (orthoses). The orthoses were full-length, with rearfoot and midfoot control, a patented support bridge, shock absorption 26
Official Journal of the American College of Sports Medicine
system and Long-Wearing Trocellen™ Foam (Superfeet Worldwide LLC, 1419 Whitehorn Street, Ferndale, WA). The order of sessions was determined by random assignment. Before performing the exercise, baseline muscle signal intensities were established. Subjects were positioned supine with the dominant lower leg within a circular coil. The lower leg length was measured. Then a dark line, indicating a location two-thirds proximal from the medial malleolus, was placed on the subject’s skin. This line referenced the pre- and postexercise positions of the imaging coil so that the reference slice could be identified. After the baseline MRI, subjects completed three sets of 30 repetitions of the above-described exercise. Each set was separated by a 1-min rest period. The exercise was completed adjacent to the scanner room to allow for immediate scanning of the subject’s leg after completion of the third set of repetitions. The time between completion of the third set of repetitions and initiation of imaging was ⬍3 min for each subject. Data analysis. An MR image of the cross-section of the lower leg, two-thirds the distance proximal from the medial malleolus to the knee joint line, was displayed on the MRI console. A circular cursor (area, 35 mm2) was used to assess the signal intensity at the region-of-interest (ROI). Cursor placement was carefully focused on the muscle of interest while avoiding fat, fascia, and vessels. Imaging software (GE system, Milwaukee, WI) was used to evaluate the signal intensity within a representative ROI for five muscle groups: tibialis posterior, medial gastrocnemius, soleus, peroneus longus, and tibialis anterior. The overall change in signal intensity was calculated using the following equation: [(SNRpost ⫺ SNRpre)/SNRpre] ⫻ 100
where SNR (signal-to-noise ratio) is signal intensity, pre is before exercise (Fig. 1B), and post is after exercise (Fig. 1C). The same investigator (KK) analyzed all MRI data. The investigator’s reliability in digitizing was 0.87 (ICC 3,1). A muscle was defined as having been active during an exercise if postexercise signal intensity exceeded the baseline level by twofold, that is, a signal intensity increase greater than 10% (18). This threshold was established after analyzing repeated signal intensity measurements recorded from nonexercised muscles. The analysis demonstrated that that signal intensity values recorded from the same persons’ nonexercised muscles varied less than 5% across days. Statistical analysis. The percent change in signal intensity from before to after exercise obtained during the barefoot and orthoses conditions was used for statistical analysis. Descriptive statistics and multivariate analyses of variance were employed to study the difference in muscle activation during the two footwear conditions. Post hoc paired t-tests were conducted when appropriate. An alpha level of 0.05 was used to test for significance. All statistical analyses were conducted using SPSS software version 10.0 (SPSS, Chicago, IL). http://www.acsm-msse.org
RESULTS A summary of the data in the form of means and standard errors of the mean signal intensity changes is presented in Figure 1D. Accompanying each bar graph, are transaxial MR images obtained before (Fig. 1B) and immediately after exercise (Fig. 1C) for a representative subject. Greater variability in the pattern of muscle recruitment was noted during the barefoot compared with orthoses condition, as evidenced by the relatively larger standard error of the means for the medial gastrocnemius (7 vs 2), anterior tibialis (4 vs 2), and soleus (3 vs 1) while barefoot. The tibialis posterior signal intensity increased after both the barefoot and orthoses exercise conditions; however, the remaining muscles displayed a less consistent pattern. While barefoot, the average increase in tibialis posterior signal intensity exceeded the 10% significance threshold in all six subjects (mean, 29% SI; range, 16 – 62% SI). Three of six subjects also displayed a significant increase in activity in the medial gastrocnemius (range, 17– 41% SI), whereas the changes recorded for the remaining three subjects did not reach the level of significance. Also, a significant postexercise increase in activity was documented in the anterior tibialis for three of six participants (range, 12–16% SI), in the soleus for two subjects (10 and 14% SI), and in the peroneus longus for one individual (12% SI) while barefoot. When wearing the orthoses, the average signal intensity increase in tibialis posterior was 54% (range, 31–90% SI) after exercise. The change in signal intensity in the medial gastrocnemius, anterior tibialis, soleus, and peroneus longus did not exceed the 10% significance threshold for any subject while wearing the orthoses. A comparison of the signal intensities recorded after each exercise condition identified that tibialis posterior activity was nearly twofold higher when exercising with an orthosis compared with barefoot (54 vs 29% SI; P ⫽ 0.019). This pattern, of higher tibialis posterior activation while wearing the orthoses, was consistently documented for all six subjects. The mean change in each of the other muscles did not differ significantly between the orthoses and barefoot exercise conditions.
