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Rocker bottom soles alter the postural response to backward translation during stance
Gait & Posture 30 (2009) 45–49
Contents lists available at ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Rocker bottom soles alter the postural response to backward translation during stance Bruce C. Albright a,*, Whitney M. Woodhull-Smith b a b
Department of Physical Therapy, College of Allied Health Sciences, Health Sciences Building, East Carolina University, Greenville, NC 27858, United States Department of Physical Therapy, College of Allied Health Sciences, East Carolina University, Greenville, NC 27858, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 September 2008 Received in revised form 13 February 2009 Accepted 22 February 2009
Shoes with rocker bottom soles are utilized by persons with diabetic peripheral neuropathy to reduce plantar pressures during gait. This population also has a high risk for falls. This study analyzed the effects of shoes with rocker bottom soles on the postural response during perturbed stance. Participants were 20 healthy subjects (16 women, 4 men) ages 22–25 years. Canvas shoes were modified by the addition of crepe sole material to represent two forms of rocker bottom shoes and a control shoe. Subjects stood on a dynamic force plate programmed to move backward at a velocity that produced an automatic postural response without stepping. Force plate data were collected for five trials per shoe type. Sway variables for center of pressure (COP) and center of mass (COM) included: mean sway amplitude, sway variance, time to peak, anterior and posterior peak velocities, functional stability margin, and peak duration time. Compared to control, both the experimental shoes had significantly larger COP and COM values for mean sway amplitude, sway variance and peak duration. The functional stability margins were significantly smaller for the experimental shoes while their anterior and posterior peak velocities were slower and time to peaks were significantly longer. In young healthy adults, shoes with rocker bottom soles had a destabilizing effect to perturbed stance, thereby increasing the potential for imbalance. These results raise concerns that footwear with rocker bottom sole modifications to accommodate an insensate foot may increase the risk of falls. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Posture control Stability Rocker bottom soles Diabetic peripheral neuropathy
1. Introduction A concern for patients with diabetic peripheral neuropathy (DPN) is the formation of foot pressure ulcers and the increased risk and incidence of lower extremity amputation [1,2]. Therapeutic footwear has addressed this concern by the incorporation of a variety of soft liners, insole and outsole components, and designs to reduce localized pressure [3–8]. Characteristically, the rocker bottom sole has a forefoot rocker pattern that functionally relocates the apex of the forefoot rocker posterior to the metatarsal heads [8]. As such, it reduces pressure on the metatarsal heads and promotes the transition from mid-stance to toe-off during gait [3,6,9]. The forefoot rocker is often combined with either a reduced heel height (negative heel) to further off-load the metatarsal heads or a mild rounded heel edge to aid the transition from initial contact to mid-stance. Sole thickness and the location of the apex alter the amplitude and distribution of plantar pressures and gait kinematics [8,10].
Persons with DPN are also prone to balance problems and falls [11–18]. Compared to normal subjects, those with DPN demonstrate increased body sway and impaired postural control [12–14]. Sway amplitude, frequency, range, velocity and the center of pressure–center of mass (COP–COM) variable have been reported to be significantly increased in subjects with DPN [12,15,19,20]. Though postural instability in persons with DPN is well documented, there is limited information on how off-loading footwear devices and shoes with rocker bottom soles affect standing postural control [21]. As an initial investigation into the effects of rocker bottom shoes on balance, the purpose of the present study was to determine the effect of shoes with rocker bottom soles on the postural response to perturbed stance in a group of young healthy adults. It was hypothesized that shoes with rocker bottom soles would have a destabilizing effect on postural control during perturbed stance compared to normal shoes. 2. Methods 2.1. Subjects
* Corresponding author. Tel.: +1 252 744 6231; fax: +1 252 744 6240. E-mail address: [email protected] (B.C. Albright). 0966-6362/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2009.02.012
A total of 20 young adults (16 women and 4 men) ages 22–25 were recruited from the general university population. Subjects were excluded if they had a neurological impairment, an orthopedic deformity of the lower extremity or an injury that required medical intervention within 6 months of their participation.
B.C. Albright, W.M. Woodhull-Smith / Gait & Posture 30 (2009) 45–49
46
Fig. 1. Photographs of shoes showing control and experimental sole patterns: (A) control shoe (CNTR); (B) rocker bottom shoe (RB); (C) negative heel shoe (NH). Subjects had to fit comfortably in either a women’s size 8 or a men’s size 10 canvas tennis shoe. This project was approved by and was in compliance with the University and Medical Center Institutional Review Board for the utilization of human subjects in research (UMCIRB # 05-0594).
3. Results
2.2. Equipment
The means and standard deviations of all variables are shown in Table 1. Backward platform translation resulted in a forward sway as evident by an initial anterior displacement of both COP and COM followed by a posterior directed recovery response (Fig. 2). The RB and NH experimental shoes responded similarly and there were no significant differences between these shoes for any of COP sway variables. Compared to the control shoe, the experimental shoes had significantly larger COP sway amplitudes and sway variances (Table 1, Figs. 2 and 3). Though the mean displacement of COP for the control shoe was greater than the experimental shoes (Fig. 2), the duration of the displacement was longer for the experimental shoes as evident by larger sway amplitudes (Figs. 2 and 3). Compared to the control shoes, the experimental shoes had a significantly longer peak duration times (Table 1 and Fig. 2). In contrast to the control shoe, plots of the COP displacement for the experimental shoes showed a distinct plateau period. Not only was the PDT significantly longer for the experimental shoes but the COP remained relatively stationary at its anterior displacement prior to initiation of a posterior recovery response (Fig. 2). There was, however, no significant difference between control and experimental shoes for sway range (Table 1).
