Safety and Feasibility of One-Repetition Maximum (1-RM) Testing in Children and Adolescents with Bilateral Spastic Cerebral Palsy

Abstract

Purpose

The purpose of this study was to standardize 1-Repetition Maximum (1-RM) testing procedures and evaluate the safety and feasibility of these procedures in youth with cerebral palsy (CP).

Methods

Youth with CP completed 1-RM testing on a leg press.

Results

Mean absolute, adjusted, and normalized 1-RM loads were 262.4 lbs ± 161.3 lbs, 127.0 lbs ± 80.2 lbs, 1.28 ± 0.51, respectively, and 67% were able to successfully lift the same or heavier load after a single failure. Cessation of testing after 1 failed attempt resulted in a 19.0% underestimation of the 1-RM.

Conclusions

1-RM testing in youth with CP is safe and feasible. Multiple attempts at a failed load should be provided to prevent underestimation in strength. The 1-RM test provides a more accurate strength assessment, which will improve the dosing for resistance training in youth with CP.

Introduction

Resistance training is one of the most common interventions for muscle weakness in individuals with Cerebral Palsy (CP).^1,2 Dosing guidelines developed by the National Strength and Conditioning Association (NSCA) for muscle strengthening, a type of resistance training, include 3 sets of 6–15 repetitions of an exercise at 60–85% of the 1-repetition maximum (1-RM) on 2–3 non-consecutive days per week to produce muscle hypertrophy.³ These recommendations were developed for participants who are typically developing (TD) but have been adapted for participants and adolescents with CP.^4,5 While volume (repetitions and sets), frequency (days/week), duration, velocity, and mode of exercise are important dosing parameters. The intensity, or load, of the resistance training program is the most critical variable because it drives the functional and architectural changes in skeletal muscle.^6,7 Thus, determining an accurate initial training load is essential for optimizing changes in muscle structure and function.

The 1-Repetition Maximum (1-RM) is a dynamic strength test which identifies the maximum load lifted 1 time and is the gold standard for determining the intensity of resistance training interventions.⁴ Methods for 1-RM testing are well established in adults⁴ and participants developing typically (TD);⁸ however, no specific guidelines have been developed for participants with CP. The 1-RM test was performed in 2 studies to evaluate strength in youth with CP,^9,10 but specific procedures and feasibility were not described. Given the physical uniqueness of youth with CP, testing procedures for TD youth may need to be adapted to ensure accuracy and optimal dosing. In addition, concern regarding the safety and appropriateness of maximal strength testing in CP needs to be addressed.

Several recent literature reviews have reported that even when strength training is prescribed at the recommended intensity, it does not result in significant improvements in functional mobility and walking capacity in participants with CP.^1,11,12 However, none of these studies used a maximal strength test to dose the intervention. Under-estimation of training load may contribute, in part, to the reduced effectiveness of strength training interventions by reducing the intensity of the exercise intervention. Other resistance training studies in youth with CP estimated maximal strength by performing repeated submaximal lifts.^13–16 Unfortunately, the accuracy of this procedure declines as more repetitions are performed.¹⁷ Because the recommended intensity of a resistance training program is based on a percentage of a child’s maximal strength, more specifically, the percentage of 1-RM, any underestimation in strength will lead to under-dosing and potential reduced effectiveness of a resistance training intervention.

Implementation of specific maximal strength testing procedures for participants with CP would increase the accuracy of strength assessment and subsequently improve prescription of training loads which are essential in driving structural and functional changes in muscle. Thus, the purpose of this study was to develop 1-RM testing guidelines in participants with CP using a leg press activity and to report on the safety and feasibility of these testing procedures. We hypothesize that 1-RM testing will be safe and feasible for participants and adolescents with CP.

Methods

Study Design

This was a prospective, nonexperimental descriptive study designed to develop and evaluate the safety and feasibility of 1-RM testing procedures using a leg press with a group of participants with CP. Participants included in this study were part of a larger randomized controlled trial (Clinicaltrials.gov: NCT 03625570) and were recruited from local physical therapy clinics and community organizations using fliers, media advertisements and attendance at cerebral palsy clinics. Prior to testing, the parent/legal guardian and the child provided their consent and assent, respectively, to participate in the study. The study protocol was approved by the appropriate university and health sciences ethics and institutional review boards.

Participants

Inclusion criteria were diagnosis of bilateral spastic CP, age 10–17 years, and Gross Motor Function Classification System (GMFCS) level of I, II or III. Participants who had orthopedic or neurosurgery within the last year, botulinum toxin injections in the last 3 months, or knee flexion contractures greater than 25 degrees were excluded from participation.

