Are trunk muscles weaker in adolescent females with adolescent idiopathic scoliosis compared with their healthy counterparts?

Abstract

BACKGROUND:

Adolescent Idiopathic Scoliosis (AIS) requires complex medical care because of multiple consequences especially on daily activities. Muscular involvement is part of the problem and may be treatable.

OBJECTIVE:

To analyze trunk muscle strength using an isokinetic dynamometer in female adolescents with AIS one year after orthopedic treatment by brace and compare the findings to a matched group of an asymptomatic cohort.

METHODS:

The trunk flexors and extensors strength was measured using an isokinetic dynamometer at 60, 90 and 120 ${}^{\circ}$ /s. Peak Moment (PM), Mean Power (MP) and the flexor/extensor ratio in 100 patients aged 14 to 18 years old were compared to a control group ( $N=$ 32) of asymptomatic age-matched females. In the AIS group, correlation analyses were computed to search for contributing factors to isokinetic performances, including morphological characteristics of patients, as well as clinical and radiological characteristics of the scoliosis.

RESULTS:

The trunk flexors in the AIS group were significantly but moderately (15%) weaker across speeds compared to their control counterparts at all speeds. No parallel weakness was noted for the extensors. While the MP of AIS patients was significantly weaker than that of the controls, 33% for flexors and by 31% for extensors, no significant differences were observed for the F/E ratios. The correlational analyses has indicated that weight and BMI were contributing factors at all speeds.

CONCLUSION:

Adolescents with AIS had weaker trunk extensors and mostly flexors compared to healthy females. Within this AIS population, weight and BMI seem to have a negative impact on muscular performances, whereas clinical and radiological characteristics of the scoliosis do not seem to contribute.

Keywords

Isokinetics adolescent idiopathic scoliosis trunk muscles brace

1. Introduction

Adolescent idiopathic scoliosis (AIS) is a significant public health issue with a global prevalence of 0.47–5.2% [1]. This static disorder causes a postural imbalance [2, 3] with females much more affected than males [4, 5, 6, 7, 8]. AIS contributes to a number of consequences, with repercussions on function, respiratory system, chronic pain and, in the adolescent population especially, esthetic and psychosocial effects [9, 10, 11, 12]. It can be responsible for back pain in adulthood and a flexed posture in the elderly [13, 14].

Trunk muscles are a major contributing factor to spinal postural balance but their strength profile in the adolescent population has been scarcely studied. Previous studies have explored muscle strength using mostly clinical static tests [15, 16] and electromyography [17, 18, 19, 20]. From these evaluations, results concerning scoliotic adolescents indicated a weaker spinal musculature.

Reliability of isokinetic measures of the scoliotic spine have been reported [21] but in the adolescent population, isokinetic studies are poor and based on small size cohorts [22, 23]. Two studies [24, 25] observed weakness of the spinal extensor muscles in adolescents with low back pain. Furthermore, weaker isokinetic values of scoliotic spine have been observed [26]. However, to the best of our knowledge, no isokinetic strength reference values are available for this adolescent population, reflected by a dearth of large size samples. Moreover, no previous study seems to have evaluated trunk muscles strength after the end of brace treatment, coinciding with dwindling medical follow-up, while this critical period between the end of the growth and adulthood is essential to avoid long-term consequences of the scoliosis. One may hypothesize a loss of motivation for reeducation and self-exercises after all the years of orthopedic treatment, leading to weakened muscles.

Thus the main objective of the present study was to analyze, using isokinetic dynamometry, the strength and power of the flexor and extensor trunk muscles of female adolescents one year after the end of brace treatment for AIS, and compare the findings with those derived from an age-matched asymptomatic population. The secondary objective was to explore contributive factors to isokinetic spinal performances within the AIS population.

2. Methods

2.1 Participants

The research cohorts consisted of 2 groups of female adolescents aged from 14 to 18 years old: the AIS group (AISG) with 100 girls and the control group (CG) with 32 age-matched girls. Inclusion criteria were as follow: timing of one year after the end of brace treatment for AISG and no AIS diagnosis for CG. For both groups, no medical contraindication for isokinetic test, and signed agreement by parents for the study. Non-inclusion criteria were recent surgery or scar next to trunk or abdomen, spinal inflammatory rheumatism, heart disease, vascular orphan disease, stress incontinence and/or pregnancy.

