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RESEARCH REPORTS

The Effects of Functional Progressive Strength and Power Training in Children With Unilateral Cerebral Palsy

Kaya Kara, Ozgun PT, PhD; Livanelioglu, Ayse PT, PhD; Yardımcı, Bilge Nur PT, MSc; Soylu, Abdullah Ruhi MD

Author Information
doi: 10.1097/PEP.0000000000000628

The effect of strength training on improvements in functional capacity and activity for individuals with cerebral palsy (CP) remains unknown. Some studies reported positive results, while others, including several randomized trials, failed to demonstrate functional improvements.1–5 Reasons for the failure to find benefits may include insufficient duration, intensity, and frequency of strength training, and too few muscles trained, or these results may be related to the type of exercises that were performed.1,6–14 Several studies have reported that specific anaerobic exercises (ie, activities requiring high power output such as step-ups, stair-climbing, squats, and leg presses) in conjunction with strength training successfully increased functional capacity in participants and adolescents with CP.15,16 In the most recent randomized controlled study on young adults with CP, Gillet et al17 concluded that the addition of functional anaerobic exercises to a 12-week strengthening program improved functional capacity, suggesting that classic progressive resistance exercise may not be sufficient.17

Daily activities such as walking include rapid joint movements occurring within 50 to 200 ms after the onset of muscle contraction.18 Rapid force development (RFD) was described as the force development that occurs during the first 300 ms after onset of muscle contraction.19 Moreau et al6 indicated that RFD was decreased by 70% in the knee extensors in their study of participants with CP, and time-dependent contractile characteristics of muscle were more highly correlated with gait and other aspects of physical function than strength measures.6 Traditional strength training programs typically involve slow and controlled movements, which have a minimal effect on RFD. High-velocity training may be more effective in increasing RFD and more likely improve function.6 Plyometric training consists of dynamic and high-velocity resistance movements. The results of high-velocity strength or power-training programs successfully demonstrated improvements in gross motor function, agility, and power in participants with CP.7

An earlier literature review reported that strength training was not effective in transferring muscle strength to improvements in functional capacity.3 In response, Verschuren et al20 recommended that strength training programs in CP be designed according to of the National Strength and Conditioning Association (NSCA) criteria strength training guidelines, which were developed for participants developing typically and adolescents. According to NSCA criteria, strengthening results from the following: (1) single and multijoint exercises incorporated together with concentric and eccentric contractions; (2) progressive resistance exercise sessions consisting of 1 to 3 sets of 6 to 15 repetitions; a training frequency of 3 times per week; (4) a duration of resistance training of at least 12 weeks; (5) a 5- to 10-minute warm-up period, and 1-minute rest intervals; (6) increasing strength gradually by 5% to 10% when the child performs the number of exercises more easily and using the correct form; and (7) participants should be at least 7 years of age.19 In many studies investigating the effects of strength training, exercises type, duration, time, and frequency are variable and are focused only on 1 muscle group and/or used exercise types (concentric or eccentric contractions) that are not consistent with the NSCA criteria.12,21,22 This might explain why previous studies have not been successful in improving mobility and gross motor function. However, some strength-training studies have been appropriate for the NSCA criteria.16,17,23

There is a need for high-quality research that investigates the effects of more complex strengthening and power-training protocols based on appropriate physiological criteria in participants with CP. Because many functional activities such as walking require muscle strength, power, and aerobic and anaerobic endurance, combined functional strength and power-training programs could provide improved transfer of increases in muscle power to improved gross motor and walking performance.

Dysfunctional postural control is a key problem that affects functional skills in daily-living activities in participants with CP because of damage to the neuromotor system.24 Pediatric physical therapists therefore aimed to improve postural control in participants with CP. Woollacott et al25 reported positive effects of balance training in participants with CP including increasing recruitment of the postural muscles. Balance training plays a central role in improving gross motor function in participants with CP.

The purpose of this study was to investigate the effects of a novel functional strength and power-training program, based on established strength and conditioning principles, on gait and gross motor function in participants with unilateral spastic CP (USCP). We hypothesized that this more complex training protocol would improve gait speed and power and gross motor function as well as dynamic balance and muscle strength in participants with USCP.