Subjects wore three types of shoes for testing (Fig. 1). Shoes were canvas with rubber soles and of the same style and brand in sizes women’s 8 and men’s 10. All shoes were modified by the addition of a 5/8th-in. thick crepe sole material to the outsole, shaped to conform to the shoes’ perimeter. Shoe soles were modified to represent either a control shoe (CNTR), a mild rocker bottom shoe (RB) or negative heel shoe (NH). In the control shoe the crepe material was full thickness throughout the length of the shoe. In the RB shoe the crepe sole was full thickness from the heel to apex and gently rounded to zero thickness at the toe. The rocker apex was positioned posterior to the ball of the shoe and within 60–65% of shoe length [8]. The heel edge was also slightly rounded. In the NH shoe the location of the rocker apex and the contour of the forefoot sole material were the same as the RB shoe. From the rocker apex to the heel, the sole material remained flat and was reduced to zero thickness at the heel. A certified orthotist designed and fabricated all shoe modifications. A dynamic dual force plate (NeuroCom International, Oregon, USA) was programmed to present a horizontal backward translation perturbation for a distance of 12.5 cm for 0.550 s at a velocity of 0.23 m/s. Platform velocity was determined prior to the study to provoke a visible postural response without stepping. 2.3. Procedures Subjects stood on the force plate with each foot in a predetermined position [22]. Subjects were asked to stand relaxed with arms by side, to look forward at an eye level target and to maintain balance without stepping. Data were collected 5 s before and after the onset of the perturbation that created an anterior body sway. The 5 s prior to the perturbation was to ensure a quiet stance and to make the onset of the perturbation less predictable. There were five perturbations for each shoe type. Shoe selection was random and the perturbations occurred at unexpected intervals. 2.4. Sway variables The NeuroCom software computed the center of mass (COM) and center of pressure (COP) based upon the subject’s height and weight. The body sway variables for anterior–posterior (A–P) COM and COP included: mean sway amplitude (SA); sway variance (SV); time to peak (T2P); anterior and posterior peak velocities; functional stability margin (FSM); and peak duration time (PDT) (Figs. 2 and 4). The sway amplitude is the mean anterior–posterior (AP) displacement of the COP or COM as a function of time. The sway variance is the standard deviation of the sway amplitude. Time to peak is the time period between perturbation onset and the maximum peak displacement. Sway range is the distance between the anterior and posterior peak displacements. Anterior and posterior peak velocities are the maximum rate of movement (meters/second) of the COP and COM toward and away from their respective maximum displacements. The functional stability margin is the numerical difference between the peak COP and peak COM. A comparatively small FSM indicates that the peak COM displacement is closer to the peak COP and may be indicative of an increased potential for postural instability [23,24]. The peak duration time is a measure of the time period involved in the stopping and reversing of the direction of movement. It was calculated as the time period when the anterior displacement of either the COP or COM was within 25% of their respective peak amplitudes. A comparatively longer PDT indicates a longer time involved to reverse the direction of body sway. 2.5. Statistical analysis Data were analyzed for 2.5 s post-perturbation. The independent variable was shoe type. The dependent body sway variables were calculated by MatLab and analyzed by MANOVA with planned post hoc comparisons using Boniferroni corrections (SPSSv 13) with a P < 0.05 level of significance. Omnibus F-test was set at P < 0.05 for all variables. Degrees of freedom (d.f.) were 2, 38 and planned post hoc comparisons were determined a priori.
3.1. Center of pressure
Table 1 Sway variable means and standard deviations for rocker bottom shoe (RB), negative heel shoe (NH) and control shoe (CNTR). RB
NH
CNTR
Sway amplitude COM 1.068 0.041 COP 1.313 0.045
1.098 0.044 1.350 0.066
0.854 0.030* 1.142 0.035*
Sway variance COM COP
1.236 0.038 1.535 0.047
1.264 0.047 1.565 0.067
1.036 0.031* 1.441 0.039*
Sway range COM COP
3.832 0.071 5.217 0.109
3.865 0.112 5.214 0.155
3.497 0.072* 5.213 0.123
Functional stability margin 0.852 0.219
0.848 0.279
1.117 0.267*
Peak duration time COM 0.458 0.103 COP 0.356 0.068
0.474 0.070 0.383 0.060
0.341 0.034* 0.323 0.071*
Time to peak COM COP
0.920 0.026 0.920 0.026
0.939 0.025 0.605 0.025
0.788 0.763* 0.470 0.013*
Ant peak velocity COM 5.892 0.126 COP 26.346 1.139
5.701 0.134 27.350 1.095
7.153 0.129* 30.586 1.028*
Post-peak velocity COM 10.866 0.559 COP 26.124 1.156
10.062 0.755 26.976 1.122
12.334 0.773y 30.564 1.032*
* Denotes significant difference (P < 0.05) between control (CNTR) and experimental shoes (RB and NH). y Denotes significance difference only between CNTR and NH shoes.
