The epidemiology of cerebral palsy (CP) is reviewed followed by an outline of the mechanisms of hypertonus. Mechanisms and clinical correlates of dynamic and passive equinus were studied in hemiplegic cerebral palsy which could not be explained solely in terms of muscle weakness or imbalance of power at the ankle or spasticity. Peripheral muscular transformation and increasing intrinsic stiffness with abnormal postures and muscle activation patterns are more likely explanations: a developmental model of equinus is advanced. Electromyographically-quiescent muscular creep in hemiplegia is demonstrated in muscles that might have been classed as 'tonically spastic', along with muscle moulding and stress-relaxation, confirming a peripheral transformation of the plastic or time-dependent properties of muscle following long-standing cerebral injury. These mechanisms are independent of reflex or supraspinal electrical 'tonus': the success of some physical therapies (stretches, orthotics and serial plastering) may depend in part on the properties of muscular creep and stress-relaxation. Reflex excitability of muscle acting at the knee and ankle was studied using manual discontinuous ramp or rhythmic sinusoidal stretches. Excitability of hamstrings and quadriceps muscles was greatest close to maximum knee extension, but grouped hemi- and nonparetic muscles behaved similarly. At the ankle, sinusoidal stretches allowed more graded reflex quantitation than ramp stretches. No muscles were excited at walking speeds of sinusoidal stretch. For more than half the cases, excitability was similar in hemi- and nonparetic limbs: in the remainder, the nonparetic reflex threshold was not reached. The surface EMG relation to standing, isometric and isotonic tasks is explored with respect to fine motor dexterity at the ankles and toes. The influence of the joint angle on stretch reflex excitability of the soleus muscle at the ankle has been studied in children and adults. For all subjects, reflex EMG and mechanical twitch torque gain were trivial at resting plantar flexion.