**1. Introduction**

Cerebral palsy has been defined as a nonprogressive disorder that affects the development of movement and posture, causing limitation of activity in the developing fetus or infant. Disturbances of sensation, cognition, communication, perception and behavior by seizure disorder are the most common disorders associated with cerebral palsy (CP) [1]. CP can be classified based on the type of movement disorder as spastic, athetoid, ataxic and mixed; CP can also be classified based on the area of the body involved as hemiplegia, diplegia and quadriplegia [2], in which spastic diplegia is the most common type [3]. Spasticity, caused by damage to the pyramidal parts of the brain, is defined as a velocity-dependent resistance to stretch [2]. Due to spasticity, the onset of postural muscle activity in children with CP is delayed compared to normally developing children. In addition, impairment was observed upon sequencing of multiple muscle; additionally, there is an increased level of co-activation of agonist and antagonist muscles at a joint, which results in reduction of balance [4].

CP is a neurological disorder that can cause secondary changes in the musculoskeletal system, such as decreased muscle strength, tightness or contractures around joints and abnormalities in both bony structures and gait [5]. Therefore, children with CP show weakened muscle due to lack of motor unit activation and thickness in 50% of small muscles, compared to normally developing children. Infants with CP have reduced knee extensor and ankle plantar flexor strength than normally developing infants [6], and CP has reduced rectus femoris thickness compared with normally developing children [7]. The thickness of the quadriceps muscle, indicative of lower extremity strength, has an e ffect on the quality of life of children with CP; indeed, a study has shown that children with thicker quadricep muscles participated more in community-related activities [8]. Based on this result, lower extremity strengthening should be emphasized in the rehabilitation of children with CP.

Many methods of muscle strengthening are recommended for children with CP, such as functional progressive resistance exercise (FPRE) [9], isokinetic training [10], bicycle and treadmill exercise [11], weight training [12], aquatic training [13], sports and recreation [14] and electrotherapy [15]. Until recently, strength training in children with CP was considered inappropriate, as it was believed to lead to increased spasticity or abnormal movement patterns.

The muscles of children with CP have an increased amount of collagen, which hinders movement. This increase in collagen is responsible for contracture development, thereby a ffecting the passive viscoelastic features of muscle and exerting an impact on the internal resistance of muscle when passive movement of the joint is performed [16]. A weak agonist muscle may not allow full lengthening of the spastic antagonist muscle, leading to contracture development, and an increase in passive tension leads to muscle weakness [17].

FPRE can improve lower limb muscle strength and improve function in children with CP without increasing spasticity [18]. Essentially, FPRE provides su fficient resistance so that a low number of repetitions (usually 8–12) can be completed before fatigue sets in [19]. FPRE includes exercises, such as sit to stand, half-kneeling standing and side step-up [20]. A study on antigravity close kinematic chain exercise [21] showed that FPRE e ffectively increases lower muscle strength, thereby facilitating lower extremity co-contraction and allowing agonist and antagonist muscles to work e ffectively; this leads to reduction of muscle tone in the lower extremity [22].

This study aims to contribute to the improvement of rehabilitation in children with spastic CP by investigating the e ffect of FPRE on knee extensor strength, myoarchitectonic of the quadriceps, muscle tone, dynamic balance and functional ability of the lower extremity.

### **2. Materials and Methods**