B.C. Albright, W.M. Woodhull-Smith / Gait & Posture 30 (2009) 45–49
Fig. 2. Plots of the A–P mean displacement of COM (A) and COP (B) over time. Xaxis = anterior and posterior movement in centimeters. Y-axis = time in seconds from onset of perturbation. In (A) note the similarity in anterior displacements of COM. The COM Time to Peak (T2P) was significantly longer for experimental shoes (P < 0.05). In (B) arrow with (*) indicates common point of slowing of the forward COP displacement for experimental shoes. Note the characteristic plateau region of the COP peak displacements for the experimental shoes.
The anterior and posterior peak velocities of the experimental shoes were significantly slower than the control shoe (Table 1 and Fig. 4). Similarly, the experimental shoes had significantly longer times to peak (Table 1). Figs. 2 and 4 indicate that the initial slowing of the anterior movement of the COP occurred at a relatively similar time and rate.
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Fig. 4. Plots of the A–P velocity of COM (A) and COP (B) over time. X-axis = meters/ second with positive values representing forward (anterior) movements; Yaxis = time in seconds from onset of perturbation. Zero line represents zero velocity. Plots show the significantly slower velocities for the experimental shoes as the COM and COP move toward and away from their respective anterior peak displacements.
experimental shoes also had a significantly greater sway range than the control shoe (Table 1). The anterior peak velocities for the experimental shoes were significantly slower than the control shoe (Table 1 and Fig. 4), but there were no significant differences in posterior peak velocities between the RB and control shoe.
3.2. Center of mass 3.3. Functional stability margin There were no significant differences between the experimental shoes for any of the COM sway variables (Table 1, Fig. 2). There were no difference between the experimental shoes for mean peak displacements, sway amplitudes, sway variances, or time to peak. These sway variables for the experimental shoes however were significantly larger than those of the control shoe (Table 1 and Figs. 2–4). Similarly, the PDTs for the experimental shoes were significantly longer than the control shoe (Table 1). Unlike the COP, the COM displacement plots were similar and did not exhibit any shoe specific distinctions such as the plateau region (Fig. 2). The
The functional stability margin is the numerical difference between the peak COP and COM anterior displacements [23,25]. The smaller the FSM value the closer the peak COM displacement is to the COP peak displacement. A comparatively smaller FSM value is indicative of a relatively increased potential for postural imbalance [25]. There was no significant difference in the FSM between the experimental shoes (Table 1). Compared to the control, the postural response of subjects wearing the experimental shoes
Fig. 3. (A) Bar graphs of sway amplitudes for COM and COP as a function of time. (B) Bar graphs of sway variances for COM and COP. (*) denotes significance (P < 0.05) between control and experimental shoes.
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B.C. Albright, W.M. Woodhull-Smith / Gait & Posture 30 (2009) 45–49
showed a larger peak displacement of the COM and a corresponding smaller FSM. The FSMs for the experimental shoes were significantly smaller than the control shoe (Table 1). This finding indicates that the experimental shoes created a greater tendency toward imbalance than the control shoe. 4. Discussion The literature strongly supports the effectiveness of custom diabetic shoes with enlarged toe boxes, soft liners, accommodative insoles and outer sole rocker patterns to reduce localized plantar pressures and to accommodate foot deformities and the insensate foot [3,6,9]. The impairment of somatosensation in persons with DPN is known to contribute to a reduction of postural control and an increase in imbalance [11–16,26]. Though the risk of falls is high in the DPN population, there is limited information on the impact of diabetic footwear on balance control [10,21,27]. Several studies have shown that soft and thick soles reduced balance control and induced instability [28,29]. Lavery et al. compared the effects of a standard canvas shoe and four common off-loading footwear devices on standing mean sway amplitude [21]. They showed that sway was greatest for the total contact cast with a heel and was least for the canvas shoe. As expected, sway was greatest in the device that had the smallest base of support. The current study showed that while wearing shoes with rocker bottom soles the postural responses of young healthy adults to a backward slide perturbation were more destabilizing and increased the potential for imbalance. The analyses of multiple sway variables were consistent and showed distinct and significant differences between the control and experimental shoes. There were no significant differences between the sway variables of the RB and NH shoes. This finding is likely due to their similarity in the location of the rocker apex and contour of the forefoot portion of the shoe. Characteristically, the experimental shoes had larger sway ranges, larger mean sway amplitudes and variances and smaller values for functional stability margins. These findings are consistent with reported postural responses that destabilize balance [30]. Peak velocities for the anterior movements of the COP and COM were slower for the experimental shoes compared to the control shoe. The onset of the slowing appeared to correspond to when the COP and COM moved over the apex of the rocker. Postural sway instability is associated with increases in sway range, variance and velocity [12,15,19,20]. The slower COP and COM velocities of the experimental shoes indicate that sway velocity was not a contributing factor to the overall destabilizing effect of the rocker bottom soles. The slower velocities may have been a stabilizing response of the subjects in anticipation of a perceived unstable event. It is also conceivable that the contour of the rocker bottom soles may have contributed to the slower velocities. There is no current information on the effects of rocker bottom soles on the postural control of persons with DPN. Given a somatosensory impairment and typically older age, it is conceivable that the diabetic with peripheral neuropathy while wearing shoes with rocker bottom soles will have postural responses that vary from those reported in this study for young healthy subjects. The experimental shoes were associated with significantly longer peak duration times. The PDT is a calculation for the time frame involved in the process of stopping and reversing the direction of movement for the COP and COM. These findings indicate that subjects wearing the experimental shoes were slower to reverse the direction of the COP and COM anterior displacement. Plots of the COP peak displacements for the experimental shoes were characterized by a distinct plateau region. The plateau indicates a prolonged period when the COP was relatively
