**4. Discussion**

### *4.1. Knee Extensor Muscle Strength*

CP affects physical activity and has a negative impact on the child's physical development. The spasticity and loss of strength experienced by children with CP results increased incidence of gait disorders and increased energy consumption in comparison with their healthy peers [9,37]. The muscle weakness in the lower extremity is particularly important for ambulation and requires strength training in children with CP [38]. Several studies have provided adequate evidence of a correlation between muscle strength and lower extremity function [39,40]. Indeed, increase in lower extremity muscle strength leads to positive effects on functional activities and flexibility [41].

In this study, FPRE strength training programs were used. Using the handheld dynamometer to examine the knee extensor strength, time factor effects were observed on the knee extensor muscle strength of the dominant side and non-dominant side (*p* < 0.05) and time × group interaction effects were observed on the knee extensor muscle strength of the dominant side (*p* < 0.05); these results are consistent with the results of previous studies, suggesting that strength training in CP leads to increased lower extremity strength [24]. The protocol for increasing muscle strength of the knee extensor in CP are numerous; however, due to the low methodological quality of previous studies, the effects of the study protocols may have been overestimated [42]. Nevertheless, we believe that organized method, resistance and repetitions of the exercises can increase lower extremity strength. Anttila et al. [43] reported that strengthen training in children with CP is not recommended, as it may increase spasticity, which can lead to reduction in range of motion—as well as difficulty with ambulation. However, recently increasing evidence and systemic reviews have shown that strength training can improve muscle strength in children with CP with no adverse effects on spasticity. These results indicate that FPRE training in CP leads to increased muscle power and lower extremity muscle strength in children with CP and could be considered for use in rehabilitation programs.

### *4.2. Structure of Quadriceps in Rehabilitative Ultrasound Imaging*

RUSI uses USI to aid rehabilitation of neuromusculoskeletal disorders. Physical therapists have used RUSI to evaluate function and related soft tissue morphology and muscle during exercise and physical tasks [30]. Muscle cross-sectional area has a direct relationship with the capacity of muscle to produce power [7]. RUSI was used to assess thickness of the quadriceps and cross-sectional area of the rectus femoris in order to provide a clinical measurement of increased quadricep volume, which indicates increase of lower extremity strength [44].

In this study, changes in thickness and cross-sectional area of the quadriceps were assessed with RUSI. The time × group interaction effects (*p* < 0.05) were observed for TQ of the dominant and non-dominant sides and CSA of the dominant side. Lee et al. [45] performed progressive functional training on 26 children with CP with spasticity. For 6 weeks, neurodevelopmental treatment and FPRE were performed in the experimental group. The muscle thickness of the quadriceps femoris (QF), cross-sectional area of the rectus femoris (RF) and pennation angle of the gastrocnemius (GCM) were measured with RUSI, and results after the intervention showed significant improvement on those variables (*p* < 0.05). In the experimental group, QF thickness increased from 1.6 cm to 1.9 cm and RF

CSA increased from 1.2 cm<sup>2</sup> to 2.1 cm<sup>2</sup> [45]. Additionally, increase in structure-related measurements is strongly correlated with increased muscle strength [7].

The outcomes may result from the fact that motor units work in an inadequate, irregular and slower than normal manner following upper motor neuron damage. Therefore, the more affected side cannot activate as normal muscle [17,46]. Thus, it is difficult to generalize this information to all pediatric cases, since this study was not conducted among normally developing children; however, increments observed in the results indicate that FPRE protocol has a positive effect on increasing muscle strength in children with spastic CP.

### *4.3. Muscle Tone According to Popliteal Angle in Passive, Speed and Active Ranges of Motion*

Individual spastic muscle fibers with increased tensile strength are stiffer than controls; therefore, to elongate a spastic muscle fiber, more force is needed [47]. Developments of contractures and passive stiffness could be the result of weakened agonist muscles that result in the inability of the spastic antagonist muscle to elongate, thereby perpetuating a pattern of weakness [48]. Popliteal angle was measured in passive, speed and active conditions to determine the impact of FPRE on muscle tone of the hamstring. The FPRE is an exercise program which allows the co-contraction of both quadriceps and hamstring, thereby increasing agonist muscle strength to enable elongation of the antagonist muscle to reduce muscle tone of the lower extremity and increase the popliteal angle.

