Any pathomechanical change in the foot or ankle is expected to cause adverse biomechanical effects on the lumbopelvic region. However, no objective data can be found in the literature regarding the effects of musculus transversus abdominis (mTrA) and musculus lumbar multifidus (mLM), which are effective muscles in lumbopelvic motor control, or regarding the extent of their effects.
Sixty-four healthy young adults were assessed by a physiotherapist (C.K.) experienced in treating feet and a radiologist (Y.D.) specialized in muscular imaging. In the determination of biomechanical properties of the foot, the navicular drop test (NDT), Foot Posture Index (FPI), pedobarographic plantar pressure analysis, and isokinetic strength dynamometer measurements were used in determining the strength of the muscles around the ankle. Ultrasonographic imaging was used to determine mTrA and mLM thicknesses.
Significant correlation was found between NDT results and mTrA and mLM thicknesses (P < .05) and between FPI results and mTrA thicknesses (P < .05). As the peak pressure of the foot medial line increased, mTrA and mLM thicknesses decreased (P < .05). Although dorsiflexion muscle strength was also effective, mTrA and mLM thicknesses were found to increase especially as plantarflexion muscle strength increased (P < .05).
These results show that the biomechanical and musculoskeletal properties of the foot-ankle are associated with lumbopelvic stability.
Low-back pain is a highly prevalent problem worldwide, with the point prevalence estimated to be approximately 18% of the general population.1 Although its etiology is still unknown, perturbed lower-limb dynamics, including joint rigidity, hip muscle stiffness or weakness, and poor postural muscle function leading to asymmetrical or abnormal mechanical loading of the lumbar spine, are considered to be some of the risk factors.2 Of these risk factors, foot and ankle postural disorders play an important role in the predisposition to low-back pain.3-5
The ability of the foot-ankle complex to fulfill stabilization and mobilization tasks depends on the plantar pressure distribution being performed correctly.6 Any incorrect distribution of the peak plantar pressures and various muscle weaknesses can adversely affect lower limbs and pelvic alignment, erector spinae and gluteal muscle activity, and lumbar spine kinematics.7-9 In particular, pronated foot, a foot deformity that alters the plantar pressure and stresses placed on soft-tissue structures around the spine,3-5 is associated with internal rotation of the tibia and pelvic ipsilateral drop in weightbearing during gait.10,11 Hindfoot pronation governs the movement of the cuboid and navicular. Navicular drop has been described as a predictor of abnormal pronation.3-5 If the distance between navicular height in neutral and the full weightbearing position is greater than 10 mm, hindfoot pronation is considered excessive.3-5 Excessive pronation of the foot has been shown to cause medial component injuries in the knee and internal rotation of the hip and thereby has been shown to increase the femoral anteversion angle and lumbar lordosis and to deteriorate lumbopelvic alignment, resulting in low-back pain and lumbopelvic instability.11-17
Three systems are influential in establishing and maintaining lumbopelvic stability: the passive system of osteoligamentous structures, the active system of local and global muscles in emergence of static and dynamic endurance, and the control mechanism of the neural system. In maintaining the multisegmental and intersegmental control of the lumbopelvic region against torsional and compressive stimulation, it is important to preserve and maintain the functions of these systems.18 The structures primarily responsible for the multisegmental and intersegmental control of the lumbopelvic region are the musculus transversus abdominis (mTrA) and the musculus lumbar multifidus (mLM). The mTrA contraction, a horizontal muscle contraction, causes tension in the thoracolumbar fascia and, thus, increases intra-abdominal pressure (Fig. 1).19 The tight interconnections of the mTrA with the thoracolumbar fascia, increased intra-abdominal pressure, and mLM function are directly influential in achieving lumbopelvic control.19,20 The proximity of the mLM deep fibers to the vertebral rotation center is an advantage for providing lumbopelvic control. The mLM has a smaller fiber length and a higher physiologic cross-sectional area than the other lumbar region muscles, which increases its strength. This suggests that the mLM is a specific muscle in providing lumbopelvic control (Fig. 2).20 In determining the strength and function of these muscles, which play an important role in ensuring lumbopelvic control, the cross-sectional thickness under ultrasound imaging was used rather than isolated muscle tests.21-24 Reductions in the cross-sectional thickness of these muscle groups have been reported to delay the prepostural adjustments that are necessary before starting the movement as well as automatic postural adjustments occurring during movement; thereby, these alterations negatively affect the control of the lumbopelvic region.25-30
Although any pathomechanical change in the foot or ankle is expected to cause adverse biomechanical effects in the lumbopelvic region, to our knowledge, there is no objective in the current literature that questions whether the mTrA and mLM, which are effective in lumbopelvic motor control, are affected by these changes. The present study aimed to investigate the interactions of foot-ankle biomechanics and lumbopelvic motor control by determining whether increased hindfoot pronation and ankle weakness are associated with thickness of the mTrA and mLM. It was hypothesized that increased hindfoot pronation and decreased ankle muscle strength are associated with mTrA and mLM thicknesses.