METHODS

The study design was a randomized controlled trial. The University Ethics Committee approved this study (project: GO 14/224) and written informed consent was obtained from each participant and/or guardian.

Participants

Forty-three participants with USCP were referred by pediatric neurologists to the department of physical therapy and rehabilitation. The inclusion criteria were between 7 and 16 years of age; classified as level I using the Gross Motor Function Classification System (GMFCS); and able to follow and accept verbal instructions. The exclusion criteria were orthopedic surgery or botulinum toxin injection in the past 6 months, participants whose parents refused permission for them to participate, and participants with epilepsy, or those who had any other disease that interfered with physical activity so that they could not continue a regular training program.

Procedures

The participants were stratified according to 3 variables: gender, Manual Ability Classification System (MACS) level (I, II-III), and age (youngest, 7-11 years; oldest, 12-16 years). They were randomized into 1 of 2 groups by an independent researcher using a random numbers allocation table. Among the 33 participants, 17 were randomized to the experimental (progressive functional strength training) group and 16 to the active-comparison group (Figure). Two participants in the experimental group withdrew from the study; 1 because of an upper respiratory illness and 1 decided to discontinue exercising. One participant in the comparison group failed to attend the last evaluation. Thus, 30 participants, 15 per group, completed the study.

Fig.
Fig.:
Follow-up diagram.

Clinical Measurements

The Gross Motor Function Classification System Expanded and Revised (GMFCS-E&R) was used to classify the gross motor function,26,27 and manual ability was classified using the MACS.28 Height was measured using a stadiometer and a standard scale was used to record weight. Body composition was assessed using the body mass index, calculated as weight in kilograms divided by the square of the height in meters. Assessor was masked to group.

Primary Outcomes (Walking and Gross Motor Capacity)

Short-term Muscle Power. Short-term muscle power was assessed using the muscle power sprint test that is reliable in participants with CP. Participants have to complete 6, 15-m runs at their maximum pace during this test.29 Mean power output was calculated in watts for each participant.

Gross Motor Function Measurement. The Gross Motor Function Measure (GMFM) is reliable and valid to assess changes in gross motor function in participants with CP aged 5 months to 16 years.30 Gross motor function was assessed using dimensions D (Standing) and E (Walking, Running and Jumping) of the Gross Motor Function Measure-88 (GMFM-88).31 The GMFM-88 was administered to each child following standardized procedures.

The 1-minute walk test (1MWT) was used to assess walking ability in participants with CP. The 1MWT is valid and reliable in participants with CP.32 Each participant walked as fast as possible without running around a 20-m oval track for a duration of 1 minute. The walk distance was used as the outcome variable.

Secondary Outcomes (Balance and Muscle Strength)

Dynamic Balance Measures.

Timed Up and Go. Functional mobility and dynamic balance were assessed using the Timed Up and Go (TUG) test, which is a reliable and valid test for participants with CP.33 Participants began the test while seated in an adjustable-height chair without an armrest or backrest, their knees and hips were flexed at 90°, and their feet were on the floor. The child was asked to stand up from the sitting position and walk 3 m as fast possible without running, and then turn around, walk back, and sit down. The test was repeated 3 times, and shorter times showed better functional ability.

Muscle Strength Measures.

Leg Press. The 1-repetition maximum (1RM) of a leg press (combined hip extension, knee extension, and ankle plantar flexion) was used to measure lower extremity strength. The 1RM can evaluate the muscle strength in people with CP with a high level of association (r = .89) and responsiveness.34,35 The leg press was adjusted according to the child's height. To evaluate the isotonic muscle strength of the lower limbs, the maximum weight (in kg) that can be lifted only once through the full range with good form using the leg press was assessed.34

Hand-held Dynamometer. Isometric muscle contraction (quadriceps femoris, hamstrings, tibialis anterior, gastrosoleus) of participants with USCP was assessed using the Power Track II Commander (JTECH Medical, Salt Lake City, Utah). The physical therapist instructed the child to press as hard as possible against the device for 1 second. Three trials were recorded for each muscle group in the affected and unaffected legs. The score was obtained by averaging the second and third trials (kg/N). This method is reliable for measuring lower extremity strength in participants with CP.36

Outcome measures of short-term muscle power, TUG, and 1MWT were evaluated at baseline and after the intervention (week 12) by examiner 1, who was an experienced physical therapist and masked to group allocation. The muscle strength measurements and GMFM were evaluated at baseline and after the intervention by examiner 2, who was an experienced physical therapist and masked to group allocation. The measurements were spread over 2 days to minimize fatigue effects. Short-term muscle power, GMFM, 1MWT, and TUG were administered on the first day and the muscle strength measurements were evaluated on the second day.