stationary at its peak displacement. Consequently, subjects wearing shoes with rocker bottom soles remained in an anteriorly displaced position for a significantly longer period of time, thus delaying the onset of the postural recovery phase. This distinct delay of the onset of a recovery phase has not been previously reported and may be specific to shoes with rocker bottom soles. The functional stability margin is the numerical calculation of the distance between the maximum displacement of COM and the COP [23,24]. The smaller the FSM, the less stable the postural response [23,24]. The smaller functional stability margins of the experimental shoes support the finding that they increased the potential for postural imbalance. The functional stability margin uses the mean of the peak anterior displacements and does not employ a root mean square conversion as does the COP–COM variable used in other reports [19]. Consequently, the comparatively smaller FSM of this study is comparable to a larger COP– COM variable reported for diabetics with peripheral neuropathy [19]. Within a given shoe size, the overall shoe lengths and available surface contact area of the control and experimental shoes were equal. Compared to the control shoe, the rocker bottom shoes had a smaller base of support due to the contour of the soles. This was most evident during quiet standing. For all shoes studied, the perturbation resulted in a postural response that involved A–P displacements of the COM and COP and a dynamic change in the location and potentially the size of the base of support. Compared to the control shoe, the greater sway range and anterior displacement of the COP and COM of the experimental shoes may be indicative of a greater utilization of the available shoe contact surface. The differences in the sway variables between the control and experimental shoes can be attributed to the collective effects of the contour of the rocker bottom soles. 5. Conclusion In young healthy adults, shoes with rocker bottom soles had a destabilizing effect to perturbed stance, thereby increasing the potential for imbalance. Recognizing the postural impairments of persons with peripheral neuropathy, footwear with modifications to accommodate the insensate foot should also be evaluated for their impact on postural control. Analysis of the effect of rocker bottom soles on postural control and fall risk in the DPN population is recommended. Acknowledgements The authors are grateful to Leslie Allison, PT, PhD and Denis Brunt, PT, EdD for their assistance with data analysis; to Justin Curlee, DPT and Amy Piner, DPT for their assistance with data collection; and to Carl Tyndall, CPO, for the rocker bottom sole design and fabrication of the control and experimental shoes. Conflict of interest
The authors have read this manuscript, agree to its content and no conflict of interest exists. References [1] Stess RM, Jensen SR, Mirmiran R. The role of dynamic plantar pressures in diabetic foot ulcers. Diabetes Care 1997;20:855–8. [2] Pecoraro RE, Reiber GE, Burgess EM. Pathways to diabetic limb amputations: basics for prevention. Diabetes Care 1990;13:513–29. [3] Praet S, Louwerens JW. The influence of shoe design on plantar pressures in neuropathic feet. Diabetes Care 2003;26:441–5. [4] Erdemir A, Saucerman JJ, Lemmon D, Loppnow B, Turso B, Ulbrecht JS, et al. Local plantar pressure relief in therapeutic footwear: design guidelines from finite element models. J Biomech 2005;38:1798–806.
B.C. Albright, W.M. Woodhull-Smith / Gait & Posture 30 (2009) 45–49 [5] Actis RL, Ventura LB, Lott DJ, Smith KE, Commean PK, Hastings MK, et al. Multiplug insole design to reduce peak plantar pressure on the diabetic foot during walking. Med Biol Eng Comput 2008;46:363–71. [6] Brown D, Wertsch J, Harris G, Klein J, Janisse D. Effect of rocker soles on plantar pressures. Arch Phys Med Rehabil 2004;85:81–6. [7] Hastings MK, Mueller MJ, Pilgram JK, Lott DJ, Commeau PK, Johnson JE. The effect of metatarsal pad placement on plantar pressure in people with diabetes mellitus and peripheral neuropathy. Foot Ankle Int 2007;1:84–8. [8] van Schie C, Ulbrecht JS, Becker MB, Cavanagh PR. Design criteria for rigid rocker shoes. Foot Ankle Int 2000;21:833–44. [9] Schaff PS, Cavanagh PR. Shoes for the insensitive foot: effect of a ‘‘rocker bottom’’ shoe medication on plantar pressure distribution. Foot Ankle 1990;3: 129–40. [10] Meyers KA, Long JT, Klein JP, Wertsch JJ, Janisse D, Harris GF. Biomechanical implications of the negative heel rocker sole shoe: gait kinematics and kinetics. Gait Posture 2006;24:323–30. [11] Simoneau GG, Ulbrecht JS, Becker MG, Cavanagh PR. Postural instability in patients with diabetic sensory neuropathy. Diabetes Care 1994;17:1411–21. [12] Simmons RW, Richardson C, Pozos R. Postural stability in diabetic patients with and without cutaneous sensory deficit in the foot. Diabetes Res Clin Pract 1997;36:153–60. [13] Yamamoto R, Kinoshita T, Momoki T, Arai T, Okamura A, Hirao K, et al. Postural sway and diabetic peripheral neuropathy. Diabetes Res Clin Pract 2001;52: 213–21. [14] Lafond D, Corriveau H, Prince F. Postural control mechanisms during quiet standing in patients with diabetic sensory neuropathy. Diabetes Care 2004;27: 173–8. [15] Boucher P, Teasdale N, Courtemanche R, Bard C, Fleury M. Postural stability in diabetic polyneuropathy. Diabetes Care 1995;18:638–45. [16] Meyer PF, Oddsson LIE, DeLuca CJ. Reduced plantar sensitivity alters postural responses to lateral perturbations of balance. Exp Brain Res 2004;27:173–8. [17] Horak FB, Nashner LM, Diener HC. Postural strategies associated with somatosensation and vestibular loss. Exp Brain Res 1990;82:167–77.
