Cerebral palsy (CP) is caused by faulty development or damage to the motor areas of the brain and disrupts the ability to effectively control movement. While spasticity and decreased range of motion may be more obvious features of CP, impaired selective voluntary motor control (SVMC) is a direct result of injury to the developing corticospinal tracts within the periventricular white matter. While several studies have found relationships between SVMC and gross motor function, no one has examined the effect of SVMC on gait mechanics in individuals with spastic CP.

The coordination of the hip and knee during the swing phase of gait was examined in individuals with spastic diplegic CP using relative phase analysis. SVMC was found to be a better predictor of the ability to extend the knee while the hip is flexing than strength or spasticity. Individuals with good SVMC were able to dissociate hip and knee movement, but those with poor SVMC were constrained to walk with mass flexor/extensor synergy patterns.

Since the hip must flex during swing in order to advance the limb, the extent to which the knee is extended dictates stride length and walking speed. Induced acceleration analysis was used to determine the joint moment contributions to swing knee extension acceleration in spastic CP. In spastic CP, swing limb joint moments played a large role in resisting knee extension. Clinically addressing these excessive contributions due to spasticity and contractures may improve knee extension as long as the patient has sufficient SVMC to dissociate hip and knee movement.

In four case studies, individuals with spastic CP underwent gait analysis before and after hamstring lengthening surgery. The two participants with higher levels of SVMC were capable of moving out of synergy during swing and can improve stride length;​ however, the participant with poor SVMC who was constrained to move in synergy was unable to improve stride length due to excessive hip extension coupled with knee extension during terminal-swing. An understanding of influence of SVMC on swing phase mechanics during gait may help establish more realistic goals for interventions, in particular hamstring lengthenings.