ASSOCIATED MOVEMENT

Associated Movement

Primary Disciplinary Field(s): Neurology, Kinesiology, Rehabilitation Medicine, Developmental Pediatrics

1. Core Definition

Associated movement refers to the involuntary and often unnecessary contraction of muscles in a body part that is not primarily involved in a specific voluntary action. This phenomenon manifests as an incidental, non-functional movement occurring secondary to an intentional motor command executed elsewhere in the body. The fundamental characteristic distinguishing associated movement is its lack of purpose in achieving the primary motor goal; it is essentially a neural overflow or leakage of excitation from the active motor pathway to adjacent or contralateral motor centers. For instance, as noted in clinical observations, an associated movement might occur when a patient attempting to lift a heavy object with one hand simultaneously experiences an involuntary flexor movement in the opposite, resting limb, or an unwanted movement in the proximal joints (like the upper arm or shoulder) of the limb executing the primary task.

While often considered pathological in adults, associated movements are a normal and expected part of early human development. In infants and young children, the corticospinal tracts responsible for fine, isolated motor control are still undergoing myelination and maturation. This immaturity means that voluntary motor commands frequently lack the precision and inhibitory control required to restrict activation purely to the target muscle groups. As the central nervous system (CNS) matures, typically throughout childhood, these overflow movements are gradually suppressed through the development of inhibitory circuits, allowing for increasingly fractionated and isolated joint movement. The persistence or re-emergence of pronounced associated movements in older children or adults, however, is a strong clinical indicator of damage or dysfunction within the motor control system, particularly involving the primary motor cortex or the descending motor pathways.

The concept of associated movement is closely related to, and sometimes used interchangeably with, the term synkinesis, particularly in the context of facial nerve palsy recovery. However, in broader neurological literature concerning limb function, associated movement typically refers to generalized, large-scale overflow (e.g., movement in the proximal joints or opposite limb during exertion), whereas synkinesis may sometimes be reserved for highly specific, abnormal connections that occur following nerve regeneration, such as smiling causing involuntary eye closure. The defining feature remains the linkage between an intended movement and an unwanted, simultaneous movement, reflecting a failure of the nervous system to isolate motor commands efficiently.

2. Neurophysiological Basis

The neurological mechanism underlying associated movement is primarily attributed to a failure of selective inhibition within the motor pathways, often involving the corticospinal tract (CST). In healthy adults, the highly organized, direct lateral CST provides the necessary input for fractionated movement, enabling an individual to contract specific muscles without recruiting neighboring or antagonist groups. This ability relies heavily on inhibitory feedback loops, involving interneurons within the spinal cord and descending modulatory input from supraspinal centers like the basal ganglia and cerebellum, which filter and refine the motor signal.

When injury occurs, such as a cerebral vascular accident (stroke), the primary motor cortex or the descending CST fibers are often damaged. This damage compromises the efficiency and integrity of the inhibitory control mechanisms. As a result, the excitation generated by the voluntary command—intended for a specific muscle group (e.g., wrist extensors)—spreads or “overflows” to non-target motor neurons, leading to widespread, non-selective muscle activation. This overflow is particularly evident during tasks requiring significant strength, effort, or concentration, as the increased level of central excitation needed to recruit the affected motor units also increases the collateral discharge to unintended pathways.

Furthermore, the manifestation of associated movements often suggests the increased reliance on alternative or less refined motor pathways. Following significant CNS injury, motor control may revert to relying on phylogenetically older, less sophisticated pathways, such as the ipsilateral CST or extrapyramidal pathways. These older systems are generally designed for mass movement patterns (synergies) rather than discrete control. When these pathways take over, motor commands inherently lack the fine-tuning capabilities of the damaged lateral CST, resulting in the obligatory co-activation of large muscle groups—the hallmark of associated movement patterns observed in conditions like hemiplegia.

3. Clinical Presentation and Types

Associated movements are highly variable in their presentation but generally fall into recognizable clinical patterns, often reflecting the severity and location of the underlying neurological impairment. One of the most common forms is Mirror Movements, where a voluntary action performed by one limb (e.g., flexing the fingers of the right hand) is involuntarily mirrored by the corresponding muscles in the contralateral limb (e.g., slight flexion of the fingers of the left hand). While mild mirror movements can sometimes be observed in healthy individuals performing very forceful actions, persistent and pronounced mirroring is a classic sign of impaired interhemispheric inhibition or structural anomalies in the corpus callosum.

Another crucial clinical presentation involves Flexor or Extensor Synergies, particularly prominent in patients recovering from stroke (hemiplegia) or those with Cerebral Palsy (CP). During attempts to execute a simple, isolated movement (e.g., reaching forward), the entire limb may involuntarily contract into a predictable, stereotypical pattern—such as the flexion synergy involving simultaneous shoulder abduction, elbow flexion, and wrist/finger flexion. These synergies are considered associated movements because they are involuntary contractions of muscles (the synergistic group) that are unnecessary for the immediate, voluntary goal, severely limiting the patient’s ability to perform functional tasks requiring isolated joint control.

