Technology and Health Care 23 (2015) 93–100 DOI 10.3233/THC-140877 IOS Press
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Biomechanical study of plantar pressures during walking in male soccer players with increased vs. normal hip alpha angles Marco Hagena,∗ , Christoph Abrahama , Andreas Ficklschererb and Matthias Lahnerc a Biomechanics
Laboratory, Department of Sport and Movement Sciences, University of Duisburg-Essen, Essen, Germany b Department of Orthopaedic Surgery, Physical Medicine and Rehabilitation, University Hospital of Munich, Munich, Germany c Department of Orthopaedic Surgery, Ruhr-University Bochum, St. Josef-Hospital, Bochum, Germany Received 15 October 2014 Accepted 27 October 2014 Abstract. BACKGROUND: Femoroacetabular impingement (FAI) is accompanied by increased hip alpha angles, in particular in athletes with high impact sports. OBJECTIVE: The aim of our study was to investigate the dynamic function of the foot during walking in male soccer players with increased versus normal alpha angles. METHODS: Plantar pressures of 20 injury-free male soccer players were recorded during barefoot walking at 1.6 m/s. Ten subjects had bilaterally increased (>55◦ ) (IA) and ten subjects normal (55◦ ); Normal alpha angle ( 55◦ ) Lahner et al. [16] found higher impact forces, reduced pronation excursion and higher peak pressures under the heel and the third metatarsal head which indicate a more rigid lower limb structure. Kennedy et al. [15] and Lahner et al. [16] label their findings on altered biomechanics during locomotion as compensatory strategy induced by restrictions in the hip joint. Since prolonged walking causes hip pain in FAI patients [17], it is important how FAI-related changes in hip and pelvic motion [15] affect foot biomechanics during gait. Previous research has shown the relationship between hip and rearfoot motion [18,19]. To our best knowledge, there is no study that has investigated plantar pressure distribution during walking in a FAI sample. It was hypothesized that the investigation of the foot rollover process during walking may identify altered foot mechanics due to functional restraints in the hip [15,16] which are acting as compensatory mechanisms in these subjects. Furthermore, we compare the plantar pressures of the kicking leg with the standing leg in male soccer players with increased alpha angles and age-matched soccer players with normal alpha angles, as we found differences between legs in a previous study [13]. 2. Material and methods 2.1. Participants In a previous investigation [16] 44 soccer players underwent clinical examinations of the mechanical leg axis and magnetic resonance imaging (MRI). On the basis of the MRI results and clinical examination, 10 soccer players with bilaterally increased hip alpha angles > 55◦ (IA) and 10 soccer players with
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Table 1 Hip alpha angles in kicking and standing leg of the study groups Hip alpha angle (◦ ) Study group (n) Soccer players IA (10) Soccer players NA (10) P Value
Kicking leg 60.6 ± 4.6 47.2 ± 2.8 < 0.001
Standing leg 58.3 ± 5.4 48.7 ± 4.1 < 0.001
Table 2 Anthropometric data of the soccer players with increased (IA) and normal (NA) hip alpha angles Study group (n) Soccer players IA (10) Soccer players NA (10) P value
BMI (kg/cm2 ) 23.4 ± 1.4 23.5 ± 2.8 0.59
Height (m) 1.80 ± 0.06 1.82 ± 0.07 0.79
Mass (kg) 76.2 ± 9.1 79.4 ± 10.8 0.31
Age (years) 22.3 ± 2.4 22.7 ± 2.7 0.46
Navicular drop (mm) 4.4 ± 1.4 4.7 ± 1.6 0.76
normal hip alpha angles < 50◦ (NA) were recruited for a following gait analysis (Table 1). Alpha angles > 50◦ and < 55◦ were considered as intermediate, and subjects with an alpha angle > 50◦ and < 55◦ were excluded from the study. The participants were recruited from teams from the fourth to the eighth German division. All participants reported on playing in competitive teams more than eight years and taking part in at least 2 training sessions (matches included) per week. In all cases, the right leg was the kicking leg and the left leg was the standing leg. The anthropometric data are presented in Table 2. All subjects were free of hip pain. For both groups, exclusion criteria were any kind of previous hip surgery in either hip joint, inflammatory or metabolic rheumatic disease or a history of haemophilia. Participants having greater than 10 mm of navicular drop [20] were excluded from the study, as this criterion has previously been used to classify participants as having excessive pronation [21,22]. Height and weight measurements were performed. Dorsal or knee pain was excluded by the clinical examination. Following the principles of the Declaration of Helsinki, all subjects voluntarily agreed to the study and gave their informed consent. All participants received and signed patient education for MRI. The study protocol was approved by the local ethical committee of the Ruhr-University Bochum (registration number 4370-12). 