Intervention

The active-comparison group continued its therapy (eg, locomotor training, improving symmetry of weight-bearing, stretching), which did not include progressive resistance strength training exercises, by the same clinical physical therapist 3 times per week for 60 minutes per session over a period of 12 weeks (totaling 36 sessions). Participants in the experimental group were treated 3 times per week for 90 minutes per session for a total of 36 sessions over a period of 12 weeks. The longer session duration was designed to allow adequate rest intervals between sets (1-3 minutes), as recommended for muscle recovery.

The experimental protocol consisted of functional strengthening using the leg press (for eccentric, concentric, and isometric contraction of quadriceps femoris, hamstrings, tibialis anterior, and gastrosoleus), plyometric exercises (including jumping), and balance training (see Supplemental Digital Content 2, available at: http://links.lww.com/PPT/A255). Before each training session, there was a warm-up period with 5 to 10 minutes of dynamic activities (eg, jogging). After training, there was a cooldown period with 5 to 10 minutes of dynamic stretching exercises. In addition to rest intervals, after each training session, there was a 48-hour rest interval to prevent muscle fatigue and injury. Details of the functional strength and power-training program are in Table 1.

TABLE 1 - Characteristics of the Functional Progressive Strength and Power-Training Program
Exercise Velocity Intensity Load Set Dose Duration Rest
Leg press with 6 different games
  1. Random reactive

  2. Isometric gate

  3. Random explosive

  4. Controlled position

  5. Random deceleration

  6. Controlled route

Speed was performed at the 50%-70% of the muscle power sprint test 60%-80% of the 1RM 10% of the 1RM/every 2 wk 3 sets for each game (2 sets with the affected leg and 1 set with the unaffected leg) 40 min (each set lasted 90 s) 3 times for 12 wk (36 sessions) 1-3 min between each set
Power exercises (Plyometric exercises)
  1. Jumping forward on 2 legs

  2. Jumping forward on the affected leg

Speed was performed at the 50%-70% of 1-min walk test 6-15 repetitions Begin with 6 repetition, 10% load was added every 2 wk on the basis of the number of repetitions 3 sets 15 min (each set lasted 30 s) 3 times for 12 wk (36 sessions) 1-3 min between each set
Balance training on unstable surfaces using a BOSU ball while throwing and catching a ball
  1. On 2 legs

  2. On the affected leg

  3. Small jumps

Try to protect stability 6-10 repetitions Begin with 6 repetition, 10% load was added every 2 wk on the basis of the number of repetitions 3 sets 15 min (each set lasted 30 s) 3 times for 12 wk (36 sessions) 1-3 min between each set
Abbreviation: 1RM, 1-repetition maximum.

Functional Strength Training With Leg Press. Functional strength training was implemented using a computer-aided horizontal leg press machine (Functional Squat System version 3.12, Monitored Rehab Systems, Haarlem, The Netherlands) and included eccentric, concentric, and isometric muscle contractions. According to the NSCA protocol, the intensity of training was set to 60% to 80% of the 1RM. Progressive increases of 10% of the 1RM were attempted every 2 weeks as tolerated if the participants were able to still perform the training easily. Each child performed the leg press exercise while playing 6 different games (Random Reactive, Isometric Gate, Random Explosive, Controlled Position, Random Deceleration, or Controlled Route) completing 3 sets per game (1 set with the affected leg, 1 set with the unaffected leg, and the last set again with the affected leg). Each set lasted 90 seconds, with a total exercise time of approximately 40 minutes per session, including rest periods.