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[18] Richardson JK, Hurvitz EA. Peripheral neuropathy: a true risk factor for fall. J Gerontol A Biol Sci Med Sci 1995;25:M211–5. [19] Corriveau H, Prince F, Hebert R, Raiche M. Evaluations of postural stability in elderly with diabetic neuropathy. Diabetes Care 2000;23:1187–91. [20] Uccioli L, Giacomini PG, Pasqualetti P, Di Girolamo S, Ferrigno P, Monticone G, et al. Contribution of central neuropathy to postural instability in IDDM patients with peripheral neuropathy. Diabetes Care 1997;20:929–34. [21] Lavery LA, Fleishli JG, Laughlin JT, Vela SA, Lavery DC, Armstrong DG. Is postural instability exacerbated by off-loading devices in high risk diabetics with foot ulcers? Ostomy Wound Manage 1998;44:26–34. [22] McIlroy WE, Maki BE. Preferred placement of the feet during quiet stance: development of a standardized foot placement for balance testing. Clin Biomech 1997;12:66–70. [23] Winter DA, Prince F, Frank JS, Powell C, Zabjek KF. Unified theory regarding A/P and M/L balance in quiet stance. J Neurophysiol 1996;75:2334–43. [24] Horak FB, Dimitrova D, Nutt JG. Direction-specific postural instability in subjects with Parkinson’s disease. Exp Neurol 2005;193:504–21. [25] Dimitrova D, Horak FB, Nutt JG. Postural responses to multidirectional translations in patients with Parkinson’s Disease. J Neurophysiol 2004;91:489–501. [26] Meyers KA, Long JT, Klein JP, Wertsch JJ, Janisse D, Harris GF. Biomechanical implications of negative heel rocker sole shoe: gait kinematics and kinetics. Gait Posture 2006;24:323–30. [27] Hijmans JM, Geertzen JHB, Dijkstra PU, Klass P. A systematic review of the effects of shoes and other ankle or foot applications on balance in older people and people with peripheral nervous system disorders. Gait Posture 2006;25:316–23. [28] Waked E, Robbins S, McClaran J. The effect of footwear midsole hardness and thickness on proprioception and stability in older men. J Test Eval 1997;25: 143–8. [29] Robbins S, Gouw GJ, McClaran J. Shoe sole thickness and hardness influence balance in older men. J Am Geriatr Soc 1992;40:1089–94. [30] Nashner LM. Practical biomechanics and physiology of balance. In: Jacobson GP, Newman CW, Kartush JM, editors. Handbook of balance and function testing. San Diego, CA: Singular Publishing Group Inc; 1997. p. 261–79.
Contents lists available at ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Rocker bottom soles alter the postural response to backward translation during stance Bruce C. Albright a,*, Whitney M. Woodhull-Smith b a b
Department of Physical Therapy, College of Allied Health Sciences, Health Sciences Building, East Carolina University, Greenville, NC 27858, United States Department of Physical Therapy, College of Allied Health Sciences, East Carolina University, Greenville, NC 27858, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 September 2008 Received in revised form 13 February 2009 Accepted 22 February 2009
Shoes with rocker bottom soles are utilized by persons with diabetic peripheral neuropathy to reduce plantar pressures during gait. This population also has a high risk for falls. This study analyzed the effects of shoes with rocker bottom soles on the postural response during perturbed stance. Participants were 20 healthy subjects (16 women, 4 men) ages 22–25 years. Canvas shoes were modified by the addition of crepe sole material to represent two forms of rocker bottom shoes and a control shoe. Subjects stood on a dynamic force plate programmed to move backward at a velocity that produced an automatic postural response without stepping. Force plate data were collected for five trials per shoe type. Sway variables for center of pressure (COP) and center of mass (COM) included: mean sway amplitude, sway variance, time to peak, anterior and posterior peak velocities, functional stability margin, and peak duration time. Compared to control, both the experimental shoes had significantly larger COP and COM values for mean sway amplitude, sway variance and peak duration. The functional stability margins were significantly smaller for the experimental shoes while their anterior and posterior peak velocities were slower and time to peaks were significantly longer. In young healthy adults, shoes with rocker bottom soles had a destabilizing effect to perturbed stance, thereby increasing the potential for imbalance. These results raise concerns that footwear with rocker bottom sole modifications to accommodate an insensate foot may increase the risk of falls. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Posture control Stability Rocker bottom soles Diabetic peripheral neuropathy
1. Introduction A concern for patients with diabetic peripheral neuropathy (DPN) is the formation of foot pressure ulcers and the increased risk and incidence of lower extremity amputation [1,2]. Therapeutic footwear has addressed this concern by the incorporation of a variety of soft liners, insole and outsole components, and designs to reduce localized pressure [3–8]. Characteristically, the rocker bottom sole has a forefoot rocker pattern that functionally relocates the apex of the forefoot rocker posterior to the metatarsal heads [8]. As such, it reduces pressure on the metatarsal heads and promotes the transition from mid-stance to toe-off during gait [3,6,9]. The forefoot rocker is often combined with either a reduced heel height (negative heel) to further off-load the metatarsal heads or a mild rounded heel edge to aid the transition from initial contact to mid-stance. Sole thickness and the location of the apex alter the amplitude and distribution of plantar pressures and gait kinematics [8,10].