2.2 Study design

This was a retrospective study conducted in 2018, from a database started for a previous study and pursued until 2018. Regarding the girls with AIS, every patient treated by brace had a regular follow-up during treatment and, systematically one year after the end of brace treatment, a half-day of ambulatory hospitalization for evaluations was organized. All the patients were automatically included in the global database by the physicians in charge of consultations, if they met the inclusion criteria and if parents have agreed and signed the information and consent documents about the study. Regarding the CG girls, recruitment of healthy adolescent females had been conducted on a voluntary basis, mostly among families of the establishment staff members as well as the families of patients. Information was given before the study to all parents. No x-rays or clinical examination had been pursued. Anthropometric details and sport participation, were recorded while the isokinetic evaluation has been strictly identical to that of the AIS group.

2.3 Evaluation procedure

At first, a spinal radiography was performed with the EOS (EOS imaging, Paris, France) for the AIS group. Several parameters were analyzed: type of scoliosis (lumbar, thoracic, thoracolumbar, double thoracic, combined); pelvic parameters [sacral slope (SS), pelvic incidence (PI), pelvic tilt (PTi)] and spinal parameters [Cobb angle (Cobb ${}_{\text{max}}$ ) and Lenke classification. Severity was rated based on the Cobb angle as follow 0–9 ${}^{\circ}$ , 10–19 ${}^{\circ}$ , 20–29 ${}^{\circ}$ , 30–39 ${}^{\circ}$ , 40–49 ${}^{\circ}$ ; a threshold of 30 ${}^{\circ}$ was also used (Cobb30) to separate scoliosis in 2 subgroups (scoliosis $<$ 30 ${}^{\circ}$ and $>$ 30 ${}^{\circ}$ ), this cut-off being known as a risk of resounding in adulthood [27]; growth indicator (Risser index) was also recorded. A 3D-reconstruction from the EOS system was performed for determination of the vertebral rotation defined as the maximal absolute angle at the scoliosis apex in axial plane (Rot ${}_{\text{max}}$ ).

A medical consultation was then performed for collecting data: anthropometric parameters (age, standing size, weight and BMI); participation in sports (school and after school, hours per week) and clinical evaluation of the scoliosis.

Finally, muscle strength was assessed using the Con-Trex ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ TP 1000 isokinetic dynamometer. The settings included were low-pass filter and no gravity correction. A warm-up of 15 min on treadmill or ergocycle was performed before the standardized standing installation. An initial learning phase for familiarization phase took place. The protocol consisted of 3 series of 4 consecutive concentric flexion-extension contractions over a 0–70 ${}^{\circ}$ anteflexion amplitude, at 3 different speeds (60, 90 and 120 ${}^{\circ}$ /s) with a rest period of one minute between each series. The test was terminated in the event of pain, fainting, vertigo or hypoglycemia.

2.4 Outcome criteria

Three main outcome measures were analyzed: the normalized peak moment PM, in Nm.kg ${}^{-1}$ ) in trunk flexion and extension at the 3 speeds; the mean power (MP, in W.kg ${}^{-1}$ ) in trunk flexion and trunk extension at 120 ${}^{\circ}$ /s and the flexion/extension ratios (R ${}_{\text{F/E}}$ ). Secondary outcomes were factors that could influence isokinetic performances: anthropometrics parameters and scoliosis parameters as previously described.

2.5 Statistical analysis

The statistical analysis was performed using R Software 3.4.2 version. The two groups were compared as follow: for quantitative variables the Student’s t-test in case of normality or by Wilcoxon’s test otherwise were applied. For qualitative variables by Chi-square test or Fisher’s exact test in case of insufficient size. Holm’s method corrections were also performed. Links between isokinetic variables and numeric or ordinal variables were studied using Spearman’s non-parametric correlation. For all tests, significance level was set at 0.05.