2.2. Biomechanical measurements A capacitive pressure distribution platform (EMED ST, Novel, Munich, Germany; 4 sensors per cm2 ), embedded in a gangway was used to collect plantar pressure patterns during barefoot walking. Walking speed was prespecified at 1.6 m/s (± 5%) and controlled by two photocells placed at equal distances before and behind the pressure platform. At a sampling frequency of 50 Hz at least five trials were collected for both the left and right foot. Trials outside the speed boundaries, featuring an irritated step rhythm onto the pressure platform or evident targeting by the subject were repeated. Data analysis was conducted with the Novel Database Pro software package (version 11.38, Novel GmbH). In order to achieve a detailed description of foot loading during walking, five parameters were investigated: contact area, peak pressure, pressure-time integral, force-time integral and relative loads, calculated as the percentage of the local force-time integral in relation to the total force-time integral [23]. These parameters were investigated in 10 areas of the foot after using the PRC mask method of subdividing the foot into anatomical areas of interest. This mask involves the regional division of the foot for data analysis purposes into lateral and medial heel, lateral and medial midfoot, lateral, central and medial forefoot, hallux, second toe and third to fifth toes. The use of this automated masking algorithm has been supported by previous research by Cavanagh and Ulbrecht [24].
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M. Hagen et al. / Plantar pressures during walking in soccer players with increased hip alpha angles
Fig. 1. Significant main effects based on alpha angle. The arrows indicate a difference in plantar pressure pattern in soccer players with increased hip alpha angles (> 55◦ ) when compared to soccer players with normal hip angles (< 50◦ ).
2.3. Statistical analysis Statistical data analysis was conducted in the statistical software package SPSS (version 18.0). Individual means of all repeated trials for each foot were calculated. Distribution of data was analyzed by the D’Agostino-Pearson test. Univariate comparisons were performed using the independent samples t test for comparison of continuous variables. A two-factor ANOVA was used to analyze statistical differences between groups (FAI vs. control) and legs (standing leg vs. kicking leg), and to reveal potential group X leg interactions. The level of significance was set at 5%. 3. Results There are no statistically significant differences concerning the mean age and the foot structure, based on the normalized navicular drop, between the groups (Table 2). The plantar pressure data are presented in Table 3. Graphical illustration of altered plantar pressure parameters in the different anatomical regions are shown in Fig. 1. In IA contact area of the hallux is about 13% reduced compared to NA (p < 0.01). In IA lower force-time-integrals of 22% (p < 0.01) and 23% (p < 0.05), and lower pressure-time-integrals of 29% (p < 0.001) and 28% (p < 0.001) are found under both medial and lateral heel, respectively. Furthermore, in IA relative loads are reduced under the medial (p < 0.05) and lateral heel (p < 0.05), and increased under the lateral midfoot (p < 0.05) and the second toe (p < 0.05). Higher loading of the lateral midfoot is also reflected in the increased force-time integral (+33%; p < 0.001). No differences between legs and no interactions, indicating a specificity in kicking or standing leg, are found. 4. Discussion Purpose of the present study was to investigate the dynamic function of the foot during walking in soccer players with increased hip alpha angles compared to soccer players with normal hip alpha angles.
