Relationship between the thickness and cross-sectional area of intrinsic and extrinsic foot muscles and arch stiffness : A cluster-based analysis and intervention study

국문 초록 (Abstract)

안쪽세로아치는 체중 부하, 충격 흡수, 균형 유지에 기여하며, 아치 강성 지수로 정량화되는 강성은 발과 발목의 구조적·기능적 특성을 모두 반영하는 지표일 수 있으며, 하지 과사용 손상과 관련된 기계적 특성을 반영하는 지표로 제시되어 왔다. 그러나 아치 강성이 낮은 개인에서, 아치 강성과 발 근육 두께 및 단면적, 발목관절 근력, 그리고 발 운동학 간의 관계는 아직 명확하지 않다. 따라서 본 박사학위논문은 (1) 정상 발 구조를 가진 성인을 대상으로 아치 강성 지수와 발의 내재근 및 외재근 두께와 단면적, 발목관절 등척성 근력 간의 관련성을 규명하고, (2) 아치 강성이 낮은 성인에서 4주간의 발 코어 운동 프로그램이 아치 강성, 발 근육 두께 및 단면적, 발목관절 근력, 발 운동학에 미치는 영향을 규명하는 것을 목적으로 하였다.
Study 1에서는 정상 발을 가진 성인 71명을 대상으로 아치 강성 지수와 무지외전근, 뒤정강근의 두께 및 단면적, 족저근막 두께, 그리고 발목관절 등척성 근력 간의 관련성을 조사하였다. 아치 구조는 아치 높이 지수와 아치 강성 지수로 평가하였고, 초음파 영상으로 근육 및 근막 형태를 측정하였으며, 핸드헬드 다이너모미터를 사용하여 발등굽힘, 발바닥굽힘, 안쪽번짐, 가쪽번짐 등척성 근력을 측정한 후, 발등굽힘/발바닥굽힘 및 가쪽번짐/안쪽번짐 근력 비율을 산출하였다. Spearman‘s 상관분석 결과, 아치 강성 지수는 뒤정강근 단면적 및 발목 안쪽번짐, 가쪽번짐 근력과 유의한 양의 상관관계를 보였으나, 무지외전근 두께 및 단면적, 뒤정강근 두께, 족저근막 두께, 발등굽힘 및 발바닥굽힘 근력과는 유의한 관련성이 없었다. 다중회귀분석에서는 발 근육 형태와 관련된 어떤 변수도 아치 강성 지수의 독립적인 예측요인으로 나타나지 않았으며, 성별과 BMI를 보정한 이후 발목관절 안쪽번짐 근력만이 아치 강성 지수의 유의한 독립 예측요인으로 확인되었고, 발등굽힘/발바닥굽힘 및 가쪽번짐/안쪽번짐 근력 비율은 유의한 예측변수로 나타나지 않았다. 이러한 결과는 아치 강성이 발과 발목의 구조적·기능적 특성 모두에 의해 영향을 받는다는 점을 보여주며, 특히 뒤정강근 단면적과 발목관절 안쪽번짐 근력의 중요성이 안쪽세로아치 안정성 이해에 핵심적임을 시사한다.
Study 2에서는 아치 강성이 낮은 성인을 대상으로 4주간의 감독하 foot-core 운동 프로그램의 효과를 검증하였다. Study 1의 참여자 중에서 아치 강성 지수와 무지외전근, 뒤정강근, 족저근막 두께를 이용한 군집분석을 통해 아치 강성이 가장 낮은 군에 해당하는 20명을 선정하여 사전–사후 중재 연구를 수행하였다. 운동 프로그램은 4주간 주 3회 실시되었으며, 발바닥 마사지, 아킬레스건 스트레칭, 블랙보드 트레이닝, 숏풋 운동, 타월 컬로 구성되었다. 비체중지지와 체중지지조건에서의 아치 높이 지수와 아치 강성 지수, 무지외전근 및 뒤정강근의 두께와 단면적, 족저근막 두께, 발목관절 등척성 근력 및 발등굽힘/발바닥굽힘, 가쪽번짐/안쪽번짐 근력 비율을 측정하였으며, 달리기 시 발 운동학은 Rizzoli foot model을 적용한 Vicon 시스템으로 기록하였다. 중재 후 체중지지조건의 아치 높이 지수와 아치 강성 지수는 유의하게 증가한 반면 비체중지지조건의 아치 높이 지수는 변화가 없어, 본 프로그램이 비체중부하 상태에서의 아치 높이 변화보다는 체중부하 시 정적 아치 강성을 주로 증가시켰음을 시사한다. 발목관절 근력은 모든 방향에서 유의하게 증가하였고, 발등굽힘/발바닥굽힘 비율은 감소하며 가쪽번짐/안쪽번짐 비율은 변화하지 않아, 발바닥굽힘근의 상대적 발달과 더불어 안쪽번짐근과 가쪽번짐근 간의 균형이 유지되었음을 보여주었다. 무지외전근과 뒤정강근의 단면적 및 족저근막 두께는 유의하게 증가하여, 발아치를 지지하는 내재근 및 외재근과 발바닥 연부조직에서 비대 적응이 일어났음을 나타낸다. 달리기 동안 안쪽세로아치에는 유의한 변화가 관찰되지 않았다. 그러나 후족에서는 가쪽번짐 및 발바닥굽힘과 관련된 움직임이 전반적으로 감소하고 그 최대 시점이 지연되었으며, 중족의 시상면상 운동 범위는 감소하였다. 전족은 비교적 제한된 범위 내에서 회전하면서 더 외측 위치를 유지하려는 경향을 보였다. 이러한 운동학적 적응은 4주간의 foot-core 프로그램이 하중에서 안쪽세로아치를 지지하는 분절 간 협응 전략을 변화시켰으나, 달리기 시 안쪽세로아치의 변화를 유도하기에는 중재 기간이 충분하지 않았음을 시사한다.
종합적으로, 본 연구는 아치 강성이 외재근 형태와 발목관절 근력 특성 모두에 의해 영향을 받으며, 낮은 아치 강성을 가진 성인에서 4주간의 foot-core 운동 프로그램이 정적 아치 강성을 증가시키고, 발목관절 근력을 향상시키며, 발 내재근 및 외재근, 그리고 족저근막에 비대성 변화를 유발할 수 있음을 보여준다. 또한, 본 프로그램은 달리기 시 후족의 외측 및 발바닥굽힘 감소, 중족 시상면 운동 감소, 전족의 상대적 외측 정렬 유지 등으로 특징지어지는 후족·중족·전족 운동 패턴의 부분적인 재조직을 유도하였지만, 안쪽세로아치의 변화까지는 이어지지 않았다. 이러한 결과는 foot-core 운동이 낮은 아치 강성을 가진 성인에서 안쪽세로아치의 구조적 지지 능력을 향상시키고, 동적 안정성에 기여하는 분절 간 협응 전략을 수정하는 표적 중재로 활용될 수 있음을 시사한다.
주제어: 안쪽세로아치, 아치 강성 지수, foot-core 운동, 근두께 및 단면적, 발목관절 근력, 발 운동학