3. Results

3.1 Characteristics of participants

The 2 groups were globally homogeneous (Table 1). Control group characteristics were similar except for weight; the healthy adolescents tended to be heavier (2.2 kg, $p=$ 0.062); results were adjusted to this variable and were comparable.

Table 1
Comparison of participants by descriptive characteristics

	AIS ( $n=$ 100)	Controls ( $n=$ 32)	$p$ value
Age (years)	16.1 $\pm$ 0.9	15.8 $\pm$ 1.4	0.14
	16 [15; 17]	16 [14; 17]
Weight (kg)	56.1 $\pm$ 10.5	58.3 $\pm$ 8.6	0.062
	53 [50; 60]	57.5 [53; 62]
Size (cm)	164.1 $\pm$ 6.2	164.2 $\pm$ 5.7	0.89
	164 [159.8; 168]	163 [161.2; 168]
BMI	20.8 $\pm$ 3.6	21.5 $\pm$ 3.4	0.14
(kg.m ${}^{-2}$ )	20.1 [18.6; 22]	21.2 [19.9; 22.3]

Data are summarized by the mean $\pm$ SD and medians [Q1; Q3]. AIS: Adolescent idiopathic scoliosis. BMI: Body Mass Index.

Concerning the radiological characteristics of scoliosis, there was 26% of each thoracic and lumbar, 25% of combined, 16% of thoracolumbar and 7% of double thoracic. Most of the participants had a growth index of Risser 5 (83%). Thirty seven percent of patients had a Cobb ${}_{30}$ subdivision $>$ 30 ${}^{\circ}$ . Rot ${}_{\text{max}}$ was on average 13.5(5.7) ${}^{\circ}$ .

3.2 Isokinetic trunk reference values

The results of the isokinetic tests are detailed in Table 2.

Table 2
Comparison of groups by isokinetic parameters

	AIS ( $n=$ 100)	Controls ( $n=$ 32)	$p$ values		Adjusted $p$ values
PMFL120	1.5 $\pm$ 0.3	1.8 $\pm$ 0.3	$<$ 0.	0001 ${}^{***}$	0.	0003 ${}^{***}$
(Nm.kg ${}^{-1}$ )	1.5 [1.4; 1.7]	1.8 [1.6; 2]
PMFL90	1.6 $\pm$ 0.3	1.8 $\pm$ 0.3	$<$ 0.	0001 ${}^{***}$	0.	0002 ${}^{***}$
(Nm.kg ${}^{-1}$ )	1.5 [1.4; 1.7]	1.8 [1.6; 2]
PMFL60	1.6 $\pm$ 0.3	1.9 $\pm$ 0.3	$<$ 0.	0001 ${}^{***}$	0.	00043 ${}^{***}$
(Nm.kg ${}^{-1}$ )	1.6 [1.4; 1.8]	1.9 [1.7; 2]
PMEXT120	1.6 $\pm$ 0.4	1.9 $\pm$ 0.6	0.	039 ${}^{*}$	0.	27
(Nm.kg ${}^{-1}$ )	1.6 [1.4; 1.9]	1.7 [1.5; 2.4]
PMEXT90	1.8 $\pm$ 0.5	2 $\pm$ 0.7	0.	067	0.	4
(Nm.kg ${}^{-1}$ )	1.7 [1.5; 2]	1.9 [1.6; 2.8]
PMEXT60	2 $\pm$ 0.6	2.1 $\pm$ 0.7	0.	17	0.	67
(Nm.kg ${}^{-1}$ )	1.8 [1.6; 2.3]	2 [1.6; 2.7]
RPM120	96.5 $\pm$ 16.6	104.9 $\pm$ 37	0.	56	1
	95.2 [84.8; 107]	98.7 [79.4; 121]
RPM90	90.1 $\pm$ 17.5	100.4 $\pm$ 39.9	0.	44	1
	89 [76.7; 99]	93.3 [73.1; 117.3]
RPM60	84.3 $\pm$ 17.1	95.7 $\pm$ 34	0.	13	0.	67
	83 [73.5; 95.1]	89.5 [72.7; 106.6]
MPFL120	0.8 $\pm$ 0.3	1.5 $\pm$ 0.7	$<$ 0.	0001 ${}^{***}$	$<$ 0.	0001 ${}^{***}$
(W.kg ${}^{-1}$ )	0.8 [0.7; 1]	1.4 [0.8; 2.1]
MPEXT120	0.9 $\pm$ 0.4	1.6 $\pm$ 1	0.	00048 ${}^{***}$	0.	0034 ${}^{**}$
(W.kg ${}^{-1}$ )	0.8 [0.7; 1.1]	1.2 [0.8; 2.3]
RMP120	98.6 $\pm$ 26.9	105.7 $\pm$ 33.3	0.	41	1
	95.8 [79; 112.5]	101 [80; 124.2]