Persons with DPN are also prone to balance problems and falls [11–18]. Compared to normal subjects, those with DPN demonstrate increased body sway and impaired postural control [12–14]. Sway amplitude, frequency, range, velocity and the center of pressure–center of mass (COP–COM) variable have been reported to be significantly increased in subjects with DPN [12,15,19,20]. Though postural instability in persons with DPN is well documented, there is limited information on how off-loading footwear devices and shoes with rocker bottom soles affect standing postural control [21]. As an initial investigation into the effects of rocker bottom shoes on balance, the purpose of the present study was to determine the effect of shoes with rocker bottom soles on the postural response to perturbed stance in a group of young healthy adults. It was hypothesized that shoes with rocker bottom soles would have a destabilizing effect on postural control during perturbed stance compared to normal shoes. 2. Methods 2.1. Subjects
* Corresponding author. Tel.: +1 252 744 6231; fax: +1 252 744 6240. E-mail address: [email protected] (B.C. Albright). 0966-6362/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2009.02.012
A total of 20 young adults (16 women and 4 men) ages 22–25 were recruited from the general university population. Subjects were excluded if they had a neurological impairment, an orthopedic deformity of the lower extremity or an injury that required medical intervention within 6 months of their participation.
B.C. Albright, W.M. Woodhull-Smith / Gait & Posture 30 (2009) 45–49
46
Fig. 1. Photographs of shoes showing control and experimental sole patterns: (A) control shoe (CNTR); (B) rocker bottom shoe (RB); (C) negative heel shoe (NH). Subjects had to fit comfortably in either a women’s size 8 or a men’s size 10 canvas tennis shoe. This project was approved by and was in compliance with the University and Medical Center Institutional Review Board for the utilization of human subjects in research (UMCIRB # 05-0594).
3. Results
2.2. Equipment
The means and standard deviations of all variables are shown in Table 1. Backward platform translation resulted in a forward sway as evident by an initial anterior displacement of both COP and COM followed by a posterior directed recovery response (Fig. 2). The RB and NH experimental shoes responded similarly and there were no significant differences between these shoes for any of COP sway variables. Compared to the control shoe, the experimental shoes had significantly larger COP sway amplitudes and sway variances (Table 1, Figs. 2 and 3). Though the mean displacement of COP for the control shoe was greater than the experimental shoes (Fig. 2), the duration of the displacement was longer for the experimental shoes as evident by larger sway amplitudes (Figs. 2 and 3). Compared to the control shoes, the experimental shoes had a significantly longer peak duration times (Table 1 and Fig. 2). In contrast to the control shoe, plots of the COP displacement for the experimental shoes showed a distinct plateau period. Not only was the PDT significantly longer for the experimental shoes but the COP remained relatively stationary at its anterior displacement prior to initiation of a posterior recovery response (Fig. 2). There was, however, no significant difference between control and experimental shoes for sway range (Table 1).
Subjects wore three types of shoes for testing (Fig. 1). Shoes were canvas with rubber soles and of the same style and brand in sizes women’s 8 and men’s 10. All shoes were modified by the addition of a 5/8th-in. thick crepe sole material to the outsole, shaped to conform to the shoes’ perimeter. Shoe soles were modified to represent either a control shoe (CNTR), a mild rocker bottom shoe (RB) or negative heel shoe (NH). In the control shoe the crepe material was full thickness throughout the length of the shoe. In the RB shoe the crepe sole was full thickness from the heel to apex and gently rounded to zero thickness at the toe. The rocker apex was positioned posterior to the ball of the shoe and within 60–65% of shoe length [8]. The heel edge was also slightly rounded. In the NH shoe the location of the rocker apex and the contour of the forefoot sole material were the same as the RB shoe. From the rocker apex to the heel, the sole material remained flat and was reduced to zero thickness at the heel. A certified orthotist designed and fabricated all shoe modifications. A dynamic dual force plate (NeuroCom International, Oregon, USA) was programmed to present a horizontal backward translation perturbation for a distance of 12.5 cm for 0.550 s at a velocity of 0.23 m/s. Platform velocity was determined prior to the study to provoke a visible postural response without stepping. 2.3. Procedures Subjects stood on the force plate with each foot in a predetermined position [22]. Subjects were asked to stand relaxed with arms by side, to look forward at an eye level target and to maintain balance without stepping. Data were collected 5 s before and after the onset of the perturbation that created an anterior body sway. The 5 s prior to the perturbation was to ensure a quiet stance and to make the onset of the perturbation less predictable. There were five perturbations for each shoe type. Shoe selection was random and the perturbations occurred at unexpected intervals. 2.4. Sway variables The NeuroCom software computed the center of mass (COM) and center of pressure (COP) based upon the subject’s height and weight. The body sway variables for anterior–posterior (A–P) COM and COP included: mean sway amplitude (SA); sway variance (SV); time to peak (T2P); anterior and posterior peak velocities; functional stability margin (FSM); and peak duration time (PDT) (Figs. 2 and 4). The sway amplitude is the mean anterior–posterior (AP) displacement of the COP or COM as a function of time. The sway variance is the standard deviation of the sway amplitude. Time to peak is the time period between perturbation onset and the maximum peak displacement. Sway range is the distance between the anterior and posterior peak displacements. Anterior and posterior peak velocities are the maximum rate of movement (meters/second) of the COP and COM toward and away from their respective maximum displacements. The functional stability margin is the numerical difference between the peak COP and peak COM. A comparatively small FSM indicates that the peak COM displacement is closer to the peak COP and may be indicative of an increased potential for postural instability [23,24]. The peak duration time is a measure of the time period involved in the stopping and reversing of the direction of movement. It was calculated as the time period when the anterior displacement of either the COP or COM was within 25% of their respective peak amplitudes. A comparatively longer PDT indicates a longer time involved to reverse the direction of body sway. 2.5. Statistical analysis Data were analyzed for 2.5 s post-perturbation. The independent variable was shoe type. The dependent body sway variables were calculated by MatLab and analyzed by MANOVA with planned post hoc comparisons using Boniferroni corrections (SPSSv 13) with a P < 0.05 level of significance. Omnibus F-test was set at P < 0.05 for all variables. Degrees of freedom (d.f.) were 2, 38 and planned post hoc comparisons were determined a priori.