다국어 초록 (Multilingual Abstract)

Aim: The medial longitudinal arch (MLA) contributes to load support, shock absorption, and balance, and its mechanical stiffness, quantified by the arch stiffness index (ASI), may reflect both structural and functional characteristics of the foot and ankle. However, the relationships among ASI, foot muscle morphology, and ankle joint strength, including strength ratios, remain unclear. Therefore, this study aimed to investigate the associations between ASI, the thickness and cross-sectional area (CSA) of intrinsic and extrinsic foot muscles, and isometric ankle joint strength and strength ratios in adults with normal feet.
Materials and Methods: This cross-sectional study included 71 adults who met predefined criteria for normal feet. Arch structure was evaluated using the arch height index (AHI) and ASI. Ultrasound imaging was used to measure the thickness and CSA of the abductor hallucis (AbH) and tibialis posterior (TP), as well as the thickness of the plantar fascia (PF). Isometric dorsiflexion, plantarflexion, inversion, and eversion strengths were measured using a handheld dynamometer and normalized to body mass; DFlex/PFlex and EV/INV strength ratios were then calculated. Spearman’s correlation analysis examined relationships between ASI and morphological or functional variables, and multiple regression analyses were conducted with sex and BMI controlled.
Results: Spearman’s correlation analysis showed that ASI was significantly and positively correlated with TP CSA (ρ = 0.284, p = 0.016) and ankle eversion (ρ = 0.291, p = 0.014) and inversion strength (ρ = 0.440, p < 0.001). No significant correlations were found between ASI and AbH thickness and CSA, TP thickness, PF thickness, and dorsiflexion and plantarflexion strength. Multiple regression analysis revealed that none of the foot muscle morphology variables and none of the strength ratios were independent predictors of ASI, whereas ankle inversion strength remained a significant independent predictor after adjusting for sex and BMI.
Conclusions: This study demonstrated that both structural and functional characteristics of the foot and ankle are associated with the mechanical stiffness of the MLA. TP CSA showed a modest association with ASI, whereas ankle inversion strength emerged as the only independent predictor, highlighting the particular relevance of extrinsic muscle morphology and invertor strength in understanding arch stability. These findings underscore the importance of considering both morphological and functional indicators especially the capacity of the ankle invertor musculature when evaluating the mechanical behavior of the MLA.
Keywords: Medial longitudinal arch, Arch stiffness index, Foot muscle morphology, Ankle joint strength