DISCUSSION Tibialis posterior activity increased during the resisted adduction exercise in persons with asymptomatic pes planus. While barefoot, however, the pattern of lower-leg muscle activity differed notably from our previous findings in individuals with a normal arch index (18). In the current study, the average increase in tibialis posterior activity in persons with pes planus was only 29% when barefoot. Additionally, five of six subjects demonstrated a significant increase in signal intensity in at least one other muscle assessed. This indicated that the tibialis posterior was not selectively activated while persons with pes planus performed the exercise barefoot. In contrast, in our previous work using similar methodology, we found that persons with a normal arch index demonstrated a robust and selecSELECTIVE LEG MUSCLE ACTIVATION IN PES PLANUS
tive pattern of activation of the tibialis posterior during the same exercise conditions (18). The average tibialis posterior signal intensity increased 50%, while nonsignificant increases (ⱕ2%) were documented for the remaining muscles (soleus, medial gastrocnemius, peroneus longus, tibialis anterior) (18). Supplemental muscle activity during the barefoot exercise condition reflected a need for additional muscles to complete the task. Activity in the medial gastrocnemius and soleus provided complementary support to the tibialis posterior for controlling ankle and hindfoot motion. Force augmentation by peroneus longus and anterior tibialis, however, had an associated penalty. During foot adduction, the 45° angulation of the Theraband relative to the longitudinal axis of the foot emphasized resistance to forefoot adduction, as well as whole foot plantar flexion and inversion. The insertion of medial gastrocnemius on the posterior aspect of the calcaneus permitted moderate activity in this muscle (range, 17– 41% SI) to directly contribute to the plantar flexion and inversion force required to complete the exercise. The low level of activity documented in the soleus muscle (10% and 14% SI) for two subjects, similarly augmented the plantar flexion and inversion force required to complete the motion. The biomechanical rationale for activating the medial gastrocnemius or soleus may have depended, in part, on variations in torsion occurring within the Achilles tendon before its insertion onto the calcaneus. Cummins et al. (6), in an investigation of 100 cadavers, identified that all specimens displayed some degree of rotation of the gastrocnemius portion of the Achilles tendon relative to the soleus component. In 35% of the subjects, this tendon torsion resulted in the gastrocnemius fibers inserting onto the lateral half of the calcaneus, while the soleus fibers attached on the medial aspect of the calcaneus (6). The torsion of the Achilles tendon was not quantified for subjects in the current study; however, variations would have altered the inversion– eversion lever arm of the muscles at the subtalar joint. The posterior course of the peroneus longus tendon relative to the ankle joint allowed augmentation of plantar flexion for one subject; however, its role as a foot evertor likely limited the use of this strategy by others (17). Although activation of the tibialis anterior provided an effective means for supplementing forefoot adduction and inversion for three subjects, this approach would have contributed an undesirable dorsiflexion force during the task. In the current study, use of a foot orthoses and shoes permitted both selective and more effective tibialis posterior activation during the exercise compared with the barefoot condition. Activity in the tibialis posterior nearly doubled (54% SI with orthoses vs 29% SI barefoot), whereas the remaining muscles were quiescent (i.e., the signal intensity did not exceed the 10% threshold used to define activation). Further, the pattern of muscle activation while using the orthoses mimicked those previously described when persons with a normal arch index performed the exercise barefoot (18). The selective activation of tibialis posterior during the orthoses condition, but not while barefoot, suggests that a low arch in persons with pes planus may be a factor conMedicine & Science in Sports & Exercise姞
27
tributing to nonselective tibialis posterior activation. While using the foot orthoses and shoe, changes in the alignment of the rearfoot and arch may have contributed to differences in muscle activation patterns observed. Klein et al. (17), for example, reported that the triceps surae changed from functioning as an invertor to an evertor as the subtalar joint moved from an everted to inverted position in an analysis of 10 human cadavers. In the current study, it is possible that changes in the alignment of the foot due to the use of orthoses and shoes may have altered the lever arms, and hence the force producing capabilities of the leg muscles. Tibialis posterior is an essential stabilizer of the foot during standing and walking. The tibialis posterior tendon provides dynamic support along the plantar aspect of the foot and arch (17), and when the muscle or tendon is deficient, decreases in longitudinal arch height often occur (11,14,16,19). During gait, eccentric activity of the tibialis posterior, which is initiated at heel contact, permits controlled foot pronation and assists with shock absorption during limb loading (23,24). In terminal stance, a second peak in tibialis posterior activity (24) contributes to the transverse tarsal joints “locking” (8). Body weight is able to progress forward over a stable foot as the heel rises from the ground (24). Dysfunction of the tibialis posterior has been identified as one of the primary causes of acquired flat foot in adults (5,7,12,15,20,27). One of the goals of rehabilitation for subjects with painful pes planus is to mechanically support their arches and then selectively strengthen the tibialis posterior muscles. From the results of this study, we suggest
that people with pes planus wear a foot orthosis, even while performing the adduction exercises. In light of limited research on selective muscle activation during exercises, this methodological approach using MRI may be used to study other muscles. Additionally, it is important to determine if altered morphology is associated with unique joint kinematics, kinetics, and muscle activation and to understand their underlying mechanisms. This knowledge will lead to better diagnostic and therapeutic decisions in treatment of persons with painful foot pathologies.