This study was designed as an observational study, and its sample frame consisted of young sedentary adults aged 18 to 25 years. Sixty-four healthy young adults who met the inclusion criteria were assessed by a physiotherapist experienced in examining feet (C.K.) and a radiologist specialized in muscular imaging (Y.D.); the radiologist was blinded to the study protocol.
This double-center study included 64 young adults. The inclusion criteria were as follows: volunteering to participate in the study, being 18 to 25 years old, being right-side dominant, and having a body mass index of 18.5 to 24.9 (calculated as the weight in kilograms divided by the square of the height in meters). The exclusion criteria were as follows: presence of a diagnosed systemic problem such as neurologic, musculoskeletal, endocrinologic, or rheumatologic problems; history of lower-extremity surgery; and presence of a diagnosed pathology involving the lower extremity and vertebrae, such as back pain, scoliosis, past surgery, or sensory loss.31,32
The study protocol was approved by the ethics committee of Kırşehir Ahi Evran University's Faculty of Medicine (Kırşehir, Turkey) and was conducted in accordance with the rules of the Declaration of Helsinki. Written and oral information were given to all of the participants before the evaluations took place, and they signed written consent forms to declare their volunteer participation in the study.
Sociodemographic data and health profiles of the participants were recorded through face-to-face interviews. For biomechanical features of the foot, the navicular drop test (NDT),33-35 the Foot Posture Index (FPI),36-38 and pedobarographic plantar pressure analysis6,39 were used. To assess the strength of the ankle muscles, isokinetic strength dynamometer measurements40-43 were taken.
The NDT was evaluated using a modification of the Brody33 process: With the participant standing barefoot on the floor, the tester marked the navicular tuberosity with a washable marker. The lateral and medial aspect of the talar dome of the foot was palpated with the thumb over the sinus talus and the index finger over the anteromedial portion of the talar dome. The foot was slowly inverted and everted until the talus was in a central position and the depressions felt under both fingers were equal. With the subtalar joint in neutral position, the distance between the navicular tuberosity and the floor was measured, in millimeters, with a ruler. The same process was repeated in nonweightbearing stance, measuring again the height of the navicular tuberosity. The NDT was the difference in the navicular tuberosity height between both measurements. The procedure was repeated three times in each participant.33
The FPI was used to evaluate foot posture. During the assessment, all of the individuals were asked to stand in the position in which they felt the most comfortable. Six different foot posture parameters were evaluated and scored between –2 and +2: the palpation of the talus head in the hindfoot with the thumb and forefinger, the slope above and beneath the lateral malleolus, calcaneal supination and pronation, domination at the talonavicular joint area in the forefoot, the structure of the medial longitudinal arc, and adduction and abduction of the forefoot compared with the hindfoot. Scores of 0 were considered neutral position, positive values represented pronation, and negative values expressed supination.36-38
Plantar pressure analyses were performed on a 3×1-m walking platform with sensors using the Diasu Digital Analysis System and Milletrix software (Diagnostic Support, Diasu Health Technologies, Rome, Italy).44 Participants were required to be barefoot and to wear comfortable clothing during the assessments and to ensure that they fully concentrated on the task. For static measurements, the participants were asked to remain stationary on the platform for 60 sec and then to adjust their feet to a comfortable position (Fig. 3A). For dynamic analysis, the individuals were asked to walk on a 3-m-long force plate–embedded walking platform at normal speed (Fig. 3B). At the end of the static and dynamic analyses, peak pressure values obtained from nine zones of the foot, including medial and lateral of the heel, the five metatarsal bones, the hallux, and the second, third, fourth, and fifth toes, were recorded (Fig. 4).6,39
Ankle plantarflexion and dorsiflexion muscle strength measurements were performed with an isokinetic strength dynamometer (System 4 Pro; Biodex Medical Systems Inc, Shirley, New York). During the test, the individuals were asked not to hold their breath or to distort their body posture. To prevent excessive movement, a thigh band tightly covered the thigh without compressing the popliteal region. The individual's body was prevented from bending forward by a trunk band. In the dynamometer corresponding to the lateral malleolus, the foot was stabilized with dorsal bands. Peak isokinetic concentric ankle plantarflexion and dorsiflexion muscle forces were tested at two different angular velocities (60°/sec and 180°/sec). The individuals performed 15 repetitions at both 60°/sec and 180°/sec.40 Isokinetic muscle strengths were calculated as peak torque to body weight ratios for inhibiting individual body weight changes (Fig. 5).40-43