Contact area (cm2 ) NA IA Medial rearfoot 18.3 ± 2.5 17.9 ± 1.6 Lateral rearfoot 18.4 ± 2.3 17.7 ± 1.5 Medial midfoot 2.1 ± 1.9 2.0 ± 1.5 Lateral midfoot 22.5 ± 9.6 20.2 ± 8.9 Medial forefoot 15.0 ± 2.0 14.4 ± 2.6 Central forefoot 12.4 ± 1.9 12.6 ± 1.7 Lateral forefoot 27.0 ± 4.0 27.5 ± 3.7 Hallux 11.8 ± 1.9 10.3 ± 1.1∗∗ 2nd toe 3.9 ± 1.2 4.7 ± 1.4 Toes 3–5 7.2 ± 2.6 8.1 ± 4.6 Peak pressure (kPa) NA IA 284.5 ± 61.9 258.6 ± 58.3 276.6 ± 57.2 254.4 ± 53.9 50.7 ± 42.5 70.8 ± 35.7 83.0 ± 34.0 102.5 ± 50.2 185.5 ± 80.8 187.8 ± 81.7 228.0 ± 60.7 241.1 ± 67.2 218.9 ± 62.4 229.2 ± 81.0 198.6 ± 89.7 224.2 ± 118.1 107.0 ± 53.3 102.8 ± 38.2 69.3 ± 36.9 61.4 ± 29.9
Pressure-time integral (kPa·s) NA IA 118.0 ± 25.3 83.3 ± 27.6∗∗ 113.7 ± 23.4 81.1 ± 26.7∗∗ 24.5 ± 21.0 23.1 ± 11.9 55.2 ± 25.6 51.3 ± 27.8 142.2 ± 57.6 127.0 ± 44.9 176.6 ± 47.1 166.3 ± 38.4 179.3 ± 47.9 170.1 ± 68.7 147.5 ± 64.2 144.8 ± 89.3 69.2 ± 38.2 67.0 ± 29.6 48.7 ± 27.2 43.1 ± 24.4
Force-time integral (N·s) NA IA 98.3 ± 29.1 76.7 ± 24.0∗∗ 83.1 ± 24.5 63.6 ± 20.9∗∗ 2.4 ± 2.9 2.0 ± 1.9 40.0 ± 31 53.1 ± 25.7∗∗ 88.5 ± 37.0 82.3 ± 35.8 101.6 ± 23.0 99.6 ± 22.9 154.9 ± 45.1 160.5 ± 46.9 52.6 ± 27.1 40.9 ± 23.4 8.0 ± 4.6 10.6 ± 6.0 8.7 ± 7.5 11.0 ± 9.2
Relative load (%) NA IA 15.4 ± 3.2 12.8 ± 3.4∗ 13.0 ± 2.5 10.6 ± 3.0∗ 0.4 ± 0.4 0.3 ± 0.3 6.3 ± 4.6 8.9 ± 5.4∗ 13.9 ± 5.9 13.7 ± 5.8 15.9 ± 4.4 16.6 ± 2.6 24.3 ± 5.4 26.7 ± 7.5 8.2 ± 3.8 6.8 ± 3.3 1.2 ± 0.7 1.8 ± 1.0∗ 1.4 ± 1.2 1.8 ± 1.4
Table 3 Means and standard deviations of the plantar pressure patterns in soccer players with increased (IA) and normal (NA) hip alpha angles. Significant differences between IA and NA are indicated by */** (P < 0.05/P < 0.01)
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To our best knowledge, this is the first gait analysis that determines how changes in hip alpha angle affect plantar pressures. Principal finding of this study is that, despite no apparent disparities in foot structure, there are differences in the foot rollover process between IA and NA. These biomechanical manifestations are not dependent on the kicking or standing leg. Lower force-time-integrals, pressure-time-integrals and relative loads under the heel as well as higher relative loads under the lateral midfoot and the central toes indicate a forward shifting of the load in IA compared to NA during walking. These findings partly support our previous study in which we also found higher pressures under the lateral midfoot and the central forefoot in IA during running [15]. In contrast to our prior research, plantar loading under the heel is reduced, while higher peak heel pressures, loading rates and tibial acceleration were previously found in professional soccer players with IA during shod running, which indicated reduced shock absorption capacity [16]. One explanation of this contradictory finding is that the present plantar pressure distribution data were recorded during barefoot walking while subjects ran in running shoes in the previous study. As we know, running and walking barefoot induce remarkably higher impact forces as compared to walking and running in shoes, respectively [25]. Therefore, habitually barefoot runners often fore-foot strike to reduce impact forces compared to habitually shod runners who generally rear-foot strike [26]. As proposed by Nigg [27], impact forces are input signals that produce muscle tuning shortly before the next contact with the ground to minimize soft tissue vibration and/or reduce joint and tendon loading. With respect to Niggs paradigm, one strategy to reduce shock wave transmission to the proximal structures is varying foot position at ground contact. In IA this strategy becomes obvious in forward shifting of the load because, as suggested by Kennedy et al. [15] and Lahner et al. [16], the lower limb is a more rigid structure. In NA there is no need for this strategy because the lower limb is more flexible, indicated by higher range of motion in hip and pelvis [14]. During shod running the foot strike strategies are not that different between IA and NA because the running shoe attenuates the impact [27]. Consequently, the IA subjects may perceive the initial heel contact during running as less uncomfortable as compared to barefoot walking across the pressure plate. Therefore, we conclude that our present results are consistent to previous findings [15,16]. However, a limitation of our study was that we had a small study group but it was not possible to acquire more study participants. Besides, another bias of our study was that we had no further control group, so that the results of the plantar pressures can only applied to soccer players. Our gait analysis is limited to plantar pressure data. Synchronous kinematic, kinetic and electromyographic measurements of locomotion and soccer-specific movements would give more detailed information about the compensatory mechanisms in athletes with increased hip alpha angles. 5. Conclusions Soccer players with increased hip alpha angles show compensatory forward shifting of their center of pressure during walking. By using readjustments of the locomotor system, plantar pressures under the heel at initial contact are reduced and, thus, the potential injury risk due to high impacts is decreased. In consistency with previous findings [9,14,16], our study suggests that functional constraints, for instance changes of the mechanical leg axis [28] and restrictions in range of hip motion [14,16], due to increased hip alpha angle lead to changes in plantar pressure patterns. However, the forces that act on the human body during walking are lower, so that the biomechanical manifestations of increased hip angles are less obvious when compared to running [16]. Future studies are needed to identify the detailed neuromuscular compensatory mechanisms in athletes with increased hip alpha angles. It is a challenge for future
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research to develop physical training programs for the prevention and rehabilitation of FAI in athletes of running-related high-impact sports.