1-RM Testing Procedure

In this study, a leg press was selected for testing because weight machines are easier to use and safer for novice lifters compared to free-weights.⁴ Squats and heel rises can be used to evaluate maximal strength but may be more challenging for those with CP given the high degree of strength and coordination required to perform these more complex movements. Individuals who are GMFCS level III or above (and in some cases, level II) may not be capable of performing squats and heel rises. A horizontal leg press with adjustable height and external loads can accommodate all GMFCS levels, although we only tested levels I-III in this feasibility study. Functional tasks (i.e., squats, chair rises and step ups) have been previously used in participants with CP,^13,16 however, these activities have to be modified using external weight vests or loaded back packs and may not be able to accommodate the intensity or external load necessary to accurately evaluate an individual’s maximal strength without compromising safety. In addition, the squat movement performed on a leg press is functional and targets the quadriceps, gluteal, and gastrocnemius-soleus muscles which are critical for walking and often weak in people with CP.^18–20 The specific device used for the 1-RM test (Total Gym® GTS, Total Gym Global Corp, Carlsbad, CA) has an adjustable incline (max level at 29.4 degrees from the horizontal), a weight bar to add external load, and an adjustable strap to support the participant’s body weight between presses (Figure 1A). These features allow the test to accommodate a wide range of functional levels.

Figure 1.

Position for 1-RM Testing

A) start position for participants B) end position for participants

A standardized starting position for each subject was used for each press with the angle of the sled at 29.4 degrees from the horizontal at the highest position: feet placed as low as possible on the foot plate while still maintaining heel contact; knee angle at 90 degrees of flexion (range: 88 to 92 degrees); the elbows straight when holding onto the handle bar (Figure 1A). This start position was used throughout testing to ensure that changes in load were the only contributor to the increasing difficulty of the test. Knee angle was measured with a goniometer prior to each press. A press was complete when the individual reached full range of knee extension (Figure 1B).

Prior to testing, each individual performed a 5-minute walking warm-up followed by familiarization or practice trials. Participants performed 1 set of 4 repetitions of familiarization trials at both light and medium external loads which were initially set at 25–50% and 50–100% bodyweight, respectively. Weight plates (external load) were added to the weight bar and adjusted based on the individual’s functional level and their feedback after the first repetition in each practice set, but the angle of the sled remained at 29.4 degrees from the horizontal throughout the familiarization and testing for all participants. The goal was to have the light loads feel easy and the medium loads feel moderately heavy. These loads are based on the equipment used for this study and loads may vary substantially on different weight machines with different angles of inclination. For each press, the tester used consistent commands of “1, 2, 3, press” and verbal feedback was given to correct any movement errors such as valgus collapse at the knees, incomplete range of motion at the knee, poor eccentric control of the load or other compensatory patterns. The verbal scripts used for familiarization and testing are provided in Supplemental Digital Content.

The 1-RM test began with a heavy load set at 100–150% body weight depending on the individual’s functional level and performance during the light and medium practice trials. Those in GMFCS levels I and II generally started at 150% body weight unless the medium weight practice trials were heavy, then the starting load was reduced to 125% body weight. Those in GMFCS level III often started testing at 100–125% body weight unless the assessor felt the individual could start testing at a heavier load. Testing loads were increased by 10–20% after each successful press based on recommendations by the ACSM for testing lower extremity muscles.⁴ A press was considered a success if the individual completed the lift through the full range of motion without compensatory movements. The press was considered a failure if they were unable to lift the load, the knee range of motion was less than the participant’s full range, or the individual performed a compensation to complete the press, such as lifting the hips off of the glide board. A decision-making algorithm was developed to guide the clinician through the 1-RM testing process as these procedures differ from traditional testing procedures in adults without disability (Figure 2). Participants were given up to 4 attempts at a given load with a 3–5 minute rest break provided after failures at the second and 4th attempts. Unlike in adults, prior studies have shown greater than 60% of participants who are TD will successfully lift a given load after a previous failure.⁸ After the 4th failed attempt at a single load, a 3–5 minute rest was provided, and the weight was reduced to a load that was between the last successful attempt and the last failed attempt. Next, a multi-repetition maximum test (MRMT) was performed if needed. If only 1 repetition was completed at the new load, that was recorded as the 1-RM. If 5 or less repetitions were performed successfully at the new load, then the Brzycki equation was used to estimate the 1-RM using the load lifted and the repetitions completed (Equation 1).²¹ This formula has been shown to be a valid and reliable method for calculating the 1-RM.²² If greater than 5 repetitions were performed, the test was stopped, and a 3–5 minute rest was given. The load was increased, and the MRMT was repeated. The justification for this is that performing greater than 5 repetitions significantly reduces the accuracy of the MRMT in predicting the 1-RM.¹⁷

$$\text{Estimated }1\text{RM}=\text{load lifted }\left(\text{lbs}\right)/\left[1.0278-\left(0.0278*\text{repetitions lifted}\right)\right]$$ (Eq. 1)

Figure 2.