3.1. Center of pressure
Table 1 Sway variable means and standard deviations for rocker bottom shoe (RB), negative heel shoe (NH) and control shoe (CNTR). RB
NH
CNTR
Sway amplitude COM 1.068 0.041 COP 1.313 0.045
1.098 0.044 1.350 0.066
0.854 0.030* 1.142 0.035*
Sway variance COM COP
1.236 0.038 1.535 0.047
1.264 0.047 1.565 0.067
1.036 0.031* 1.441 0.039*
Sway range COM COP
3.832 0.071 5.217 0.109
3.865 0.112 5.214 0.155
3.497 0.072* 5.213 0.123
Functional stability margin 0.852 0.219
0.848 0.279
1.117 0.267*
Peak duration time COM 0.458 0.103 COP 0.356 0.068
0.474 0.070 0.383 0.060
0.341 0.034* 0.323 0.071*
Time to peak COM COP
0.920 0.026 0.920 0.026
0.939 0.025 0.605 0.025
0.788 0.763* 0.470 0.013*
Ant peak velocity COM 5.892 0.126 COP 26.346 1.139
5.701 0.134 27.350 1.095
7.153 0.129* 30.586 1.028*
Post-peak velocity COM 10.866 0.559 COP 26.124 1.156
10.062 0.755 26.976 1.122
12.334 0.773y 30.564 1.032*
* Denotes significant difference (P < 0.05) between control (CNTR) and experimental shoes (RB and NH). y Denotes significance difference only between CNTR and NH shoes.
B.C. Albright, W.M. Woodhull-Smith / Gait & Posture 30 (2009) 45–49
Fig. 2. Plots of the A–P mean displacement of COM (A) and COP (B) over time. Xaxis = anterior and posterior movement in centimeters. Y-axis = time in seconds from onset of perturbation. In (A) note the similarity in anterior displacements of COM. The COM Time to Peak (T2P) was significantly longer for experimental shoes (P < 0.05). In (B) arrow with (*) indicates common point of slowing of the forward COP displacement for experimental shoes. Note the characteristic plateau region of the COP peak displacements for the experimental shoes.
The anterior and posterior peak velocities of the experimental shoes were significantly slower than the control shoe (Table 1 and Fig. 4). Similarly, the experimental shoes had significantly longer times to peak (Table 1). Figs. 2 and 4 indicate that the initial slowing of the anterior movement of the COP occurred at a relatively similar time and rate.
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Fig. 4. Plots of the A–P velocity of COM (A) and COP (B) over time. X-axis = meters/ second with positive values representing forward (anterior) movements; Yaxis = time in seconds from onset of perturbation. Zero line represents zero velocity. Plots show the significantly slower velocities for the experimental shoes as the COM and COP move toward and away from their respective anterior peak displacements.
experimental shoes also had a significantly greater sway range than the control shoe (Table 1). The anterior peak velocities for the experimental shoes were significantly slower than the control shoe (Table 1 and Fig. 4), but there were no significant differences in posterior peak velocities between the RB and control shoe.
3.2. Center of mass 3.3. Functional stability margin There were no significant differences between the experimental shoes for any of the COM sway variables (Table 1, Fig. 2). There were no difference between the experimental shoes for mean peak displacements, sway amplitudes, sway variances, or time to peak. These sway variables for the experimental shoes however were significantly larger than those of the control shoe (Table 1 and Figs. 2–4). Similarly, the PDTs for the experimental shoes were significantly longer than the control shoe (Table 1). Unlike the COP, the COM displacement plots were similar and did not exhibit any shoe specific distinctions such as the plateau region (Fig. 2). The
The functional stability margin is the numerical difference between the peak COP and COM anterior displacements [23,25]. The smaller the FSM value the closer the peak COM displacement is to the COP peak displacement. A comparatively smaller FSM value is indicative of a relatively increased potential for postural imbalance [25]. There was no significant difference in the FSM between the experimental shoes (Table 1). Compared to the control, the postural response of subjects wearing the experimental shoes
Fig. 3. (A) Bar graphs of sway amplitudes for COM and COP as a function of time. (B) Bar graphs of sway variances for COM and COP. (*) denotes significance (P < 0.05) between control and experimental shoes.