Plyometric Exercises. According to the NSCA protocol, 3 sets of 6 repetitions for each exercise were performed including jumping forward on 2 legs and jumping forward on the affected leg with therapist support if necessary. The number of repetitions was increased every 2 weeks up to a maximum of 15 repetitions if the participants were able to perform the training easily. Total dose of plyometric exercises in 1 session was approximately 15 minutes.

Balance Training. Balance training exercises included (1) standing on a BOSU ball while throwing and catching a ball; (2) 1-legged standing (affected leg) on the BOSU ball while throwing and catching a ball with therapist support; and (3) small jumps on the BOSU ball.14 Three sets of 6 repetitions for each exercise were performed for both repetitions and were increased by 10% every 2 weeks up to a maximum of 10 repetitions if the participants were still able to perform the training easily. Total dose of balance training in 1 session was 15 minutes.

Statistical Analysis

The Statistical Package for the Social Sciences (SPSS) version 21 for the Macintosh (IBM SPSS Statistics; IBM Corporation, Armonk, New York) was used for analyses. One-sample Kolmogorov-Smirnov tests were used to evaluate distribution of variables before test selection. The sample size calculation was based on an observed effect size of d = 1.11, which was reported for the 6-repetition maximum on the leg press after resistance training for participants with CP.5 To achieve 80% power to detect a difference with a 95% confidence interval using a 2-tailed test, a sample size of 11 participants was required for each group not including loss to follow-up.

Descriptive analyses were presented using the median and interquartile range for the nonnormally distributed and ordinal variables. Physical characteristics in the experimental and comparison groups were compared using the χ2 test for categorical variables (gender, hemiplegic side, MACS, and GMFCS) and the Mann-Whitney U test for continuous variables (age, height, weight, and body mass index).

Baseline, postintervention, and change scores were calculated. The Wilcoxon signed rank test was used to compare the difference in dimensions D and E for the GMFM, muscle power, TUG, 1MWT, muscle strength with the dynamometer, and the 1RM leg press value between baseline and postintervention scores within groups.

The Mann-Whitney U test was used to compare changes in the GMFM dimensions D and E scores, muscle power, TUG, 1MWT, muscle strength with the dynamometer, and the 1RM leg press value between groups.

Effect sizes (ESs) were calculated with GPower V.3.1.7 (University of Kiel, Kiel, Germany) using the mean and standard deviation of the change scores for each group. Effect size was classified as follows: a large ES when greater than 0.8, a medium ES when 0.5 to 0.8, and a small ES when less than 0.5. Mean differences were analyzed using paired and independent t tests. The statistical significance level was P < .05.

RESULTS

Participants

Descriptive statistics are in Table 2. There were no statistical differences between the groups at baseline for age, height, weight, body mass index, sex, hemiplegic side, or functional level. Participants completed the 36 training sessions.

TABLE 2 - Characteristics of Participantsa
Experimental Group (n = 15), No Title ± SD Control Group (n = 15), No Title ± SD Pb,c
Age, y 11.8 ± 2.95 11.26 ± 3.28 .645b
Weight, kg 44.26 ± 14.08 37.86 ± 13.48 .254b
Height, cm 148.86 ± 17.45 139.6 ± 17.88 .106b
Leg length, cm 78.6 ± 9.95 73.3 ± 10.77 .105b
BMI, kg/m2 19.27 ± 3.73 18.70 ± 3.57 .82b
n (%) n (%)
Gender 1.00c
Female 8 (53.3) 8 (53.3)
Male 7 (46.7) 7 (46.7)
Hemiplegic side .713c
Right 8 (53.3) 9 (60)
Left 7 (46.7) 6 (40)
MACS level .881c
I 7 (46.7) 7 (46.7)
II 5 (33.3) 4 (26.7)
III 3 (20) 4 (26.7)
Abbreviations: BMI, body mass index; MACS; manual ability classification system.
aValues are presented as the mean (standard deviation) for continuous variables and as the frequency for categorical variables. P values of less than .05 were considered significant.
bP Mann-Whitney U test for continuous variable.
cP χ2 test for categorical variables.