1-RM Decision Tree

Each participant’s 1-RM was recorded in pounds at completion of the test.

Outcome Measures

Safety was evaluated by tracking the occurrence of adverse events or complications during and after 1-RM testing. Feasibility analyses were reported as the number of participants who were able to complete the 1-RM test as described in the protocol.

Maximal strength was evaluated using the 1-RM test on a leg press. Testing was performed by 2 examiners (physical therapists) who were trained to use the testing protocol. Pounds lifted at completion of the test was the absolute load lifted. The adjusted 1-RM was calculated by multiplying the load by the sine of sled angle (29.4 degrees). The adjusted 1-RM was also normalized by body weight for each participant for individual comparison. In addition, the total number of attempts, and lifts were recorded. Attempts per person were defined as the total number of successes and failures for each participant. Lifts per person were the total number of successful presses which occurred during a single 1-RM test.

Data Analysis

Descriptive statistics (means and standard deviations) of participant demographics and frequency counts for the number of males and females, the number of participants per GMFCS level, and the number of participants by diagnosis were reported. Descriptive statistics were calculated for the group and by GMFCS level for outcomes of the 1-RM test: absolute, adjusted and normalized loads lifted, attempts per person, and lifts per person.

Results

Characteristics of Participants

Twenty-four participants with a diagnosis of bilateral spastic cerebral palsy between the ages of 10–17 years (mean age 13.25 ± 2.4 years) participated in the study. Because this study is descriptive, our sample size was based on similarly designed feasibility studies in participants with disabilities.^23,24 Demographic features of the participants are in Table 1.

Table 1.

Participant Demographics

Characteristics	Result
Diagnosis (Diplegia/Triplegia/Quadriplegia)	20/3/1
Sex (male/female)	14/10
Age, mean (SD), [range], y	13.3 (2.4), [10–17]
Height, mean (SD), [range], m	1.48 (0.1), [1.3–1.7]
Weight, mean (SD), [range], lbs	94.8 (28.3) [63.5–143.5]
GMFCS (I/II/III)	2/19/3

Abbreviations: GMFCS – Gross Motor Classification System; lbs – pounds; y – years; m – meters

Safety and Feasibility

All participants completed the 1-RM test as described in the protocol and procedures were well-tolerated by all participants based on subjective report by the participants during and after the test and observation of their performance by the examiner. One mild adverse event, a muscle cramp, occurred in 1 participant. After a few minutes of rest, the individual was able to complete testing. No severe muscle soreness which limited daily activity was reported by any participant.

Clinical Outcome Measure: 1-RM Test

Average loads (absolute, adjusted, and normalized), attempts, and lifts per person are presented as a whole and by GMFCS level for participants with CP in Table 2. Those individuals in GMFCS I and II were combined as a single group because the loads lifted across individuals in these functional groups were similar and there were only 2 participants classified as GMFCS level I.

Table 2.

Results

Result	Group (n=24)	GMFCS I&II (n=21)	GMFCS III (n=3)
Absolute load lifted (lbs)(Mean ± SD)	262.4 ± 161.3	277.52 ± 162.39	156.67 ± 127.12
Adjusted load lifted (lbs)(Mean ± SD)	127.0 ± 80.2	134.17 ± 81.09	77.06 ± 62.64
Normalized load lifted* (Mean ± SD)	1.28 ± 0.5	1.35 ± 0.48	0.85 ± 0.53
Average attempts/person ± SD	12.7 ± 3.2	12.76 ± 3.0	12.33 ± 5.13
Average lifts/person ± SD	6.96 ± 2.68	7.19 ± 2.46	5.33 ± 4.16

Abbreviations: lbs – pounds

^*data was normalized by body weight in pounds

Sixteen of 24 participants (66.7%) successfully lifted the same or heavier load after 1 failed attempt and 12 of 24 (50%) were able to lift the same or heavier load after a second failure. Eight of 24 participants (33.3%) were able to lift the same or heavier load after their third and fourth failures. A MRMT was used to estimate the 1-RM in 10 of 24 participants (41.7%). All participants who completed the MRMT performed the test to muscle failure meaning that they completed leg presses until they could no longer lift the load. The average number of presses performed during the MRMT test was 4 ± 1.5.

