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CONTENTS ABSTRACT .................................................................................................................................

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CONTENTS

ABSTRACT ... ACKNOWLEDGEMENTS ... CONTENTS ... 1 LIST OF PUBLICATIONS ... 5 LIST OF PRESENTATIONS ... 6 LIST OF ABBREVIATIONS ... 7 INTRODUCTION ... 9 LITERATURE REVIEW ... 11

I. THE MOTOR SYSTEM ... 11

1. The descending pathways ... 11

2. Motor units ... 13

3. The muscle-tendon complex ... 13

a. The muscle-tendon complex structure ... 14

i. The muscle ... 14

ii. The in vivo recording of muscle architecture ... 14

iii. The tendon and aponeurosis ... 15

b. The muscle-tendon complex function ... 15

i. The muscle mechanics ... 16

Force-length relationship ... 16

Force-velocity relationship ... 17

ii. The tendon mechanics ... 19

Tendon stiffness ... 19

Young’s Modulus ... 20

Hysteresis ... 21

II. AGING OF THE NEURO-MECHANICAL SYSTEM ... 21

1. Dynapenia ... 21

2. The muscular mechanisms of dynapenia ... 22

3. The neural mechanisms of dynapenia ... 24

4. The changes in tendon properties ... 25

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1. The visual system ... 27

2. The vestibular system ... 28

3. The proprioceptive system ... 28

a. The Golgi tendon organs and articular receptors ... 28

b. The cutaneous receptors ... 29

c. The muscle spindles ... 29

i. The Ia fibers ... 30

ii. The group II afferent fibers ... 30

iii. The gamma (γ) motoneuron ... 31

4. The role of muscle spindle afferents in the control of upright standing ... 31

a. The Hoffmann (H) reflex ... 32

b. Methodological considerations for the H-reflex recording. ... 32

c. The modulation of the H-reflex during postural tasks ... 35

d. The cortico-spinal pathway during upright standing ... 36

e. The muscle-tendon interaction during upright standing ... 39

IV. LOCOMOTION AT GROUND LEVEL ... 40

1. The gait cycle ... 40

2. Kinematics and Kinetics ... 40

a. Joint Angles ... 40

b. Joint Moments ... 41

c. Joint Powers ... 41

3. Modulation of the H-reflex during walking ... 42

4. Modulation of the MEP during walking ... 43

5. The muscle-tendon interaction during level walking ... 44

V. STAIR LOCOMOTION ... 45

1. The stair gait cycle ... 45

2. Stair kinetics and kinematics ... 46

a. Joint Angles ... 47

b. Joint Moments ... 48

c. Joint Powers ... 49

3. Muscle activity ... 50

VI. PURPOSE OF THE DOCTORAL THESIS ... 51

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SPINAL AND CORTICOSPINAL PATHWAYS ARE DIFFERENTLY MODULATED WHEN STANDING AT THE BOTTOM AND THE TOP OF A 3-STEP STAIRCASE IN

YOUNG AND OLDER ADULTS ... 55

1. Introduction ... 55

2. Methods ... 56

3. Experimental results ... 62

4. Discussion ... 68

PROJECT 2: ... 73

MODULATION OF THE HOFFMANN REFLEXES IN SOLEUS AND MEDIAL GASTROCNEMIUS DURING STAIR ASCENT AND DESCENT IN YOUNG AND OLDER ADULTS ... 73 1. Introduction ... 73 2. Methods ... 74 3. Experimental results ... 81 4. Discussion ... 86 PROJECT 3: ... 91

AGE-RELATED CHANGES IN MUSCLE FASCICLES AND TENDON LENGTH DURING STAIR ASCENT AND DESCENT ... 91

1. Introduction ... 91 2. Methods ... 92 3. Experimental results ... 98 4. Discussion ... 108 GENERAL DISCUSSION ... 113 1. Methodological considerations ... 113

2. Stair locomotion kinematics in older adults ... 114

3. The neural modulation during stair locomotion ... 116

4. The muscular and tendon length changes during stair locomotion ... 119

5. Interaction between fascicles length change and H-reflex modulation………122

6. Healthy aging ... 123

BIBLIOGRAPHY ... 125

ANNEX 1: THE STAIRCASE DEVICE ... 157

ANNEX 2: BEHAVIOUR OF THE MUSCLE-TENDON UNIT OF THE GASTROCNEMIUS MEDIALIS AND TIBIALIS ANTERIOR DURING FORWARD AND BACKWARD SWAYS. ... 159

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ANNEX 4: DO MECHANICAL PROPERTIES OF ACHILLES TENDON INFLUENCE TORQUE STEADINESS? ... 175 ANNEX 5: SPINAL AND CORTICOSPINAL PATHWAYS ARE DIFFERENTLY

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