다국어 초록 (Multilingual Abstract)

Aim: This study aimed to investigate the effects of a 4-week foot-core exercise program on arch stiffness, foot muscle thickness and cross-sectional area (CSA), ankle joint strength, and multi-segment foot kinematics during running in adults with low arch stiffness.
Materials and Methods: From the participants in Study 1, 20 adults who belonged to the lowest arch stiffness cluster, classified using the arch stiffness index (ASI) and the thickness of the abductor hallucis (AbH), tibialis posterior (TP), and plantar fascia (PF), were included in this pre-post intervention study. The supervised 4-week foot-core exercise program, performed three times per week, consisted of plantar massage, Achilles tendon stretching, blackboard training, short foot exercise, and towel curl. Standing and sitting arch height index (AHI) and ASI were measured, and the thickness and CSA of the AbH, TP, and PF were assessed using ultrasound imaging. Isometric ankle joint strength in dorsiflexion, plantarflexion, inversion, and eversion was measured with a handheld dynamometer, normalized to body mass, and used to calculate DFlex/PFlex and EV/INV strength ratios. Three dimensional kinematics of the rearfoot, midfoot, forefoot, and MLA angle during running were recorded using a Vicon motion capture system with a modified Rizzoli multi-segment foot model. Paired t-tests and Wilcoxon signed-rank tests with an alpha level of 0.05 were performed using SPSS and Matlab to compare pre and post intervention measurements.
Results: After the intervention, standing AHI (p = 0.001) and ASI (p < 0.001) increased significantly, whereas sitting AHI did not change. Ankle joint strength increased significantly in all directions, accompanied by a decrease in the DFlex/PFlex ratio, while the EV/INV ratio remained unchanged. The CSA of the AbH and TP and PF thickness increased significantly, whereas AbH and TP thickness showed no significant changes. During running, no significant changes were observed in MLA angle. However, rearfoot motions associated with eversion and plantarflexion were generally reduced and their maximum timing was delayed, the sagittal plane excursion of the midfoot decreased, and the forefoot tended to remain in a more lateral position relative to the midfoot and rearfoot while rotating within a relatively constrained range.
Conclusions: A 4-week foot-core exercise program in individuals with low arch stiffness increased static arch stiffness, enhanced ankle joint strength, and induced changes in the CSA of intrinsic and extrinsic foot muscles and the thickness of the plantar fascia, but did not produce significant alterations in MLA angle patterns during running. These structural and functional changes were partially reflected as a reorganization of intersegmental foot kinematics during running, characterized by reduced lateral and plantarflexion components of the rearfoot, decreased sagittal plane motion of the midfoot, and a forefoot strategy that favored a relatively lateral position. These findings suggest that foot-core exercise may serve as a targeted intervention in individuals with low arch stiffness to enhance the structural support capacity of the MLA and to modify foot segment coordination strategies that contribute to dynamic stability.
Keywords: Foot-core exercise, Medial longitudinal arch, Arch stiffness, Foot muscle thickness and CSA, Ankle joint strength, Foot kinematics

다국어 초록 (Multilingual Abstract)