Discussion

This study documented a standardized 1-RM testing protocol in children and adolescents with CP and who could walk. Secondly, the 1-RM testing protocol was safe and feasible for participants and adolescents with CP during a leg press exercise. Based on our observations, modifications to adult protocols, such as the number of attempts at a given load, are needed for children with CP. The standardized testing procedures presented here will improve clinical implementation and reliability of the 1-RM test in youth with CP.

One common concern regarding maximal strength testing in children is musculoskeletal injury or injury to the growth plate. Several case studies in children who are TD have reported epiphyseal plate injury with heavy resistance training, but these injuries were attributed to excessive loads, inappropriate lifting and training techniques, and lack of adult supervision.³ There have been no reports of growth plate injury in any prospective resistance training studies using a 1-RM in TD youth.^8,25,26 While 1 participant in this study experienced a muscle cramp, no individual experienced serious injury with testing, which is consistent with previous reports using the 1-RM.^8,9 All participants in this study completed testing and most enjoyed the activity.

Current adult strength testing guidelines suggest the 1-RM should be reached within 5 progressive lifts,⁴ which is less than what has been observed in children who are TD.⁸ Faigenbaum et. al performed 1-RM testing on a cohort of 96 children and noted that a range of 7–11 lifts were needed to reach their 1-RM. The authors attribute this finding to the inexperience of the participants with weight training and stressed the importance of providing multiple attempts at a given load.⁸ We reported a slightly wider range, 4–13 lifts, to reach the 1-RM in our cohort. This is not surprising given the heterogeneity in motor function for individuals with CP in our sample (GMFCS levels I-III). In addition to being novice lifters, children with CP may have deficits in motor control, mobility, and cognition which may impact their performance on a 1-RM test. Thus, giving multiple attempts at a given load is important to ensure accurate results because a child with CP may be more inconsistent with performance than an adult or child without CP. Our data support that 67% of participants with CP were able to successfully lift a given load after one failed attempt and 50% were successful after 2 failed attempts. Terminating the test after the first failed press, as recommended in adult protocols, would have resulted in a 19.0% underestimation in the 1-RM. This equates to an underestimation, on average, of 40 pounds which would reduce the prescribed load for a resistance training intervention.

A MRMT was used to estimate the 1-RM in 42% of the participants tested in this study, when following the decision tree algorithm (Figure 2). Because participants had up to 4 attempts to lift a given load, the MRMT was incorporated into the algorithm to improve efficiency as the 1-RM can be time consuming, when factoring in rest breaks. Estimations of maximal strength using a MRMT have been reported to be more accurate when 5 or less repetitions are performed to muscle failure.^4,17,27 Those who used the MRMT in this study completed an average of 4 presses before they were unable to lift the load successfully (muscle failure), which maximized the accuracy of this test for determining the 1-RM. The protocol and decision tree were designed to increase the chances of being able to complete 5 or fewer presses at a given load during the MRMT.

Variations of the MRMT reported in this study have been used in those with CP using a 6-, 8- or 12-repetition maximum^13,16 during the performance of functional tasks, such as the sit-to-stand, lateral step-up, or half knee-rise, using loaded weight vests. While these are appropriate estimations of maximal strength, we developed a protocol to achieve muscle failure with 5 or less repetitions as this results in the most accurate measure of maximal strength. If clinicians use a MRMT using different equipment or during functional tasks, we recommend performing the lowest number of repetitions possible to failure to achieve the most accurate estimation. Repetition maximum tests should be distinguished from functional strength “tests”, such as the sit to stand test, lateral step up test, and half kneel to stand.^28,29 These tests require participants to perform as many repetitions as possible in a set amount of time²⁸ without external load or measure the amount of time to perform a task (i.e., 5-repetition sit-to-stand test).²⁹ These functional strength tests are clinical outcome measures that are reliable and can measure change following an intervention, but they cannot be used to dose an intervention unless modified to be used in a MRMT protocol with external weight vests or weighted backpacks. However, we recommend a partial gravity-eliminated position, such as shown here, for safety and feasibility.