Ultrasonographic imaging was used to determine the thickness of the mTrA and mLM, which are the primary muscles responsible for lumbopelvic motor control. A linear probe (Aplio 500; Toshiba Inc, Nasu, Japan) set at 4 to 11 MHz was used for ultrasonographic imaging of muscle thickness. Before the test, all of the individuals were taught the abdominal hallowing maneuver involving pulling the abdomen upward and inward without any excessive movement in the superficial abdominal muscles. This maneuver activates the mTrA and enables co-contraction of the muscles responsible for stabilization. For the movement to be successful, the participants needed to develop the perception of achievement. For this purpose, the basic anatomy of the muscle was explained to the participants using an image. The difference between trunk movement and abdominal hallowing was explained. For full performance during contraction, the participants were requested to concentrate on the lower abdominal muscles.23,24,45,46
The mTrA muscle thickness was assessed by placing the ultrasonic probe at the imaginary line through the lower angle of the thoracic cage at the anterolateral part of the abdominal wall and the medial half of the iliac crest. Muscle thickness was measured while the individual was laying on his or her back, knees at 45° flexion, and performing the abdominal hallowing movement enabling mTrA activation (Fig. 6A). The ultrasonic probe was placed on lumbar vertebrae levels 3 and 4 to determine mLM thickness. The thickness of the mLM was obtained by measuring the anteroposterior diameter of the muscle in the supine position and during the abdominal hallowing movement (Fig. 6B).21-24
To our knowledge, the relationship between the biomechanical properties of the foot and ankle and mTrA and mLM thicknesses has not previously been investigated. Therefore, we found no studies in the current literature associated with correlation analysis to calculate sample size. Duval et al10 examined the association of mechanical relationships among the rearfoot, pelvis, and lower back. Based on the findings of their study, the effect size was determined. The minimum required sample size for a correlation analysis was calculated to be 64 participants for a significance of α = .05, the anticipated effect size to be 0.30, and the statistical power level to be 80% (β = 0.20) using G*Power software (version 184.108.40.206).
IBM SPSS Statistics for Windows, Version 20.0 (IBM Corp, Armonk, New York) was used to analyze the data. The variables were investigated using visual (histograms, probability plots) and analytical (Kolmogorov-Smirnov/Shapiro-Wilk test) methods to determine whether they were normally distributed.47 Values are expressed as mean ± SD for continuous variables and as ratio (%) for categorical variables. Spearman correlation coefficients were used to examine the correlations between biomechanical features of the foot and ankle and lumbopelvic motor control. The correlation coefficients were set as less than 0.5 for a strong correlation, 0.3 to 0.5 for a moderate correlation, and 0.2 to 0.3 for a weak correlation.48 The level of significance was set at P < .05.
The present study included 64 healthy young adults (26 women and 38 men) with a mean ± SD age of 21.89 ± 1.21 years. The demographic characteristics of the participants are presented in Table 1.
A negative correlation was found between the NDT results and the mTrA and mLM thicknesses (P < .05). According to the results of the FPI, which investigates the increased pronation in hindfoot, mTrA and mLM thicknesses decreased as pronation increased (P < .05). The relationships between NDT and FPI results and lumbopelvic muscle thicknesses are shown in Table 2 and Fig. 7.
For the right foot, in the static plantar pressure analysis, a significant correlation was found between the peak plantar pressure of the medial of the heel and first metatarsal bone and the mLM thickness and between the peak plantar pressure of the fifth metatarsal bone and the mTrA thickness (P < .05).
For the left foot, in the static plantar pressure analysis, a significant correlation was found between the peak plantar pressure of the medial of the heel and the mLM thickness and between the peak plantar pressure of the fifth metatarsal bone and the mTrA thickness (P < .05).
For the right foot, in the dynamic plantar pressure analysis, a statistically significant relationship was found between the peak pressure of the fifth metatarsal bone and the mTrA and mLM thicknesses and between the medial of the heel and lesser toes and the mLM thickness (P < .05).
For the left foot, in the dynamic plantar pressure analysis, a significant relationship was found between the peak pressure of the fifth metatarsal bone and the mTrA thickness (P < .05).