Primary Outcomes

The primary outcomes are in Table 3. There were no statistically significant differences between the experimental and comparison groups for the baseline scores. Only the experimental group had significant improvements in the GMFM E score, muscle power, and 1MWT scores after 12 weeks. These change scores differed significantly from the active-comparison group (Table 4).

TABLE 3 - Before and After Treatment Valuesa
Experimental Group (n = 15) Control Group (n = 15) Comparison of Baseline Scores
Before Median (Minimum-Maximum) After Median (Minimum-Maximum) Z Pb Mean Difference Effect Size Before Median (Minimum-Maximum) After Median (Minimum-Maximum) Z Pb Z Pc
Primary outcomes (walking capacity)
Muscle Power Sprint Test, s 4.54 (3.05-6.23) 4.24 (2.94-5.8) −3.408 .001d 0.37 2.05 4.7 (3.71-7.38) 4.44 (3.56-7.48) −1.421 .155 −0.664 .507
Muscle Power Sprint Test, watt 48.47 (12.74-170.1) 57.42 (15.79-189.92) −3.408 .001d 15.28 1.52 25.81 (10.36-93.06) 46.28 (9.95-113.2) −1.250 .211 −0.892 .373
GMFM-D 100 (97.4-100) 100 (100-100) −1.000 .317 0.17 0.25 100 (94.87-100) 100 (94.43-100) −0.365 .715 −1.438 .151
GMFM-E 94.44 (88.88-100) 97.22 (91.66-100) −2.952 .003d 2.31 1.05 95.83 (93.05-100) 95.83 (88.88-100) −0.042 .967 −0.865 .387
1 min walk 94 (80-116) 102.5 (89-118.5) −3.353 .001d 7.76 1.103 92 (79-103) 90 (80-110) −0.429 .668 −1.37 .171
Secondary outcomes
Dynamic balance
TUG 6.26 (4.76-7.47) 4.94 (4.15-5.97) −3.408 .001d 10.2 2.26 6.01 (4.96-7.76) 5.82 (5.05-7.62) −0.852 .394 −0.519 .604
Muscle strength
Affected lower extremity
1RM, kg 25 (10-70) 80 (35-100) −3.415 .001d 51.33 2.39 35 (5-100) 35 (5-80) −1.342 .18 −1.021 .307
Quadriceps Femoris, N/kg 22.33 (5.33-50.5) 27.5 (15.66-50.5) −2.731 .006d 5.54 0.87 22 (10-53.16) 23.83 (10.83-54.33) −0.398 .691 −0.353 .724
Hamstring, N/kg 14.5 (5.5-32.83) 21.16 (7.5-43.16) −2.897 .004d 6 0.76 16.16 (9-49.5) 17 (8.33-49) −0.426 .67 −0.29 .771
Dorsiflexors, N/kg 15.16 (6.83-36) 19.66 (10-41.66) −3.039 .002d 4.62 1.11 14.16 (6.88-31) 13 (5.33-30.16) −1.364 .173 −0.477 .633
Plantar flexors, N/kg 19 (10.5-37.33) 33 (23.16-52.16) −3.408 .001d 11.91 1.54 21.33 (14.16-46.5) 21 (15-46.16) −0.199 .842 −0.249 .803
Unaffected lower extremity
1RM, kg 35 (20-90) 90 (45-100) −3.416 .001d 43.66 1.94 50 (5-100) 45 (5-100) −0.632 .527 −1.147 .251
Quadriceps Femoris, N/kg 30.5 (9.16-44.16) 34.16 (16.16-49) −2.669 .008d 4.55 0.78 25 (13.83-59) 24.83 (14-60.83) −1.307 .191 −0.373 .709
Hamstring, N/kg 21.83 (7-32.33) 27 (8.66-47.83) −2.556 .011d 5.01 0.806 21 (9.5-53.33) 21.16 (9.83-53.5) −0.313 .754 −0.187 .852
Dorsiflexors, N/kg 19.5 (8.33-37.16) 28 (10-52.5) −2.499 .012d 4.92 0.77 18 (10.33-34) 17.33 (10-35.16) −0.440 .66 −1.058 .29
Plantarflexors, N/kg 30.66 (15.16-46.83) 45.5 (22-54.83) −3.352 .001d 11.17 1.507 28.66 (21-50) 27.83 (20.33-49.83) −0.853 .394 −0.332 .74
Abbreviations: GMFM, Gross Motor Function Measurement; N/kg, Newton/kilogram; 1RM, 1-repetition maximum; TUG, Timed Up and Go.
aValues are median (minimum, maximum).
bP values for within-group change calculated using the Wilcoxon signed rank test.
cP values for between-group difference in baseline scores were calculated using the Mann-Whitney U test.
dStatistically significant at P < .05.