The medial longitudinal arch (MLA) contributes to load support, shock absorption, and balance, and its mechanical stiffness, quantified by the arch stiffness index (ASI), may reflect both structural and functional characteristics of the foot and ankle that are related to lower extremity overuse injuries. However, the relationships among arch stiffness, foot muscle thickness and cross-sectional area (CSA), ankle joint strength, and dynamic multi-segment foot function remain unclear, particularly in individuals with low arch stiffness. This dissertation therefore aimed to (1) examine the associations between ASI, the thickness and CSA of the intrinsic and extrinsic foot muscles, and isometric ankle joint strength in adults with normal feet, and (2) determine how a 4-week foot-core exercise program affects arch stiffness, foot muscle thickness and CSA, ankle joint strength, and foot kinematics during running in adults with low arch stiffness.
Study I investigated the associations among ASI, the thickness and CSA of the abductor hallucis (AbH) and tibialis posterior (TP), plantar fascia (PF) thickness, and isometric ankle joint strength in 71 adults with normal feet. Arch structure was assessed using the arch height index (AHI) and ASI, ultrasound imaging was used to measure muscle and fascia morphology, and a hand-held dynamometer was used to quantify dorsiflexion, plantarflexion, inversion, and eversion strength, from which DFlex/PFlex and EV/INV strength ratios were calculated. Spearman’s correlation analysis showed that ASI was significantly and positively correlated with TP CSA and ankle eversion and inversion strength, whereas AbH thickness and CSA, TP thickness, PF thickness, and dorsiflexion and plantarflexion strength were not associated with ASI. Multiple regression analysis revealed that none of the foot muscle morphology variables were independent predictors of ASI, and that ankle inversion strength was the only significant independent predictor after adjusting for sex and BMI, whereas the DFlex/PFlex and EV/INV strength ratios were not significant predictors. These findings indicate that both structural and functional characteristics of the foot and ankle contribute to arch stiffness, and highlight the particular relevance of extrinsic muscle morphology especially TP CSA, and ankle inversion strength in understanding MLA stability.
Study II examined the effects of a 4-week supervised foot-core exercise program on adults with low arch stiffness. From the participants in Study I, 20 adults belonging to the lowest arch stiffness cluster, classified using ASI and AbH, TP, and PF thickness, were enrolled in a pre–post intervention study. The program, performed three times per week for 4 weeks, consisted of plantar massage, Achilles tendon stretching, blackboard training, short foot exercise, and towel curl. Standing and sitting AHI and ASI, AbH and TP thickness and CSA, PF thickness, and isometric ankle joint strengths and DFlex/PFlex and EV/INV ratios were measured. Foot kinematics of the rearfoot, midfoot, forefoot, and MLA angle during running were recorded using a Vicon system with a modified Rizzoli multi-segment foot model. After the intervention, standing AHI and ASI increased significantly, whereas sitting AHI remained unchanged, indicating that the program primarily increased static arch stiffness under load rather than altering arch height in the non–weight-bearing condition. Ankle joint strength increased significantly in all directions, accompanied by a decrease in the DFlex/PFlex ratio and no change in the EV/INV ratio, suggesting relatively greater development of the plantarflexors while maintaining the balance between evertor and invertor strength. The CSA of the AbH and TP and PF thickness increased significantly, indicating hypertrophic adaptations in both intrinsic and extrinsic muscles and plantar soft tissues that support the arch. During running, no significant changes were observed in MLA angle. However, rearfoot motions associated with eversion and plantarflexion were generally reduced and their maximum timing was delayed, the sagittal plane range of motion of the midfoot decreased, and the forefoot tended to remain in a more lateral position while rotating within a relatively constrained range. These kinematic adaptations suggest that the 4-week foot-core exercise program modified intersegmental coordination strategies that support the MLA under load, even though the intervention period was not sufficient to induce clear changes in MLA angle patterns during running.
Collectively, these studies demonstrate that the stiffness of the arch is influenced by both extrinsic muscle morphology and ankle strength characteristics, and that a 4-week foot-core exercise program in individuals with low arch stiffness can increase static arch stiffness, enhance ankle joint strength, and induce hypertrophic changes in intrinsic and extrinsic foot muscles and the plantar fascia. In addition, the program partially reorganized rearfoot, midfoot, and forefoot kinematics during running toward a pattern characterized by smaller rearfoot lateral and plantarflexion components, reduced midfoot sagittal plane motion, and a forefoot that remained relatively lateral, without measurable changes in MLA angle. These findings suggest that foot-core exercise may serve as a useful intervention to enhance the structural support of the MLA and to modify segmental coordination strategies that contribute to dynamic stability, and that they provide foundational data for understanding and evaluating mechanical factors associated with overuse injury risk in adults with low arch stiffness.
Keywords: Medial longitudinal arch, Arch stiffness index, Foot-core exercise, Muscle thickness and cross-sectional area, Ankle joint strength, Foot kinematics.