According to static and dynamic pedobarographic analyses, as the peak plantar pressure of the medial line of the foot increased, mTrA and mLM thicknesses were found to decrease. The relationship between the results of static plantar pressure analysis and mTrA and mLM thicknesses and the relationship between the results of dynamic plantar pressure analysis and mTrA and mLM thicknesses are presented in Tables 3 and 4, respectively.
A significant correlation was found between right- and left-side ankle plantar flexor muscle strength and mTrA and mLM thicknesses at 60°/sec and 180°/sec (according to ankle muscle strength measurement performed with a System 4 Pro isokinetic strength dynamometer [P < .05]), and a significant correlation was found between left-side mLM thickness and dorsiflexion muscle strength at 60°/sec.
Thicknesses of the mTrA and mLM were found to increase, especially as the plantarflexion muscle strength increased, at 60°/sec and 180°/sec. The relationships between plantarflexion and dorsiflexion isokinetic muscle strengths and mTrA and mLM thicknesses are presented in Table 5 and Fig. 8.
In this study, the relationship between foot-ankle characteristics and lumbopelvic motor control was examined in young healthy adults. The results of the study showed that the biomechanical and musculoskeletal properties of the foot-ankle complex, which are distal structures, were associated with the thickness of the muscles primarily responsible for lumbopelvic motor control.
The efficacy of mTrA and mLM has been demonstrated in stabilizing the lumbopelvic region by counteracting gravity and compensating loads during extremity movements.49,50 Along with the function of the diaphragm and pelvic floor muscles, these two muscles' contractions contribute to stabilization by increasing the intra-abdominal pressure.43,44 In addition, the contraction of the mTrA increases tension of the thoracolumbar fascia, creating an extensor momentum.49,50 This passive momentum results in activation of the mLM, and, thereby, lumbopelvic stability is achieved.49,50
Various pathomechanical changes related to the lower extremity affecting lumbopelvic stability have been reported.2-5 There are studies reporting that lumbopelvic stability is adversely affected by increased hindfoot pronation.11-17 In particular, increased pronation causes internal rotation of the hips, and lumbar lordosis increases due to increased femoral anteversion angle. This condition can reduce lumbopelvic stability and lead to mechanical back pain.11-17 To our knowledge, although the biomechanical interaction between lumbopelvic stability and the lower extremities and foot-ankle biomechanics has been emphasized, no evidence involving mTrA and mLM thickness has been reported to date. The present study is the first to demonstrate a relationship between foot-ankle characteristics and lumbopelvic motor control with ultrasonographic imaging. In terms of the biomechanical properties of the foot and ankle, the present study is compatible with studies demonstrating that foot-ankle biomechanical disorders adversely affect lumbopelvic stability.7-9 We believe that the study's results are important in terms of demonstrating a relationship between these parameters and ultrasonographic imaging.
The physiologic function of the subtalar joint plays an effective role in the correct distribution of the plantar pressures. It is important for a healthy gait that the pronation and supination movements of the joint occur within normal limits and in time. Hence, increased and prolonged hindfoot pronation causes increased contact and pressure in the medial line of the foot. This may adversely affect proximal segments due to biomechanical alignment occurring during static postures such as standing and dynamic activities such as walking.12,15 In the present study, based on the NDT results, a decrease in mTrA and mLM thicknesses was found as hindfoot pronation increased. The FPI results showed that increased pronation was associated with decreased mTrA and mLM thicknesses but that mLM was the most affected. Plantar pressure analysis results supported the NDT and FPI results. According to the static and dynamic pedobarographic analyses, as the peak plantar pressure of the medial line of the foot (especially the medial of the heel and the first metatarsal bone) increased, mTrA and mLM thickness decreased. This biomechanical relationship between the proximal and distal segments becomes increasingly more effective due to the increased dynamic moments that are activated during daily activities such as walking and running.17 Dananberg and Guiliano51 emphasized that a significant portion of low-back pain symptoms are related more to the abnormal stresses applied during the gait cycle than to specific anatomical abnormality of the spine itself. This study outcome shows the importance of examining foot posture as a possible cause of chronic or acute recurrent low-back pain. Proper treatment with custom-made foot orthoses can be more effective for improving symptoms of low-back pain than is treatment with standard care methods, and the symptoms remain improved for longer periods. Participants in this prospective study experienced more than twice the improvement in alleviation of pain, and for twice as long, compared with individuals in a study using traditional back pain treatment.51
Walking requires tonic activation of local muscles, such as mTrA and mLM, and phasic activation of superficial abdominal and paraspinal muscles.52,53 Maximum contractions of local and global muscles occur mostly during heel strike. The ground reaction forces met by a heel strike are transmitted to the lumbopelvic region through the lower extremity.51-53 Although the pronation movement is important for normal transmission of these forces, increased and prolonged hindfoot pronation causes increased contact and pressure in the medial line of the foot. This adversely affects proximal segments based on kinetic chain principles.12,15 Increased adduction and internal rotation of the femur increases the valgus angle and reduces lumbopelvic stability.54 Previous studies have shown that increased foot pronation causes an increase in valgus with loading on the medial compartment of the knee, and also causes femoral adduction and internal rotation.7,16,17 The present results on the biomechanical properties of the foot were found to be parallel with the results of these previous research studies.