TABLE 4 - Change in Outcomes From Baseline to 12 Weeks
Differences Between Baseline and 12 wka Experimental Group ( No Title ± SD) Control Group ( No Title ± SD) Z Pb Mean Difference Effect Size
Primary outcomes (walking capacity)
Muscle power sprint test, s −0.37 ± 0.18 −0.09 ± 0.21 −3.132 .002c −0.27 1.39d
Muscle power sprint test, watt −15.28 ± 10.04 −3.807 ± 10.18 −2.924 .003c −11.47 1.13d
GMFM-D 0.17 ± 0.67 0.32 ± 1.42 −0.479 .632 −0.15 0.13
GMFM-E 2.31 ± 2.20 −0.37 ± 2.59 −2.849 .004c 2.68 1.11d
1-min walk 7.76 ± 7.03 0.53 ± 3.37 −3.391 .001c 7.23 1.31d
Secondary outcomes
Dynamic balance
TUG −1.02 ± 0.45 0.08 ± 0.45 −4.169 <.001c −1.10 2.42d
Muscle strength
Affected lower extremity
1RM, kg 51.33 ± 21.41 −3 ± 10.31 −4.874 <.001c 54.33 3.23d
Quadriceps femoris, N/kg 5.54 ± 6.33 0.05 ± 1.59 −2.551 .011c 5.48 1.18d
Hamstring, N/kg 6.00 ± 7.86 −0.09 ± 1.02 −3.405 .001c 6.09 1.08d
Dorsiflexors, N/kg 4.62 ± 4.16 −0.29 ± 0.87 −3.734 <.001c 4.92 1.63d
Plantarflexors, N/kg 11.91 ± 7.72 −0.007 ± 1.21 −4.668 <.001c 11.91 2.15d
Unaffected lower extremity
1RM, kg 43.66 ± 22.47 −0.66 ± 4.16 −4.661 <.001c 44.33 2.74d
Quadriceps femoris, N/kg 4.55 ± 5.77 0.41 ± 1.19 −2.718 .007c 4.13 0.99d
Hamstring, N/kg 5.01 ± 6.21 −0.08 ± 0.94 −2.926 .003c 5.09 1.14d
Dorsiflexors, N/kg 4.92 ± 6.36 −0.19 ± 1.006 −2.803 .005c 5.12 1.12d
Plantarflexors, N/kg 11.17 ± 7.41 0.31 ± 1.29 −3.88 <.001c 10.86 2.04d
Abbreviations: GMFM, gross motor function measurement; 1RM, 1-repetition maximum; TUG, Timed Up and Go.
aPostintervention change calculated by subtracting baseline value from postsession value.
bP value for between-group difference calculated using the Mann-Whitney U tests.
cStatistically significant at P < .05.
dd > 0.80.

Secondary Outcomes

At baseline, there were no statistically significant differences between the experimental and comparison groups. The median scores before and after treatment and baseline score comparisons are in Table 3.

After 12 weeks, the experimental group had significantly decreased TUG values, increased 1RM values for the affected and unaffected legs, and increased muscle strength in the quadriceps femoris, hamstrings, dorsiflexor, and plantar flexor muscles on both legs. However, there were no changes in the comparison group.

The experimental group had significant differences compared with the active-comparison group in the TUG, 1RM of the affected and unaffected legs, and muscle strength of the affected and unaffected legs for the quadriceps femoris, hamstring, dorsiflexors, and plantar flexors muscles (Table 4) from baseline to 12 weeks.

Adverse Events

There were no adverse events associated with the training protocol. None of the participants reported discomfort or muscle soreness and all participated in their activities of daily living during the study. One participant in the experimental group had ankle pain at week 6 because of a fall on the playground when playing basketball. The doctor indicated that there was no serious injury preventing him from exercising.