Acknowledgments The authors thank Dr. Andreas Falarzik for his assistance in the study. The authors thank Coach André Pawlak and the soccer players of the Sportgemeinschaft Wattenscheid 09 for contributing to the study.
Competing interests The authors declare that they have no competing interests.
References [1]
Beck M, Kalhor M, Leunig M, Ganz R. Hip morphology influences the pattern of damage to the acetabular cartilage: femoroacetabular impingement as a cause of early osteoarthritits of the hip. J Bone Joint Surg Br. 2005; 87(7):1012-1018. [2] Ganz R, Leunig M, Leunig-Ganz K, Harris WH. The etiology of osteoarthritis of the hip: an integrated mechanical concept. Clin Orthop Relat Res. 2008; 466(2):264-272. [3] Ganz R, Parvizi J, Beck M, Leunig M, Nötzli H, Siebenrock KA. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clin Orthop Relat Res. 2003; 417:112-120. [4] Meyer DC, Beck M, Ellis T, Ganz R, Leunig. Comparison of six radiographic projections to assess femoral head/neck asphericity. Clin Orthop Relat Res. 2006; 455:181-185. [5] Locher S, Werlen S, Leunig M, Ganz R. MR-arthrography with radial sequence for visualization of early hip pathology not visible on plain radiographs. Z Orthop Ihre Grenzgeb. 2002; 140(1):52-57. [6] Pfirrmann CW, Mengiardi B, Dora C, Kalberer F, Zanetti M, Hodler J. Cam and pincer femoroacetabular impingement: characteristic MR arthrographic findings in 50 patients. Radiology. 2006; 240(3):778-785. [7] Nötzli HP, Wyss TF, Stoecklin CH, Schmid MR, Treiber K, Hodler J. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg Br. 2002; 84(4):556-560. [8] Clohisy JC, Nunley RM, Otto RJ, Schoenecker PL: The frog-leg lateral radiographic accurately visualized hip cam impingement abnormalities. Clin Orthop Relat Res. 2007; 462:115-121. [9] Johnson AC, Sharman MA, Ryan TG. Femoroacetabular impingement in former high-level youth soccer players. Am J Sports Med. 2012; 40(6):1342-1346. [10] Naal FD, Miozzari HH, Wyss TF, Nötzli HP. Surgical hip dislocation for the treatment of femoroacetabular impingement in high-level athletes. Am J Sports Med. 2011; 39(3):544-550. [11] Philippon M, Schenker M, Briggs K, Kuppersmith D. Femoroacetabular impingement in 45 professional athletes: associated pathologies and return to sport following arthroscopic decompression. Knee Surg Sports Traumatol Arthrosc. 2007; 15(7):908-914. [12] Philippon MJ, Weiss DR, Kuppersmith DA, Briggs KK, Hay CJ. Arthroscopic labral repair and treatment of femoroacetabular impingement in professional hockey players. Am J Sports Med. 2010; 38(1):99-104. [13] Lahner M, Walter PA, von Schulze Pellengahr C, Hagen M, von Engelhardt LV, Lukas C. Comparative study of femoroacetabular impingement (FAI) prevalence in male semiprofessional and amateur soccer players. Arch Orthop Trauma Surg. 2014; 134(8):1135-41. [14] Lamontagne M, Kennedy MJ, Beaulé PE. The effect of cam FAI on hip and pelvic motion during maximum squat. Clin Orthop Relat Res. 2009; 467:645-650. [15] Kennedy MJ, Lamontagne M, Beaulé PE. Femoracetabular impingement alters hip and pelvic biomechanics during gait walking biomechanics of FAI. Gait Posture. 2009; 30:41-44. [16] Lahner M, von Schulze Pellengahr C, Walter PA, Lukas C, Falarzik A, Daniildis K, von Engelhardt LV, Abraham C, Hennig EM, Hagen M. Biomechanical and functional indicators in male semiprofessional soccer players with increased hip alpha angles vs. amateur soccer players. BMC Musculoskelet Disord. 2014; 15:88. [17] Beaule PE, LeDuff MJ, Zaragoza E. Quality of life following femoral head-neck osteochondroplasty for femoroacetabular impingement. J Bone Joint Surg Am. 2007; 89:773-779.