Limited data exists for comparison as only 3 studies have performed 1-RM testing in individuals with CP.^9,10,30 Taylor et. al reported normalized 1-RM values of 1.69 in a strength training study in youth with CP.¹⁰ In an earlier study, Taylor et. al found normalized 1-RM strength values to be 0.68 in adults with CP prior to participation in a community strength training program.³⁰ While Kaya Kara et. al also performed a 1-RM in youth with CP, testing was unilateral, therefore, measures cannot be compared to our values or those previously published.⁹ Our study found mean normalized strength value of 1.28 which is different than values previously reported. The discrepancies among normalized strength values across studies may be explained, in part, by differences in age, GMFCS levels, equipment type, method of testing, or the anthropometric factor used for normalization of strength data. Data from our laboratory supported that age, height and weight significantly contributed to the variability in knee extensor muscle strength in a group of 53 participants with CP (r² = .10 p = .024; r² = .45 p<.001; r² = .38 p<.001, respectively) (unpublished data). While body weight is most commonly used to normalize lower extremity strength data, factors such as height and age may also contribute to the variability in lower extremity strength scores in youth with CP.

Participants with CP have lower extremity muscle strength deficits that are 50–64% lower than TD peers.^18,20,31 Moreover, those with greater mobility limitations have more profound muscle weakness.^18,32 Our study found similar trends when we examined strength data by functional level. Individuals categorized in GFMCS level III had normalized strength values that were 43.7% less than those classified in GMFCS levels I/II. While there were only 3 individuals in the GMFCS III group, these participant’s strength values were nearly half of their peers with greater mobility levels. We believe this finding was significant to report despite the small sample size. It should also be noted that significant differences may be present between those in GMFCS levels I and II, however, there were not enough individuals classified as GMFCS level I (n=2) for us to complete this sub-analysis.

Muscle strength is significantly related to walking capacity,^18,31 thus, many resistance training interventions target muscle strength in hopes of improving functional mobility in those with CP. Unfortunately, previous reviews have identified the limited effectiveness of strength training interventions at improving walking ability and partly attributed these outcomes to issues with dosing, more specifically, exercise intensity.^2,11,12 While many cite the appropriate exercise intensity of 60–85% of 1RM, the majority of strength training studies do not use a maximal strength test to dose resistance interventions.^{9,10,13–16,30} Most perform submaximal estimations of the 1-RM using greater than 5 lifts which compromise the accuracy of the strength assessment.¹⁷ Only 3 studies (2 in youth^9,10 and 1 in adults³⁰) have performed 1-RM testing in individuals with CP^9,10,30 and only 1 of these studies used the results of the test to dose the resistance training intervention.⁹ It is likely that studies which did not perform maximal strength testing to determine the training load in participants with CP were under-dosing the intervention, which may partially explain the lack of significant findings in these studies.^11,12 The intensity, or load, of a resistance training program is the most important factor in driving change in muscle structure and function. Therefore, we believe that performing a maximal strength test like the 1-RM is critical in determining the initial training load because it provides the most accurate measure of muscle strength.

Limitations

Several limitations should be mentioned in this study. First, a single 1-RM test is somewhat time intensive taking 30–45 minutes to complete. This can be accomplished in a single physical therapy session; however, it would consume the entire visit. Therefore, we incorporated the MRMT into the 1-RM protocol for improved efficiency. Given the importance of exercise intensity in driving changes in muscle, we believe that using an accurate test like the 1-RM is important for achieving the appropriate dose and optimizing outcomes for resistance training interventions. Second, our specific leg press device requires participants to grip a handle bar to support their load during the test. This may be a difficult activity for people with more upper extremity involvement. For some of our participants with upper extremity weakness, we used weight lifting hooks, which strap around the wrist and distribute the load through the entire upper extremity, to allow them to manage the heavier loads. In addition, procedures for maximal strength testing for the upper extremity should be developed and evaluated in participants with CP.

Conclusion

This study determined the safety and feasibility of 1-RM testing using a leg press activity in participants with CP. Our findings are consistent with previous reports that performed maximal strength testing in children who are TD without adverse effects.⁸ Future studies should examine the safety and feasibility of 1-RM testing using other weight machines or modes of testing (functional tasks, free weights, etc.) and in different age groups of individuals with CP. Participants with CP should be given multiple attempts at a given load. Cessation of testing after 1 failure leads to an underestimation in strength of nearly 20%, as 67% of youth continued lifting heavier loads if given additional attempts. Use of a gold standard maximal strength assessment like the 1-RM may prevent under-dosing and improve the efficacy of resistance training interventions in children with CP.

What This Adds to the Evidence

This study provides a step by step guide for clinicians and researchers to perform 1-RM testing in youth with CP. Although we used a leg press in this study, the 1-RM and MRMT protocol can be applied to functional tasks using weight vests, free weights, or other activities. In addition, performing a 1-RM on leg press machine is a safe and feasible method for measuring maximal strength in youth with CP. Participants with CP need multiple attempts at a given load to successfully perform 1-RM testing.