Plantar flexor and dorsiflexor muscle strength, as well as vertical and lateral oscillations of the pelvis, are important for absorption of ground reaction forces. Loss of strength in these muscles and biomechanical disturbances seen in the pelvis may cause mechanical back pain.54-56 Owing to the lack of studies investigating the association between ankle muscle strength and the lumbopelvic region, to compare our results, we analyzed a study examining the relationship between range of motion and lumbopelvic complex. In 204 participants, Brantingham et al57 examined the association between navicular drop, calcaneal eversion, and low-back pain. They reported that individuals with mechanical back pain had 2.2° less dorsiflexion on the right ankle and 1.7° on the left ankle.57 According to the results of the present study, mTrA and mLM thicknesses were correlated with plantarflexion and dorsiflexion muscle strengths. Because foot pronation is often associated with loss of ankle joint dorsiflexion, the findings of Brantingham et al57 would be very consistent with the findings of the present study; increases in foot pronation correlated with chronic postural complaints.
Certain limitations should be mentioned in connection with the present study. First, the study included only healthy young adults. Therefore, the results are not generalizable to all age groups and some symptomatic patients. Second, because it was not a longitudinal study, there was no investigation into how many of the currently asymptomatic participants with excess pronation would develop back-related problems in the future. Third, the relationship between ankle muscle strength and lumbopelvic motor control was investigated, but individuals could be weaker in the lower limb overall due to lower muscle tone. Fourth, participants diagnosed as having lumbopelvic stability disorder have not been compared with healthy individuals in terms of the predetermined foot and ankle parameters.
The results of the present study show that increased contact and peak plantar pressure in the medial region of the foot in parallel with increased pronation, and especially the decrease in ankle plantarflexion muscle strength, are associated with a decrease in mTrA and mLM thicknesses. Although joint pronation movement is important for normal transmission of forces, increased and prolonged hindfoot pronation causes increased contact and pressure in the medial line of the foot. This adversely affects proximal segments based on kinetic chain principles. In particular, increased pronation causes internal rotation of the hips, and lumbar lordosis increases due to increased femoral anteversion angle. This condition can reduce lumbopelvic stability and lead to mechanical back pain. Furthermore, plantar flexor and dorsiflexor muscle strength and vertical and lateral oscillations of the pelvis are important for absorption of ground reaction forces. Loss of strength in these muscles and biomechanical disturbances seen in the pelvis may contribute to mechanical back pain. It is known that the mTrA and mLM rapidly undergo atrophy due to reflex inhibition based on mechanical problems in cases such as long-term bed rest, sway-back posture, lumbar surgeries, lumbar disc herniations, facet joint problems, and lower-extremity problems. Although the effects of pathophysiologic changes of the foot and ankle on the biomechanics of the lumbopelvic region have been previously reported, the characteristics of the muscular state, especially that of mTrA and mLM thickness, were not examined before the present study. The structure of these muscles was demonstrated by means of ultrasonographic imaging, one of the methods associated with the highest validity and reliability, and it was shown that both the mTrA and mLM thicknesses were associated with increased pronation and ankle muscle strength. The present results indicate that further studies on lumbopelvic region pathologic disorders should also focus on the foot-ankle complex.
In conclusion, it should be kept in mind that the examination of foot-ankle biomechanics is important in cases in which lumbopelvic stability is adversely affected, and it is important that treatment applications related to foot and ankle training can be added for the treatment of symptoms related to lumbopelvic motor control.
Financial Disclosure: None reported.
Conflict of Interest: None reported.
Department of Physiotherapy and Rehabilitation, Kırşehir Ahi Evran University, Kırşehir, Turkey.
Department of Physiotherapy and Rehabilitation, Hacettepe University, Ankara, Turkey.
Department of Radiology, Kırşehir Ahi Evran University, Education and Research Hospital, Kırşehir, Turkey.