A synergy pattern is a specific, coordinated movement involving the simultaneous and involuntary contraction of multiple muscle groups across several joints. This pattern emerges primarily after damage to the central nervous system, such as following a stroke or traumatic brain injury. Instead of moving a single joint in isolation, the nervous system defaults to this fixed, mass movement. The lower extremity extensor synergy pattern is a common example, characterized by the leg forcefully straightening and pointing downward. These patterns are considered abnormal because they limit the selective movements necessary for daily activities.
Neurological Basis of Synergy Patterns
Synergy patterns arise from damage to the descending motor pathways, the nerve tracts that carry movement commands from the brain to the spinal cord. The most significant pathway affected is often the corticospinal tract, which is responsible for individual joint movement and selective motor control. When this tract is compromised, the brain loses its ability to send precise, isolated signals to specific muscles.
The central nervous system compensates for this loss of fine control by relying on more primitive, less-selective motor pathways. One such pathway is the cortico-reticulospinal tract, which activates large groups of muscles together in a coupled, “all-or-nothing” fashion. This shift results in a lack of fractionation, meaning the individual cannot separate the movement of one joint from another.
These coupled movements are part of a predictable sequence of motor recovery after a neurological event. Movement initially emerges only in these mass patterns, ideally progressing toward more voluntary and isolated control. The presence of the synergy, while limiting, indicates that the motor system is beginning to regain some connection and function.
Key Movements in the Extensor Synergy
The extensor synergy is defined by movements that occur simultaneously across the hip, knee, and ankle joints. At the hip, the pattern involves powerful extension (moving the leg backward) combined with adduction (pulling the leg inward toward the midline). This is often coupled with internal rotation, causing the entire leg to turn slightly inward.
At the knee, the core component is forceful extension, causing the leg to straighten completely. This extension can sometimes cause hyperextension, or “locking,” of the joint. This fixed posture of the knee and hip is optimized for weight bearing, but hinders advancing the leg during walking.
The ankle and foot complete the pattern by moving into plantarflexion and inversion. Plantarflexion causes the foot to point downward, while inversion turns the sole of the foot inward. This combination results in a pointed, inwardly-turned foot that severely impacts functional mobility.
This rigid pattern creates significant challenges during the swing phase of walking. Since the hip is adducted and the ankle is plantarflexed, the leg becomes too long, making it difficult to lift the foot without dragging the toes. Individuals often compensate by adopting a circumduction gait, swinging the leg out in a semi-circle to clear the ground.
Therapeutic Approaches to Management
Management focuses on encouraging the nervous system to regain selective motor control and preventing secondary complications. Physical therapy uses exercises designed to “break” the synergy by demanding isolated joint movement. For example, a therapist might practice knee extension while actively preventing the ankle from moving into plantarflexion, uncoupling the two movements.
Repetitive, goal-oriented practice facilitates neuroplasticity, the brain’s ability to reorganize and form new neural connections. By consistently practicing movements outside the established synergy pattern, the brain gradually learns to bypass damaged pathways. This training often involves isolating hip flexion to lift the leg, which opposes the extensor synergy’s dominant components.
Assistive devices play a significant role in managing the effects of the synergy, especially during gait. Ankle-Foot Orthoses (AFOs) are commonly prescribed to keep the ankle in a neutral position, preventing the foot from dropping into plantarflexion and inversion. By mechanically stabilizing the ankle, the AFO helps the foot clear the ground during the swing phase, making walking safer and more efficient.
Preventing muscle contractures and tissue shortening is crucial. Due to the prolonged, forceful muscle contractions associated with the extensor pattern, regular stretching and proper positioning maintain muscle length and joint mobility. For severe spasticity that impedes function or causes pain, medical interventions are used. Targeted injections of botulinum toxin (Botox) can temporarily reduce the overactivity of involved muscles, such as the calf or thigh adductors.