DISCUSSION

In a recent study, Vulpen et al16 investigated the effects of a functional power-training program, including high-velocity resistance exercises, on walking capacity and plantarflexor muscle strength in participants with CP, aged 4 to 10 years (3 times a week for 60 minutes over 14 weeks). They reported an increase of 83% in the muscle power sprint test after training.16 Their larger increase in muscle power sprint might have been because of the inclusion of younger participants with CP who may have more muscle and neural plasticity or the greater singular focus on power exercises. In addition, Gillet et al17 examined the efficacy or a combined functional anaerobic and strength-training program in young adults with CP (aged 15-30 years, 3 times a week for 12 weeks) in a randomized controlled trial. They reported improvement of 8.3% in the muscle power sprint test.17 This difference may be because of the inclusion of older participants with CP as well as the greater variation in the training protocol. Task specificity in training, that is, a greater focus on power training, can influence outcomes as well as age; however, high-intensity exercises are recommended only by NSCA for those aged 7 years and older for safety reasons.

Our results demonstrate a 9.0% increase in the 1MWT, which was one of the primary outcomes that indicated an improvement in walking capacity after training. Vulpen et al16 and Moreau et al23 reported 13.0% and 9.3% increases, respectively, in the 1MWT after high-velocity exercises. In addition, Gillet et al17 demonstrated a 6.1% improvement in walking capacity after a functional anaerobic and strength-training program. The current literature, including our study, supports the conclusion that power exercises are effective for improving walking capacity.16,23 Our results further support previous findings that high-velocity exercises provide greater functional improvements than other strength-training programs.23 The increase in walking capacity may improve the important functional daily activity of walking as well as the participation of participants with CP in sports and games with their peers. The main differences between these functional power-training programs and our progressive functional strength-training protocol are that our study included plyometric exercises using the NSCA criteria, and we combined the plyometric exercises with isometric, concentric, and eccentric training using the leg press and balance training, and we used a computer-based device that was focused on squatting exercises with varying movement velocities and degrees of difficulty.

The other functional improvement in our study was a 2.9% increase in the gross motor capacity that was measured by the GMFM E scores. Several previous studies have demonstrated improvements in strength but have not been successful at transferring improvements in strength to gross motor function. Taylor et al1 investigated the effects of progressive resistance training on functional mobility in 47 participants with spastic CP (mean age: 18 years) who were GMFCS levels II and III, 23 of whom performed strength training using a leg press twice per week for 12 weeks. Their results showed a 27% (95% CI: 8-46) increase in muscle strength, a 14.8 kg (95% CI: 4.3-25.3) increase in the leg press maximum, and a 0.8 unit increase in the functional mobility scale (95% CI: 0.1-1.6, P = .02) in the treatment group but no differences in the 6-minute walk test (mean difference, 0.1 m; 95% CI: −20.6 to −20.9), 10-m walk test (mean difference, 0.01 m/s; 95% CI: −0.06 to 0.07), and the GMFM E score (mean difference, 0.9%, 95% CI: −3.0 to 4.7).37 In another study, Scholtes et al5 investigated the effects of progressive functional strength training 3 times per week for 12 weeks in participants with spastic unilateral and bilateral CP (mean age of 10 years; GMFCS levels I-II). Participants performed 1 exercise on the leg press in addition to wearing a weighted backpack during functional exercises (sit-to-stand, half knee rise, and lateral step-up). They reported an 8% (1.30 N/kg) increase in muscle strength tested with a hand-held dynamometer and a 14% increase in 6 maximum repetitions on the leg press, but there was no statistically significant difference in GMFM-66, sit-to-stand, and lateral step-up tests.38 Peungsuwan et al39 showed improved walking endurance, walking speed, leg muscle strength, and physical balance in participants with hemiplegic or diplegic CP after an 8-week combination of exercises. Another quantitative study demonstrated a significant (20%) increase in the 1RM leg press in the training group compared with the comparison group in adolescent and young adults with bilateral spastic CP.35 Our results indicated a 26.7% improvement in muscle strength after training. Based on the literature, we recommend that physical therapy programs include high-velocity exercises and varied strengthening activities to increase functional capacity in participants with CP.