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M. Hagen et al. / Plantar pressures during walking in soccer players with increased hip alpha angles Knutzen K, Price A. Lower extremity static and dynamic relationships with rearfoot motion in gait. J Am Pod Med Assoc. 1994; 84:171-180. Snyder KR, Earl JE, O’Connor KM, Ebersole KT. Resistance training is accompanied by increases in hip strength and changes in lower extremity biomechanics during running. Clin Biomech. 2009; 24:26-34. Brody D. Techniques in the evaluation and treatment of the injured runner. Orthop Clin North Am. 1982; 13:541-558. Menz H. Alternative techniques for the clinical assessment of foot pronation. J Am Pod Med Assoc. 1998; 88:119-129. Mueller M, Host J, Norton B. Navicular drop as a composite measure of excessive pronation. J Am Pod Med Assoc. 1993; 83:198-202. Hennig EM, Rosenbaum D. Pressure distribution patterns under the feet of children in comparison to adults. Foot Ankle. 1991; 11:306-311. Cavanagh PR, Ulbrecht JS. Clinical plantar pressure measurement in diabetes: rationale and methodology. Foot. 1994; 4:123-35. Lafortune MA, Hennig EM. Cushioning properties of footwear during walking: accelerometer and force platform measurements. Clin Biomech. 1992; 7(3):181-184. Lieberman DE, Venkadesan M, Werbel WA, Daoud AI, D’Andrea S, Davis IS, Mang’eni RO, Pitsiladis Y. Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature. 2010; 463:531-535. Nigg, BM: The role of impact forces and foot pronation: a new paradigm. Clin J Sport Med. 2001; 11:2-9. Lahner M, Jahnke NL, Zirke S, Teske W, Vetter G, von Schulze Pellengahr C, Daniilidis K, Hagen M, von Engelhardt LV. The deviation of the mechanical leg axis correlates with an increased hip alpha angle and could be a predictor of femoroacetabular impingement. Int Orthop. 2014; 38:19-25.
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Biomechanical study of plantar pressures during walking in male soccer players with increased vs. normal hip alpha angles Marco Hagena,∗ , Christoph Abrahama , Andreas Ficklschererb and Matthias Lahnerc a Biomechanics
Laboratory, Department of Sport and Movement Sciences, University of Duisburg-Essen, Essen, Germany b Department of Orthopaedic Surgery, Physical Medicine and Rehabilitation, University Hospital of Munich, Munich, Germany c Department of Orthopaedic Surgery, Ruhr-University Bochum, St. Josef-Hospital, Bochum, Germany Received 15 October 2014 Accepted 27 October 2014 Abstract. BACKGROUND: Femoroacetabular impingement (FAI) is accompanied by increased hip alpha angles, in particular in athletes with high impact sports. OBJECTIVE: The aim of our study was to investigate the dynamic function of the foot during walking in male soccer players with increased versus normal alpha angles. METHODS: Plantar pressures of 20 injury-free male soccer players were recorded during barefoot walking at 1.6 m/s. Ten subjects had bilaterally increased (>55◦ ) (IA) and ten subjects normal (55◦ ); Normal alpha angle ( 55◦ ) Lahner et al. [16] found higher impact forces, reduced pronation excursion and higher peak pressures under the heel and the third metatarsal head which indicate a more rigid lower limb structure. Kennedy et al. [15] and Lahner et al. [16] label their findings on altered biomechanics during locomotion as compensatory strategy induced by restrictions in the hip joint. Since prolonged walking causes hip pain in FAI patients [17], it is important how FAI-related changes in hip and pelvic motion [15] affect foot biomechanics during gait. Previous research has shown the relationship between hip and rearfoot motion [18,19]. To our best knowledge, there is no study that has investigated plantar pressure distribution during walking in a FAI sample. It was hypothesized that the investigation of the foot rollover process during walking may identify altered foot mechanics due to functional restraints in the hip [15,16] which are acting as compensatory mechanisms in these subjects. Furthermore, we compare the plantar pressures of the kicking leg with the standing leg in male soccer players with increased alpha angles and age-matched soccer players with normal alpha angles, as we found differences between legs in a previous study [13]. 2. Material and methods 2.1. Participants In a previous investigation [16] 44 soccer players underwent clinical examinations of the mechanical leg axis and magnetic resonance imaging (MRI). On the basis of the MRI results and clinical examination, 10 soccer players with bilaterally increased hip alpha angles > 55◦ (IA) and 10 soccer players with