Data are summarized by the mean $\pm$ SD and medians [Q1; Q3]. AIS: Adolescent idiopathic scoliosis. PMFL120: Peak Moment for flexion at 120 ${}^{\circ}$ .sec ${}^{-1}$ . PMFL90: Peak Moment for flexion at 90 ${}^{\circ}$ .sec ${}^{-1}$ . PMFL60: Peak Moment for flexion at 60 ${}^{\circ}$ .sec ${}^{-1}$ . PMEXT120: Peak Moment for extension at 120 ${}^{\circ}$ .sec ${}^{-1}$ . PMEXT90: Peak Moment for extension at 90 ${}^{\circ}$ .sec ${}^{-1}$ . PMEXT60: Peak Moment for extension at 60 ${}^{\circ}$ .sec ${}^{-1}$ . RPM120: ratio PMFL120/ PMEXT120. RPM90: ratio PMFL90/PMEXT90. RPM60: ratio PMFL60/PMEXT60. MPFL120: Mean Power for flexion at 120 ${}^{\circ}$ .sec ${}^{-1}$ . MPEXT120: Mean Power for extension at 120 ${}^{\circ}$ .sec ${}^{-1}$ . RMP120: ratio MPFL120/MPEXT120. Nm.kg ${}^{-1}$ : Newton-meter per kg. W.kg ${}^{-1}$ : Watts per kg. Significance level: ${}^{*}<$ 0.05; ${}^{**}<$ 0.01; ${}^{***}<$ 0.001.

3.2.1 PM values

For AIS teens, the normalized values were on average between 1.5 and 1.6 Nm.kg ${}^{-1}$ for flexors and 1.6–2 Nm.kg ${}^{-1}$ for extensors. For the control group, the respective ranges were 1.8–1.9 Nm.kg ${}^{-1}$ and 1.9–2.1 Nm.kg ${}^{-1}$ .

3.2.2 MP values

For AIS teens, the mean normalized values were 0.8 and 0.9 W.kg ${}^{-1}$ for the flexors and extensors, respectively whereas for the control group they were 1.5 W.kg ${}^{-1}$ and 1.6 W.kg ${}^{-1}$ , respectively.

3.2.3 $R_{\textit{F/E}}$ values

For the AIS participants, the mean ratios were 0.84–0.96 for PM and around 0.99 for MP. For the control group, they were respectively 0.96–1.05, and around 1.06.

3.3 AIS values vs control values

Concerning the PM values, there were significant differences between the groups for trunk flexors, weaker in AIS participants, but no difference for trunk extensors. As for the MP values, there were significant differences between the groups for both trunk flexors and extensors, with weaker performances for AIS participants. Concerning ratios of PM and MP, no significant differences were found between the groups, in terms of the ratios.

3.4 Relationship between the isokinetic and scoliosis parameters

The findings are detailed in Table 3.

There was an impact of 2 major variables on isokinetic performances, especially for high speeds and for the extensors: weight and BMI. This correlation was negative, higher weight or BMI were associated with weaker performance during isokinetic exercises.

Regarding weight, there was a significant correlation in PM for trunk extensors and flexors especially for high speeds, and also for trunk extensors at low speed. No significant correlation was found between MP and weight. There was nonetheless a significant impact of weight on $R_{\text{F/E}}$ for PM and MP at high speed.