In this study, popliteal angles in passive, speed and active ranges of motion were assessed in order to evaluate the effects of FPRE on lower leg range of motion and strength. Time × group interaction effects (*p* < 0.05) were observed for the PA-P, PA-S and PA-A of the dominant and non-dominant side. Results of studies by Stubbs, P.W et al. [49] and Scholtes, V.A., et al. [50] imply that muscle strengthening does not increase muscle tone. A prior study has shown that muscle tone was significantly lower after the intervention in the training group (median 1, 0/7; *p* < 0.01) and control group (median 0, 0/4; *p* = 0.02) after implementing a muscle strengthening exercise on children with CP and spasticity, p. A prior study on dynamic strength exercises of the knee extensor showed statistically significant changes (pre: 64.4 to post: 92.6) in the training group and (pre: 60.8 to post: 65.3) control group (*p* < 0.05) [49,50]. Generally, the muscle tension in children with CP develops due to elongated sarcomeres, with decreased action and myosin interaction, which limits the number of cross-bridges causing reduced force production capability [17]. This biomechanical disadvantage may disrupt the ability of the muscle to sufficiently contract to produce the required functional movement.

This study suggests that an increase in passive range of motion allows for adequate muscle length to produce maximum muscle contraction, which could be the reason for an increment in active range of motion. An increase in agonist muscle strength may be strongly correlated with increment in speed range of motion due to co-activation of the thigh muscle. This result provides valid evidence of the effect of FPRE, which can be used in the future to treat children with CP.

### *4.4. Dynamic Balance*

A comparison of normally developing children-with-children with CP reveals that children with CP have delayed onset of postural muscle activity. In addition, there is a high level of co-activation of agonist and antagonist muscles at a joint and multiple muscle action sequences are impaired. This can cause difficulty in balance control in CP [4,51]. In this study, dynamic balance was examined using the FRT. An increase in lower extremity muscle strength was shown to be closely correlated with increased dynamic balance ability, which was assessed with the modified FRT [52].

In this study, dynamic balance was assessed with the modified functional reach test in two different positions: forward reaching position and side reaching position. Time effects were observed with the S-FRT (*p* < 0.05), while time × group interaction effects were observed with the F-FRT and S-FRT (*p* < 0.05). FPRE program comprises exercises such as sit to stand and side step-up with load. This exercise program includes voluntary co-contraction of both lower extremity muscles, quadriceps and hamstring muscles [53]. After performing the sit to stand exercise, the FRT value increased in the exercise group; results for the asymmetric paretic limb position in the control group were pre 19.36 ± 6.42 to post 23.43 ± 3.85, while in the training group results were pre 14.91 ± 6.11 to post 21.81 ± 4.59, which indicates that the training group showed statistical improvement after intervention t(*p*); −2.287(0.034) (*p* < 0.05).

In our FPRE study, the FRT was measured in two positions, position forward (F) reaching and side (S) reaching in the FPRE group. These results imply that the exercise protocol of the FPRE, which includes co-activation and functional strengthening, can have a positive effect on dynamic balance.

### *4.5. GMFM in Functional Ability*

It is important to accurately measure changes in the acquisition of total motor skills to determine the impact on rehabilitation and the effectiveness of the intervention program in children with CP. The GMFM-88 is an effective measuring tool to detect changes in gross motor function in children with CP [35]. In spastic diplegia CP, strength was highly related to functional abilities [54].

In this study, functional ability was assessed with the GMFM-88. A paired *t*-test revealed a statistically significant increase after the intervention on the GMFM score in the FPRE group (*p* < 0.05).

Ross et al. [54] indicates that lower extremity strength has a strong correlation with functional ability. They conducted a study to determine the relationship between strength and GMFM-66 on CP. The study included 49 boys and 48 girls; mean age ± standard deviation, 9.11 ± 4.8 years. Aggregate strength consisting of values for the ankle dorsiflexors and plantar flexors, knee extensors and flexors and hip abductors and adductors averaged across sides was strongly correlated to the GMFM-66 (r.83). In this study, the GMFM-88 score of the FPRE group increased from a pre-mean value of 69.98 ± 21.55 to a post mean value of 71.78 ± 21.05, (0.019, *p* < 0.05) compared to the control group in which there was a reduction in the GMFM-88 score from 68.15 ± 27.15 to 63.48 ± 27.48 after the intervention. Although an increase was shown in the FPRE group, the change between the two groups was not statistically significant. This result may be due to the duration of the intervention. The results of the study conducted by Bryant et al. [55] indicate that a six-week program can show significant difference on GMFM-88D scores, but not on GMFM-66 or GMFM-88E scores.

The RUSI is an effective assessment device, which can successfully measure thickness and cross-sectional area of the quadriceps, which are associated with lower extremity strength [30]. Quadriceps thickness can also be an indicator of muscle strength, Ohata et al. [8] identified a relationship between thickness of the quadriceps and activity limitation in children and adolescents with CP. Muscle thickness of the quadriceps showed a significant correlation with the GMFM-66 score (*r* = 0.52, *p* = 0.001, 95% CI 0.24 to 0.72). This result suggests that lower extremity muscle thickness may be strongly correlated with functional ability of the child with CP.

This study has the following limitations: a short 6-week intervention period and a small sample size. This makes it difficult to generalize the findings to all children with CP. It is also difficult to control all the factors that may affect the child's activities of daily living.