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Table 1 Hip alpha angles in kicking and standing leg of the study groups Hip alpha angle (◦ ) Study group (n) Soccer players IA (10) Soccer players NA (10) P Value
Kicking leg 60.6 ± 4.6 47.2 ± 2.8 < 0.001
Standing leg 58.3 ± 5.4 48.7 ± 4.1 < 0.001
Table 2 Anthropometric data of the soccer players with increased (IA) and normal (NA) hip alpha angles Study group (n) Soccer players IA (10) Soccer players NA (10) P value
BMI (kg/cm2 ) 23.4 ± 1.4 23.5 ± 2.8 0.59
Height (m) 1.80 ± 0.06 1.82 ± 0.07 0.79
Mass (kg) 76.2 ± 9.1 79.4 ± 10.8 0.31
Age (years) 22.3 ± 2.4 22.7 ± 2.7 0.46
Navicular drop (mm) 4.4 ± 1.4 4.7 ± 1.6 0.76
normal hip alpha angles < 50◦ (NA) were recruited for a following gait analysis (Table 1). Alpha angles > 50◦ and < 55◦ were considered as intermediate, and subjects with an alpha angle > 50◦ and < 55◦ were excluded from the study. The participants were recruited from teams from the fourth to the eighth German division. All participants reported on playing in competitive teams more than eight years and taking part in at least 2 training sessions (matches included) per week. In all cases, the right leg was the kicking leg and the left leg was the standing leg. The anthropometric data are presented in Table 2. All subjects were free of hip pain. For both groups, exclusion criteria were any kind of previous hip surgery in either hip joint, inflammatory or metabolic rheumatic disease or a history of haemophilia. Participants having greater than 10 mm of navicular drop [20] were excluded from the study, as this criterion has previously been used to classify participants as having excessive pronation [21,22]. Height and weight measurements were performed. Dorsal or knee pain was excluded by the clinical examination. Following the principles of the Declaration of Helsinki, all subjects voluntarily agreed to the study and gave their informed consent. All participants received and signed patient education for MRI. The study protocol was approved by the local ethical committee of the Ruhr-University Bochum (registration number 4370-12). 2.2. Biomechanical measurements A capacitive pressure distribution platform (EMED ST, Novel, Munich, Germany; 4 sensors per cm2 ), embedded in a gangway was used to collect plantar pressure patterns during barefoot walking. Walking speed was prespecified at 1.6 m/s (± 5%) and controlled by two photocells placed at equal distances before and behind the pressure platform. At a sampling frequency of 50 Hz at least five trials were collected for both the left and right foot. Trials outside the speed boundaries, featuring an irritated step rhythm onto the pressure platform or evident targeting by the subject were repeated. Data analysis was conducted with the Novel Database Pro software package (version 11.38, Novel GmbH). In order to achieve a detailed description of foot loading during walking, five parameters were investigated: contact area, peak pressure, pressure-time integral, force-time integral and relative loads, calculated as the percentage of the local force-time integral in relation to the total force-time integral [23]. These parameters were investigated in 10 areas of the foot after using the PRC mask method of subdividing the foot into anatomical areas of interest. This mask involves the regional division of the foot for data analysis purposes into lateral and medial heel, lateral and medial midfoot, lateral, central and medial forefoot, hallux, second toe and third to fifth toes. The use of this automated masking algorithm has been supported by previous research by Cavanagh and Ulbrecht [24].