Specific associated reactions, sometimes categorized separately but sharing the same underlying mechanism, include the Souques’ phenomenon and the Raimiste’s phenomenon. Souques’ phenomenon describes the involuntary extension and abduction of the fingers when the hemiplegic arm is passively elevated above the horizontal plane. Raimiste’s phenomenon involves resistance to abduction or adduction of the sound leg leading to a similar movement in the paralyzed leg. These specific reflexes demonstrate how central motor overflow and lack of inhibition cause predictable, generalized muscle activity linked to effort or movement elsewhere in the body, providing valuable diagnostic information regarding the integrity of descending motor control.

4. Developmental Context and Maturation

The study of associated movements provides important insight into typical motor development. In healthy neonates and infants, global, mass movements dominate, and associated movements are the norm rather than the exception. When an infant grasps a toy, they often exhibit associated movements in the mouth, trunk, or opposite limb. This prevalence reflects the dominance of subcortical and primitive reflex pathways before the lateral corticospinal tract achieves functional maturity.

As a child develops, the progressive myelination and synaptogenesis within the corticospinal system, coupled with the development of the basal ganglia’s inhibitory functions, gradually suppress these overflow movements. The ability to perform fractionated movements—such as isolating finger extension from wrist extension—is a key marker of neurological maturation. Pediatric neurologists often assess the presence and intensity of associated movements during standardized motor examinations to gauge the child’s neurological age and identify potential delays or abnormalities, such as those associated with periventricular leukomalacia or other forms of brain injury sustained early in life.

The persistence of pronounced associated movements beyond the age of 5 to 7 years is typically considered pathological, suggesting that the normal inhibitory developmental process has been hindered. In a small subset of the population, a benign condition known as Hereditary Benign Congenital Mirror Movements (HBCMM) exists, where individuals exhibit persistent mirror movements without other neurological deficits, highlighting that in rare cases, the isolated failure of interhemispheric inhibition may not necessarily indicate widespread neurological injury, although it remains an anomaly of motor control.

5. Diagnostic and Prognostic Significance

In adult neurology, the presence and severity of associated movements hold significant diagnostic and prognostic value, particularly following acquired brain injuries. Clinically, the re-emergence of associated movements in an adult who previously had mature, fractionated control is a reliable sign of upper motor neuron (UMN) damage, frequently seen following stroke, traumatic brain injury (TBI), or multiple sclerosis (MS). Their intensity can often correlate with the extent of corticospinal injury.

From a prognostic perspective, the gradual reduction and eventual disappearance of pathological associated movements during rehabilitation are often tracked as markers of neurological recovery and reorganization. As the brain undergoes neuroplastic changes, and alternative motor pathways are refined or inhibitory control is partially restored, the reliance on mass synergy patterns diminishes. Physical and occupational therapists utilize specialized tests to quantify associated movements, helping to tailor rehabilitation strategies. The persistence of severe associated movements months after a stroke typically suggests a poorer long-term functional prognosis regarding the recovery of fine motor skills and independent limb function.

Furthermore, in the diagnosis of specific conditions like Parkinson’s disease, involuntary movements such as synkinetic movements (sometimes categorized broadly as associated movements) are noted. While distinct from the resting tremor or bradykinesia, these involuntary co-contractions reflect the dysfunction of the basal ganglia’s modulatory role in filtering and scaling motor commands, reinforcing the link between these involuntary phenomena and global central nervous system control failure.

6. Therapeutic Approaches

Managing associated movements is a core objective in neurological rehabilitation, as these involuntary contractions hinder the patient’s ability to perform necessary daily tasks requiring isolated movement. Therapeutic strategies are centered on two main goals: suppressing the unwanted overflow and strengthening the ability to perform fractionated, goal-directed movements.

One common approach involves Constraint-Induced Movement Therapy (CIMT), which aims to force the use of the affected limb while restricting the movement of the unaffected limb. Although CIMT primarily targets learned non-use, the intense, repetitive practice of functional tasks can facilitate neuroplasticity and the refinement of inhibitory control, potentially reducing the reliance on synergistic, associated movement patterns. Biofeedback and electromyography (EMG) training are also employed, allowing patients to visually or audibly monitor the unwanted muscle activity associated with movement, thereby teaching them to consciously inhibit the overflow.

Modern rehabilitation techniques emphasize task-specific training and movement decomposition. Therapists work to break down complex actions into isolated joint movements, requiring the patient to concentrate specifically on avoiding the associated contraction. For instance, in a patient exhibiting strong elbow flexion synergy, exercises might focus exclusively on controlled wrist extension, often with the proximal joints stabilized. This targeted training helps the nervous system re-learn how to activate specific motor units without triggering the widespread neural excitation that characterizes associated movements. The ultimate goal is to restore the independence of movement essential for high-level motor function.

Further Reading

• Associated Movements in Neurological Disorders (ScienceDirect)
• Corticospinal Tract Function and Damage (Wikipedia)
• Associated Reactions and Synkinesis in Clinical Practice (UpToDate – requires subscription/access)
• Motor Control and Recovery after Stroke (NCBI Review)