What is the physiology of locomotion?

Physiology of Locomotion

Neural Control Architecture

Locomotion is controlled through a hierarchical neural system spanning from the spinal cord to the cortex, with the spinal central pattern generators (CPGs) forming the core propulsive mechanism. 1

Spinal Level Control

• Spinal CPG networks generate the fundamental rhythmic patterns that control muscle timing and coordination during locomotion, operating as the primary propulsive control system 1
• These networks can produce basic locomotor patterns even without descending input from higher brain centers 1
• Sensory feedback from the periphery continuously modulates CPG output to compensate for perturbations and environmental changes 1

Brainstem Command Systems

• Brainstem centers control the level of CPG activity and regulate locomotor speed 1
• These systems integrate postural control with propulsive movements to maintain body orientation during locomotion 1

Forebrain and Basal Ganglia

• The basal ganglia determine which motor programs should be recruited at any given time 1
• These structures can both initiate and terminate locomotor activity based on behavioral goals 1

Biomechanical Principles

Human locomotion operates through a spring-mass system model in both sagittal and horizontal planes, representing the simplest mechanical template that captures essential locomotor behavior. 2

Mechanical Templates

• Diverse species with different skeletal structures, leg numbers, and postures run using similar spring-mass dynamics 2
• This template model reduces the complexity of multiple legs, joints, and muscles by identifying fundamental synergies and symmetries 2

Musculoskeletal Coordination

• Movement results from highly coordinated mechanical interactions between bones, muscles, ligaments, and joints under nervous system control 3
• The human leg contains over 50 muscles, yet muscle activity patterns during normal walking are remarkably stereotyped despite this redundancy 4
• Spatiotemporal maps of spinal motoneuron output show both stereotypical features and functional reorganization capacity 4

Cardiovascular Response

During locomotion, cardiac output increases initially through stroke volume augmentation via the Frank-Starling mechanism, then primarily through heart rate elevation in later phases. 5

Hemodynamic Adaptations

• At submaximal workloads below ventilatory threshold, steady-state conditions are reached within minutes, maintaining constant heart rate, cardiac output, blood pressure, and pulmonary ventilation 5
• Sympathetic discharge becomes maximal during strenuous exertion while parasympathetic stimulation is withdrawn 5
• Vasoconstriction occurs in most body systems except exercising muscle, cerebral, and coronary circulations 5
• Skeletal muscle blood flow increases, oxygen extraction increases up to 3-fold, and total peripheral resistance decreases as exercise progresses 5

Muscle Activation Patterns

Locomotion requires coordinated activation of specific muscle groups, with the adductor muscles playing a critical role in maintaining leg position and stability. 5

Key Muscle Groups

• The adductor muscle group (adductor brevis, longus, magnus, minimus, pectineus, gracilis, and obturator externus) maintains leg positioning during locomotion 5
• Gluteal muscles, iliopsoas, and triceps surae contribute to hip and leg movement control 5
• Hip stabilizing muscles, particularly gluteus medius, increase activity to maintain pelvic stability in the coronal plane during altered locomotion conditions 6

Gait Mechanics

Normal gait involves specific kinematic variables including stride length, trunk gradient, knee angles during non-support and stance phases, all of which affect locomotor economy. 5

Stride Characteristics

• Stride frequency and length are primary determinants of gait efficiency 5, 6
• Hip and ankle vertical oscillation, along with thigh, knee, and trunk angles at different phases, characterize normal gait patterns 5

Postural Control

• Balance during locomotion is maintained through integration of feedback from otolithic, visual, and somatosensory systems 5
• Proprioceptive feedback from locomotor activity persists briefly after cessation, requiring neurosensory adaptation during transitions between activities 5, 7

Control Mechanisms

During rapid, rhythmic locomotion, control resides primarily within the mechanical system through passive dynamic self-stabilization, while slow, variable-frequency locomotion is dominated by nervous system control. 2

Feedforward vs. Feedback Control

• Passive, dynamic self-stabilization from a feedforward, tuned mechanical system can reject rapid perturbations and simplify control requirements 2
• Neural and mechanical systems are dynamically coupled, with both playing essential roles in locomotor control 2

Neurotrophic Factors and Plasticity

Physical exercise during locomotion promotes CNS regeneration through elaboration of neurotrophic factors, particularly brain-derived neurotrophic factor (BDNF). 5

• Exercise increases neurotrophic factor production that promotes neuronal recovery through mechanisms including axonal regeneration 5
• This physiological basis supports the importance of early, vigorous rehabilitation to maximize neurological recovery after injury 5