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M. Hagen et al. / Plantar pressures during walking in soccer players with increased hip alpha angles
Fig. 1. Significant main effects based on alpha angle. The arrows indicate a difference in plantar pressure pattern in soccer players with increased hip alpha angles (> 55◦ ) when compared to soccer players with normal hip angles (< 50◦ ).
2.3. Statistical analysis Statistical data analysis was conducted in the statistical software package SPSS (version 18.0). Individual means of all repeated trials for each foot were calculated. Distribution of data was analyzed by the D’Agostino-Pearson test. Univariate comparisons were performed using the independent samples t test for comparison of continuous variables. A two-factor ANOVA was used to analyze statistical differences between groups (FAI vs. control) and legs (standing leg vs. kicking leg), and to reveal potential group X leg interactions. The level of significance was set at 5%. 3. Results There are no statistically significant differences concerning the mean age and the foot structure, based on the normalized navicular drop, between the groups (Table 2). The plantar pressure data are presented in Table 3. Graphical illustration of altered plantar pressure parameters in the different anatomical regions are shown in Fig. 1. In IA contact area of the hallux is about 13% reduced compared to NA (p < 0.01). In IA lower force-time-integrals of 22% (p < 0.01) and 23% (p < 0.05), and lower pressure-time-integrals of 29% (p < 0.001) and 28% (p < 0.001) are found under both medial and lateral heel, respectively. Furthermore, in IA relative loads are reduced under the medial (p < 0.05) and lateral heel (p < 0.05), and increased under the lateral midfoot (p < 0.05) and the second toe (p < 0.05). Higher loading of the lateral midfoot is also reflected in the increased force-time integral (+33%; p < 0.001). No differences between legs and no interactions, indicating a specificity in kicking or standing leg, are found. 4. Discussion Purpose of the present study was to investigate the dynamic function of the foot during walking in soccer players with increased hip alpha angles compared to soccer players with normal hip alpha angles.
Contact area (cm2 ) NA IA Medial rearfoot 18.3 ± 2.5 17.9 ± 1.6 Lateral rearfoot 18.4 ± 2.3 17.7 ± 1.5 Medial midfoot 2.1 ± 1.9 2.0 ± 1.5 Lateral midfoot 22.5 ± 9.6 20.2 ± 8.9 Medial forefoot 15.0 ± 2.0 14.4 ± 2.6 Central forefoot 12.4 ± 1.9 12.6 ± 1.7 Lateral forefoot 27.0 ± 4.0 27.5 ± 3.7 Hallux 11.8 ± 1.9 10.3 ± 1.1∗∗ 2nd toe 3.9 ± 1.2 4.7 ± 1.4 Toes 3–5 7.2 ± 2.6 8.1 ± 4.6 Peak pressure (kPa) NA IA 284.5 ± 61.9 258.6 ± 58.3 276.6 ± 57.2 254.4 ± 53.9 50.7 ± 42.5 70.8 ± 35.7 83.0 ± 34.0 102.5 ± 50.2 185.5 ± 80.8 187.8 ± 81.7 228.0 ± 60.7 241.1 ± 67.2 218.9 ± 62.4 229.2 ± 81.0 198.6 ± 89.7 224.2 ± 118.1 107.0 ± 53.3 102.8 ± 38.2 69.3 ± 36.9 61.4 ± 29.9
Pressure-time integral (kPa·s) NA IA 118.0 ± 25.3 83.3 ± 27.6∗∗ 113.7 ± 23.4 81.1 ± 26.7∗∗ 24.5 ± 21.0 23.1 ± 11.9 55.2 ± 25.6 51.3 ± 27.8 142.2 ± 57.6 127.0 ± 44.9 176.6 ± 47.1 166.3 ± 38.4 179.3 ± 47.9 170.1 ± 68.7 147.5 ± 64.2 144.8 ± 89.3 69.2 ± 38.2 67.0 ± 29.6 48.7 ± 27.2 43.1 ± 24.4
Force-time integral (N·s) NA IA 98.3 ± 29.1 76.7 ± 24.0∗∗ 83.1 ± 24.5 63.6 ± 20.9∗∗ 2.4 ± 2.9 2.0 ± 1.9 40.0 ± 31 53.1 ± 25.7∗∗ 88.5 ± 37.0 82.3 ± 35.8 101.6 ± 23.0 99.6 ± 22.9 154.9 ± 45.1 160.5 ± 46.9 52.6 ± 27.1 40.9 ± 23.4 8.0 ± 4.6 10.6 ± 6.0 8.7 ± 7.5 11.0 ± 9.2
Relative load (%) NA IA 15.4 ± 3.2 12.8 ± 3.4∗ 13.0 ± 2.5 10.6 ± 3.0∗ 0.4 ± 0.4 0.3 ± 0.3 6.3 ± 4.6 8.9 ± 5.4∗ 13.9 ± 5.9 13.7 ± 5.8 15.9 ± 4.4 16.6 ± 2.6 24.3 ± 5.4 26.7 ± 7.5 8.2 ± 3.8 6.8 ± 3.3 1.2 ± 0.7 1.8 ± 1.0∗ 1.4 ± 1.2 1.8 ± 1.4
Table 3 Means and standard deviations of the plantar pressure patterns in soccer players with increased (IA) and normal (NA) hip alpha angles. Significant differences between IA and NA are indicated by */** (P < 0.05/P < 0.01)
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M. Hagen et al. / Plantar pressures during walking in soccer players with increased hip alpha angles