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showed a larger peak displacement of the COM and a corresponding smaller FSM. The FSMs for the experimental shoes were significantly smaller than the control shoe (Table 1). This finding indicates that the experimental shoes created a greater tendency toward imbalance than the control shoe. 4. Discussion The literature strongly supports the effectiveness of custom diabetic shoes with enlarged toe boxes, soft liners, accommodative insoles and outer sole rocker patterns to reduce localized plantar pressures and to accommodate foot deformities and the insensate foot [3,6,9]. The impairment of somatosensation in persons with DPN is known to contribute to a reduction of postural control and an increase in imbalance [11–16,26]. Though the risk of falls is high in the DPN population, there is limited information on the impact of diabetic footwear on balance control [10,21,27]. Several studies have shown that soft and thick soles reduced balance control and induced instability [28,29]. Lavery et al. compared the effects of a standard canvas shoe and four common off-loading footwear devices on standing mean sway amplitude [21]. They showed that sway was greatest for the total contact cast with a heel and was least for the canvas shoe. As expected, sway was greatest in the device that had the smallest base of support. The current study showed that while wearing shoes with rocker bottom soles the postural responses of young healthy adults to a backward slide perturbation were more destabilizing and increased the potential for imbalance. The analyses of multiple sway variables were consistent and showed distinct and significant differences between the control and experimental shoes. There were no significant differences between the sway variables of the RB and NH shoes. This finding is likely due to their similarity in the location of the rocker apex and contour of the forefoot portion of the shoe. Characteristically, the experimental shoes had larger sway ranges, larger mean sway amplitudes and variances and smaller values for functional stability margins. These findings are consistent with reported postural responses that destabilize balance [30]. Peak velocities for the anterior movements of the COP and COM were slower for the experimental shoes compared to the control shoe. The onset of the slowing appeared to correspond to when the COP and COM moved over the apex of the rocker. Postural sway instability is associated with increases in sway range, variance and velocity [12,15,19,20]. The slower COP and COM velocities of the experimental shoes indicate that sway velocity was not a contributing factor to the overall destabilizing effect of the rocker bottom soles. The slower velocities may have been a stabilizing response of the subjects in anticipation of a perceived unstable event. It is also conceivable that the contour of the rocker bottom soles may have contributed to the slower velocities. There is no current information on the effects of rocker bottom soles on the postural control of persons with DPN. Given a somatosensory impairment and typically older age, it is conceivable that the diabetic with peripheral neuropathy while wearing shoes with rocker bottom soles will have postural responses that vary from those reported in this study for young healthy subjects. The experimental shoes were associated with significantly longer peak duration times. The PDT is a calculation for the time frame involved in the process of stopping and reversing the direction of movement for the COP and COM. These findings indicate that subjects wearing the experimental shoes were slower to reverse the direction of the COP and COM anterior displacement. Plots of the COP peak displacements for the experimental shoes were characterized by a distinct plateau region. The plateau indicates a prolonged period when the COP was relatively
stationary at its peak displacement. Consequently, subjects wearing shoes with rocker bottom soles remained in an anteriorly displaced position for a significantly longer period of time, thus delaying the onset of the postural recovery phase. This distinct delay of the onset of a recovery phase has not been previously reported and may be specific to shoes with rocker bottom soles. The functional stability margin is the numerical calculation of the distance between the maximum displacement of COM and the COP [23,24]. The smaller the FSM, the less stable the postural response [23,24]. The smaller functional stability margins of the experimental shoes support the finding that they increased the potential for postural imbalance. The functional stability margin uses the mean of the peak anterior displacements and does not employ a root mean square conversion as does the COP–COM variable used in other reports [19]. Consequently, the comparatively smaller FSM of this study is comparable to a larger COP– COM variable reported for diabetics with peripheral neuropathy [19]. Within a given shoe size, the overall shoe lengths and available surface contact area of the control and experimental shoes were equal. Compared to the control shoe, the rocker bottom shoes had a smaller base of support due to the contour of the soles. This was most evident during quiet standing. For all shoes studied, the perturbation resulted in a postural response that involved A–P displacements of the COM and COP and a dynamic change in the location and potentially the size of the base of support. Compared to the control shoe, the greater sway range and anterior displacement of the COP and COM of the experimental shoes may be indicative of a greater utilization of the available shoe contact surface. The differences in the sway variables between the control and experimental shoes can be attributed to the collective effects of the contour of the rocker bottom soles. 5. Conclusion In young healthy adults, shoes with rocker bottom soles had a destabilizing effect to perturbed stance, thereby increasing the potential for imbalance. Recognizing the postural impairments of persons with peripheral neuropathy, footwear with modifications to accommodate the insensate foot should also be evaluated for their impact on postural control. Analysis of the effect of rocker bottom soles on postural control and fall risk in the DPN population is recommended. Acknowledgements The authors are grateful to Leslie Allison, PT, PhD and Denis Brunt, PT, EdD for their assistance with data analysis; to Justin Curlee, DPT and Amy Piner, DPT for their assistance with data collection; and to Carl Tyndall, CPO, for the rocker bottom sole design and fabrication of the control and experimental shoes. Conflict of interest
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