There was a 21% decrease in TUG after training in our study. Moreau et al23 showed a significant (11.6%) decrease in TUG after high-velocity training only.23 The reason for the greater improvement in dynamic balance in our study may have been because we combined power exercises with balance training.

A limitation of this study was that changes in muscle architecture were not assessed. Lieber and Friden40 identified that muscle architecture is the main determining factor of the muscle function. Specifically, sarcomere length, cross-sectional area,41 fascicle length, and number of fiber types are some of the predictors of the muscle force, and they can be used to predict muscle function.40,42 There is a decreased fascicle length, muscle volume, and cross-sectional area (CSA), and increased sarcomere length and adipose tissue infiltration in participants with CP compared with healthy controls.43–45 In addition, Marbini et al46 demonstrated that type II fibers were hypotrophic and type I fibers were predominant in the adductor longus and triceps surae muscles of participants with CP. Ito et al47 reported that type I fibers increased in participants with CP. When the number of type I fibers is increased, skeletal muscles tend to be more effective at producing lower, more sustained muscle contractions rather than high-speed and high-intensity contractions. For example, the gastrocnemius muscle typically includes a high proportion of fast-twitch type II fibers,48 which are essential for forward propulsion during walking and for jumping.47,48 Participants with CP have been shown to have 50% less propulsive force in the late stance phase of walking than their peers without CP because of adverse changes in muscle architecture, motor unit recruitment, and force-generating capacity in participants with CP.37 Lieber and Friden40 stated that increased fascicle length is related to an increased maximum contraction velocity. Thus, longer muscle fibers tend to generate a higher force.40 Another of the main mechanisms for increasing muscle power is an increase in the CSA of type II muscle fibers.49 Macaluso et al50 demonstrated that plyometric exercises directly increase the size of type II fibers and plyometric jumping is a commonly used power-training method in healthy adults. Bodgoins et al51 also showed that RFD and CSA in all muscle types are increased in healthy participants after plyometric training. Racil et al52 recommended the use of the plyometric exercises combined with high-intensity interval training to reduce body mass. Plyometric exercises may lead to decreased adipose tissue in participants with CP. Reid et al13 showed that maximal torque produced was increased in participants with CP after eccentric strength training. Moreau et al23 showed that rectus femoris fascicle length and CSA increased after velocity training. Future studies should investigate the effects of plyometric exercises on type of muscle fiber and adipose tissue infiltration of skeletal muscle in participants with CP.

The significant limitation of this study was that the duration (90 minutes) of therapy in the experimental group was longer than that of the comparison group (60 minutes). Another limitation of this study was that it focused only on those with minimal functional impairments (GMFCS level I). In addition, an active comparison group that did not include strengthening was used in this study. For future studies, a comparison group including muscle strengthening without power training would more clearly demonstrate the differences between different muscle-training protocols.

In this study, participants exercised 3 times per week for 12 weeks, and thus we do not know whether similar benefits would be gained with a shorter duration (eg, 8-10 weeks) because no interim assessments were included. There was also no long-term follow-up, so it is not possible to determine whether the program led to any lasting changes once it was discontinued. However, it is likely that strength training and power training need to be continued on a regular basis for the benefits to be sustained. It is also possible that longer programs will lead to even greater benefits over time than those reported here.

Clinical Message

  • The intervention administered in this randomized clinical trial was progressive functional strength training combined with plyometric exercises and balance training 3 times a week for 12 weeks.
  • The intervention consisted of different lower limb strengthening techniques, including exercises that were specifically based on national strength and conditioning guidelines.
  • Consequently, this program increased muscle strength and power in the lower extremities, balance, and gross motor function, adding to the evidence base that supports the use of muscle strengthening in participants with spastic CP.

What This Adds to the Evidence

  • The 36 sessions (90 minutes, 3 times per week) of progressive functional strength training led to an increase in gait function, gross motor capacity, dynamic balance, and muscle strength and power.

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Keywords:

cerebral palsy; power; strength; training; unilateral

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