SUMMARY Tibialis posterior was preferentially recruited during a resisted foot adduction exercise in persons with pes planus. While barefoot, however, subjects activated additional lower-leg muscles to complete the exercise. In contrast, the tibialis posterior was selectively activated when subjects performed the exercise while wearing the arch supporting foot orthoses and shoes. This latter muscle activation pattern is similar to that previously documented when adults with a normal arch index completed the exercise while barefoot (18). The findings of this study suggest that in cases where selective activation of TP is desirable, such as persons with flat feet or TP tendon dysfunction, use of shoes and an arch-supporting foot orthosis may enhance selective activation of the muscle. The authors of this paper would like to gratefully acknowledge the contributions of Grace Liao, B.A., for her assistance with data processing. This work was supported, in part, by a grant from the Orthopaedic Section of the American Physical Therapy Association.
REFERENCES 1. ADAMS, G. R., M. R. DUVOISIN, and G. A. DUDLEY. Magnetic resonance imaging and electromyography as indexes of muscle function. J. Appl. Physiol. 73:1578 –1583, 1992. 2. AUGUSTIN, J. F., S. L. SHELDON, W. S. BERBERIAN, and J. E. JOHNSON. Nonoperative treatment of adult acquired flat foot with the Arizona brace. Foot Ankle Clin. North Am. 8:491–502, 2003. 3. BLAKE, R. L., K. ANDERSON, and H. FERGUSON. Posterior tibial tendinitis: a literature review with case reports. J. Am. Podiatr. Med. Assoc. 84:141–149, 1994. 4. CHAO, W., K. L. WAPNER, T. H. LEE, J. ADAMS, and P. J. HECHT. Nonoperative management of posterior tibial tendon dysfunction. Foot Ankle Int. 17:736 –741, 1996. 5. CHURCHILL, R. S., and J. J. SFERRA. Posterior tibial tendon insufficiency: its diagnosis, management, and treatment. Am. J. Orthop. 27:339 –347, 1998. 6. CUMMINS, E. J., B. J. ANSON, B. W. CARR, and R. R. WRIGHT. The structure of the calcaneal tendon (of Achilles) in relation to orthopedic surgery with additional observations on the plantaris muscle. Surg. Gynecol. Obstet. 99:107–116, 1955. 7. DYAL, C. M., J. FEDER, J. T. DELAND, and F. M. THOMPSON. Pes planus in patients with posterior tibial tendon insufficiency: asymptomatic versus symptomatic foot. Foot Ankle Int. 18:85– 88, 1997. 8. ELFTMAN, H. Transverse tarsal joint and its control. Clin. Orthop. 16:41– 46, 1960. 9. ELFTMAN, N. W. Nonsurgical treatment of adult acquired flat foot deformity. Foot Ankle Clin. North Am. 8:473– 489, 2003.