목차 (Table of Contents)

• ABSTRACT ⅷ
• [Chapter Ⅰ] GENERAL INTRODUCTION 1
• 1.1 Significance of the Study 1
• 1.2 Purpose 5
• 1.2.1 Research purpose of the Study Ⅰ 5
• 1.2.2 Research purpose of the Study Ⅱ 5
• 1.3 Research Hypotheses 6
• 1.3.1 Research hypotheses of the Study Ⅰ 6
• 1.3.2 Research hypotheses of the Study Ⅱ 6
• [Chapter Ⅱ] BACKGROUND 7
• 2.1 Foot arch biomechanics 7
• 2.1.1 Anatomical structure of the foot arch 7
• 2.1.2 Functional role of the foot arch 7
• 2.1.3 Arch stiffness and biomechanical implications 9
• 2.2 Ankle joint complex 10
• 2.2.1 Talocrural joint 11
• 2.2.2 Subtalar joint 12
• 2.2.3 Distal tibiofibular joint 13
• 2.3 Biomechanical characteristics of foot arch and function 14
• 2.3.1 Arch height index 14
• 2.3.2 Arch stiffness index 14
• 2.3.3 Ankle joint strength 15
• 2.4 Foot arch supporting structures 16
• 2.4.1 Abductor hallucis 16
• 2.4.2 Tibialis posterior 16
• 2.4.3 Plantar fascia 17
• 2.4.4 Muscle thickness and cross-sectional area 17
• 2.5 Intervention 18
• 2.5.1 Plantar massage 18
• 2.5.2 Achilles tendon stretching 19
• 2.5.3 Short foot exercise 20
• 2.5.4 Towel curl exercise 21
• 2.5.5 Blackboard training 22
• [Chapter Ⅲ] MANUSCRIPT 1 23
• 1. Introduction 25
• 2. Method 28
• 2.1 Participants 28
• 2.2 Instruments 29
• 2.3 Procedures 30
• 2.4 Data processing 31
• 2.4.1 Measurement of variables 31
• 2.5 Statistical analysis 34
• 3. Result 35
• 3.1 Participants’ characteristics 35
• 3.2 Arch characteristics 35
• 3.3 Thickness and CSA of intrinsic and extrinsic foot muscles 36
• 3.4 Ankle joint strength 36
• 3.5 Correlation between ASI, intrinsic and extrinsic foot muscles, and
• ankle joint strength 37
• 3.6 Multiple regression analysis for ASI, intrinsic and extrinsic foot
• muscles 38
• 3.7 Multiple regression analysis for ASI and ankle joint strength 38
• 4. Discussion 39
• 5. Conclusion 44
• [Chapter Ⅳ] MANUSCRIPT 2 45
• 1. Introduction 48
• 2. Method 51
• 2.1 Participants 51
• 2.2 Instruments 52
• 2.3 Procedures 52
• 2.4 Intervention 53
• 2.5 Data processing 54
• 2.5.1 Marker sets 55
• 2.5.2 Gait analysis 58
• 2.6 Statistical analysis 59
• 3. Result 60
• 3.1 Participants’ characteristics 60
• 3.2 Arch height and stiffness index 61
• 3.3 Ankle joint strength 62
• 3.4 Intrinsic and extrinsic foot muscles 63
• 3.5 Rearfoot (calcaneus relative to shank) kinematics 64
• 3.6 Midfoot (midfoot relative to calcaneus) kinematics 67
• 3.7 Forefoot1 (forefoot relative to midfoot) kinematics 70
• 3.8 Forefoot2 (forefoot relative to rearfoot) kinematics 73
• 3.9 Medial longitudinal arch angle 76
• 4. Discussion 77
• 5. Conclusion 83
• [Chapter Ⅴ] GENERAL DISCUSSION 85
• [Chapter Ⅵ] GENERAL CONCLUSIONS 87
• References 88
• Korean abstract 97
• Appendix 1 100
• Appendix 2 101
• Appendix 3 102