To our best knowledge, this is the first gait analysis that determines how changes in hip alpha angle affect plantar pressures. Principal finding of this study is that, despite no apparent disparities in foot structure, there are differences in the foot rollover process between IA and NA. These biomechanical manifestations are not dependent on the kicking or standing leg. Lower force-time-integrals, pressure-time-integrals and relative loads under the heel as well as higher relative loads under the lateral midfoot and the central toes indicate a forward shifting of the load in IA compared to NA during walking. These findings partly support our previous study in which we also found higher pressures under the lateral midfoot and the central forefoot in IA during running [15]. In contrast to our prior research, plantar loading under the heel is reduced, while higher peak heel pressures, loading rates and tibial acceleration were previously found in professional soccer players with IA during shod running, which indicated reduced shock absorption capacity [16]. One explanation of this contradictory finding is that the present plantar pressure distribution data were recorded during barefoot walking while subjects ran in running shoes in the previous study. As we know, running and walking barefoot induce remarkably higher impact forces as compared to walking and running in shoes, respectively [25]. Therefore, habitually barefoot runners often fore-foot strike to reduce impact forces compared to habitually shod runners who generally rear-foot strike [26]. As proposed by Nigg [27], impact forces are input signals that produce muscle tuning shortly before the next contact with the ground to minimize soft tissue vibration and/or reduce joint and tendon loading. With respect to Niggs paradigm, one strategy to reduce shock wave transmission to the proximal structures is varying foot position at ground contact. In IA this strategy becomes obvious in forward shifting of the load because, as suggested by Kennedy et al. [15] and Lahner et al. [16], the lower limb is a more rigid structure. In NA there is no need for this strategy because the lower limb is more flexible, indicated by higher range of motion in hip and pelvis [14]. During shod running the foot strike strategies are not that different between IA and NA because the running shoe attenuates the impact [27]. Consequently, the IA subjects may perceive the initial heel contact during running as less uncomfortable as compared to barefoot walking across the pressure plate. Therefore, we conclude that our present results are consistent to previous findings [15,16]. However, a limitation of our study was that we had a small study group but it was not possible to acquire more study participants. Besides, another bias of our study was that we had no further control group, so that the results of the plantar pressures can only applied to soccer players. Our gait analysis is limited to plantar pressure data. Synchronous kinematic, kinetic and electromyographic measurements of locomotion and soccer-specific movements would give more detailed information about the compensatory mechanisms in athletes with increased hip alpha angles. 5. Conclusions Soccer players with increased hip alpha angles show compensatory forward shifting of their center of pressure during walking. By using readjustments of the locomotor system, plantar pressures under the heel at initial contact are reduced and, thus, the potential injury risk due to high impacts is decreased. In consistency with previous findings [9,14,16], our study suggests that functional constraints, for instance changes of the mechanical leg axis [28] and restrictions in range of hip motion [14,16], due to increased hip alpha angle lead to changes in plantar pressure patterns. However, the forces that act on the human body during walking are lower, so that the biomechanical manifestations of increased hip angles are less obvious when compared to running [16]. Future studies are needed to identify the detailed neuromuscular compensatory mechanisms in athletes with increased hip alpha angles. It is a challenge for future
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research to develop physical training programs for the prevention and rehabilitation of FAI in athletes of running-related high-impact sports.
Acknowledgments The authors thank Dr. Andreas Falarzik for his assistance in the study. The authors thank Coach André Pawlak and the soccer players of the Sportgemeinschaft Wattenscheid 09 for contributing to the study.
Competing interests The authors declare that they have no competing interests.
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