28
Official Journal of the American College of Sports Medicine
10. FLECKENSTEIN, J. L., R. C. CANBY, R. W. PARKE, and R. M. PESHOCK. Acute effects of exercise on MR imaging of skeletal muscle in normal volunteers. Am. J. Roent. 151:231–237, 1988. 11. FUNK, D. A., J. R. CASS, and K. A. JOHNSON. Acquired flat foot deformity secondary to posterior tibial tendon pathology. J. Bone Joint Surg. Am. 68:95–102, 1986. 12. GEIDEMAN, W. M., and J. E. JOHNSON. Posterior tibial tendon dysfunction. J. Orthop. Sports Phys Ther. 30:68 –77, 2000. 13. HORRIGAN, J. M., F. G. SHELLOCK, J. H. MINK, and A. L. DEUTSCH. Magnetic resonance imaging evaluation of muscle usage associated with three exercises for rotator cuff rehabilitation. Med. Sci. Sports Exerc. 31:1361–1366, 1999. 14. JOHNSON, K. A. Tibialis posterior tendon rupture. Clin. Orthop. 177:140 –147, 1983. 15. KATCHIS, S. D. Posterior tibial tendon dysfunction. In: Disorders of the Heel, Rearfoot and Ankle, C. S. Ranawat and R. G. Positano (Eds.). New York: Churchill Livingstone, 1999, pp. 415– 422. 16. KITAOKA, H. B., Z. P. LUO, and K. N. AN. Effect of the posterior tibial tendon on the arch of the foot during simulated weightbearing: biomechanical analysis. Foot Ankle Int. 18:43– 46, 1997. 17. KLEIN, P., S. MATTYS, and M. ROOZE. Moment arm length variations of selected muscles acting on talocrural and subtalar joints during movement: an in vitro study. J. Biomech. 29:21– 30, 1996. 18. KULIG, K., J. M. BURNFIELD, S. M. REQUEJO, M. SPERRY, and M. TERK. Selective activation of tibialis posterior: Evaluation by maghttp://www.acsm-msse.org
19. 20. 21. 22. 23. 24. 25.
netic resonance imaging. Med. Sci. Sports Exerc. 36:862– 867, 2004. MANN, R. A., and F. M. THOMPSON. Rupture of the posterior tibial tendon causing flat foot: surgical treatment. J. Bone Joint Surg. Am. 67:556 –561, 1985. MENDICINO, S. S. Posterior tibial tendon dysfunction: diagnosis, evaluation, and treatment. Clin. Podiatr. Med. Surg. 17:33–54, vi, 2000. MEYER, R. A., and B. M. PRIOR. Functional magnetic resonance imaging of muscle. Exerc. Sport Sci. Rev. 28:89 –92, 2000. PEDOWITZ, W. J., and P. KOVATIS. Flatfoot in the adult. J. Am. Acad. Orthop. Surg. 3:293–302, 1995. PERRY, J. Anatomy and biomechanics of the hindfoot. Clin. Orthop. 177:9 –16, 1983. PERRY, J. Gait Analysis, Normal and Pathological Function. Thorofare, NJ: Charles B. Slack, 1992, pp 69 – 80. POMEROY, G. C., R. H. PIKE, T. C. BEALS, and A. MANOLI II. Acquired flatfoot in adults due to dysfunction of the posterior tibial tendon. J. Bone Joint Surg. Am. 81:1173–1182, 1999.
SELECTIVE LEG MUSCLE ACTIVATION IN PES PLANUS
26. SHELLOCK, F. G. Pocket Guide to MR Procedures and Metallic Objects. Baltimore: Williams & Wilkins, 1999, pp. 250 –256. 27. SUPPLE, K. M., J. R. HANFT, B. J. MURPHY, C. J. JANECKI, and G. F. KOGLER. Posterior tibial tendon dysfunction. Semin. Arthritis Rheum. 22:106 –113, 1992. 28. TRNKA, H.-J., M. E. EASLEY, and M. S. MYERSON. The role of calcaneal osteotomies for correction of adult flatfoot. Clin. Orthop. Relat. Res. 365:50 – 64, 1999. 29. VAN BOERUM, D. H. and B. J. SANGEORZAN. Biomechanics and pathophysiology of flat foot. Foot Ankle Clin. North Am. 8:419 – 430, 2003. 30. WAPNER, K. L. and W. CHAO. Nonoperative treatment of posterior tibial tendon dysfunction. Clin. Orthop. 365:39 – 45, 1999. 31. WILLIAMS, D. S., and I. S. MCCLAY. Measurements used to characterize the foot and the medial longitudinal arch: reliability and validity. Phys. Ther. 80:864 – 871, 2000.
Medicine & Science in Sports & Exercise姞
29