Neurophysiological basis and regulation of locomotion
The physiology of walking is based on a complex system of connections between neurons located in various segments of the spinal cord. It is this spinal network that provides basic coordination—the timely and alternating contraction of flexor and extensor muscles.
Higher control over locomotion is exercised by cortical and brainstem structures of the brain. They are responsible for adapting movements, in particular, regulating step length and the frequency of muscle contractions. Descending motor pathways running from the brain to the spinal cord are functionally divided into two separate systems. Each of these systems assumes control over strictly defined muscle groups involved in moving the body through space.
Cyclicity and periods of walking
Walking movements are not chaotic; they form a strict, constantly repeating sequence—the walking cycle. This cycle consists of alternating periods that smoothly transition into one another:
- Double-support period: the initial phase in which both legs simultaneously contact the supporting surface.
- Asymmetrical phase: the left leg enters the single-support period (bears weight), while the right leg is in the swing period (moves forward).
- Second double-support period: a brief return to support on both limbs.
- Role reversal: the right leg enters the single-support period, and the left leg performs the swing.
Interestingly, when changing pace (speeding up), the total duration of the cycle predictably decreases. However, this reduction occurs unevenly: time is saved exclusively due to a decrease in the stance period, whereas the duration of the swing phase remains practically constant.
Biomechanics of periods: stance and swing
Each period of the walking cycle is characterized by a unique pattern of muscle activity and biomechanical changes.
Stance period (pillar-leg phenomenon) In this phase, the trunk rests on the limb, and the leg straightens completely, bearing the body weight. Key roles are played by:
- Plantar flexors, which provide the necessary rotation of the foot around the ankle.
- Quadriceps femoris, responsible for the powerful extension of the knee joint.
Just before ground contact, both flexors and extensors are activated. Their combined (cocontraction) contraction gives the leg maximum rigidity, turning it into a reliable support—a pillar.
Swing period This phase combines active and passive biomechanical processes:
- Active movement: occurs due to the contraction of the iliopsoas muscle, which pulls the leg forward.
- Passive movement: in parallel, passive flexion of the limb in the knee and hip joints is observed.
The end of the swing phase is of particular importance. To prevent the leg from continuing to move forward by inertia, a sharp contraction of the biceps femoris muscle (m. biceps femoris) occurs. It acts as a physiological brake, stopping the limb's movement right before its contact with the support.
Briefly: answers to topical questions
Into what two separate systems are the descending motor pathways controlling locomotion divided?
Descending motor pathways of the spinal cord are functionally divided into two systems that control different muscle groups during walking.
- Medial motor system—innervates motor neurons located more medially in the anterior horns of the spinal cord; is responsible primarily for trunk and leg movements, ensuring standing and walking.
- Lateral system—is responsible for smooth and precise movements of the forearms and hands.
Which spinal centers and generators form the locomotor rhythm (stepping reflex)?
The flexion phase during locomotion is largely determined by the spinal locomotor generator. There are several such generators per limb; their activity rapidly adapts under the influence of afferent impulses entering the spinal cord.
At the spinal level, the transmission of motor commands involves:
- spinal interneurons—intermediate links through which many descending pathways switch to motor neurons;
- spinal alpha motor neuron—the "final common path" innervating skeletal muscle.
During voluntary stepping, initial commands come from the motor areas of the cerebral cortex directly to the flexor and extensor motor neurons.
To memorize the biomechanics of acceleration: imagine that the ground becomes "hot" when running. You try to remove your leg faster (the stance period is shortened), but the flight time of the leg in the air (swing) remains stable.
Class and exam questions
How is the walking cycle shortened when accelerating pace?
During acceleration, the cycle duration is reduced only by shortening the stance period. The duration of the swing phase remains practically constant.
Which muscle stops the leg at the end of the swing period?
The forward movement of the leg before ground contact is stopped by the contraction of the biceps femoris muscle (m. biceps femoris).
What is the essence of the "pillar-leg" phenomenon?
This is a biomechanical state in which the simultaneous contraction of flexor and extensor muscles makes the leg rigid to reliably support the weight of the trunk.
Which parts of the central nervous system regulate step length and frequency?
These motor activity parameters are controlled by cortical and brainstem structures of the brain.
What else to study on this topic
- Functional division of descending spinal motor pathways during locomotion
- Detailed biomechanics of the iliopsoas muscle
- Interaction of segmental spinal neurons
- Mechanisms of passive flexion in the hip and knee joints
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