Regarding BMI, there was a significant correlation in PM for trunk flexors and mainly for extensors, at all speeds. Correlation was only significant with MP for extensors. Concerning ratios, BMI was highly correlated to $R_{\text{F/E}}$ for PM and MP at high speed, and correlations were decreasing when speeds were lowering for PM.

Ages classes (14–16 and 17–18 years old) came out also with significant correlation for PM and PM ratios, with best ratios (lower) for the 17–18 class. Some correlations were found with size, but haphazardly, and so without any clinical meaning. No other clinical or radiological variables were correlated with the isokinetic parameters.

4. Discussion

To the best of our knowledge, this is the first study which explores isokinetic strength parameters in a large cohort of scoliotic female adolescent evaluated one year following the termination of brace treatment with an additional objective of setting reference isokinetic values.

Table 3
Correlation analysis between isokinetic values and AIS quantitative variables

	PMFL120 (Nm.kg ${}^{-1}$ )	PMFL90 (Nm.kg ${}^{-1}$ )	PMFL60 (Nm.kg ${}^{-1}$ )	PMEXT120 (Nm.kg ${}^{-1}$ )	PMEXT90 (Nm.kg ${}^{-1}$ )	PMEXT60 (Nm.kg ${}^{-1}$ )	RPM120	RPM90	RPM60	MPFL120 (W.kg ${}^{-1}$ )	MPEXT120 (W.kg ${}^{-1}$ )	RMP120
Ag (Years)	NS	NS	NS	$p<$ 0.05	NS	NS	$p<$ 0.05	$p<$ 0.05	$p<$ 0.01	NS	NS	NS
Weight (Kg)	$p<$ 0.05	$p<$ 0.05	NS	$p<$ 0.00	$p<$ 0.00	$p<$ 0.05	$p<$ 0.01	NS	NS	NS	NS	$p<$ 0.01
Size (cm)	NS	NS	NS	NS	NS	NS	NS	NS	$p<$ 0.05	$p<$ 0.05	$p<$ 0.05	NS
BMI (kg.m ${}^{-2}$ )	$p<$ 0.05	$p<$ 0.01	$p<$ 0.05	$p<$ 0.00	$p<$ 0.00	$p<$ 0.00	$p<$ 0.00	$p<$ 0.01	$p<$ 0.05	NS	$p<$ 0.05	$p<$ 0.00
Risser	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
SS (degrees)	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
PI (degrees)	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
PTi (degrees)	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
Cobb ${}_{\text{max}}$ (degrees)	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
Cobb ${}_{\text{grp10}}$	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
Rot ${}_{\text{max}}$ (degrees)	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS

Spearman correlation test. AIS: adolescent idiopathic scoliosis. BMI: Body Mass Index. Risser: growth index on X-rays. SS: Sacral Slope. PI: Pelvic Incidence. PTi: Pelvic Tilt. Cobb ${}_{\text{max}}$ : maximal angle of scoliosis. Cobb ${}_{\text{grp10}}$ : classification of scoliosis severity by intervals of 10 ${}^{\circ}$ . Rot ${}_{\text{max}}$ : maximal rotation angle of vertebra in axial view. PMFL120: Peak Moment for flexion at 120 ${}^{\circ}$ .sec ${}^{-1}$ . PMFL90: Peak Moment for flexion at 90 ${}^{\circ}$ .sec ${}^{-1}$ . PMFL60: Peak Moment for flexion at 60 ${}^{\circ}$ .sec ${}^{-1}$ . PMEXT120: Peak Moment for extension at 120 ${}^{\circ}$ .sec ${}^{-1}$ . PMEXT90: Peak Moment for extension at 90 ${}^{\circ}$ .sec ${}^{-1}$ . PMEXT60: Peak Moment for extension at 60 ${}^{\circ}$ .sec ${}^{-1}$ . RPM120: ratio PMFL120/ PMEXT120. RPM90: ratio PMFL90/PMEXT90. RPM60: ratio PMFL60/PMEXT60. MPFL120: Mean Power for flexion at 120 ${}^{\circ}$ .sec ${}^{-1}$ . MPEXT120: Mean Power for extension at 120 ${}^{\circ}$ .sec ${}^{-1}$ . RMP120: ratio MPFL120/MPEXT120. Nm.kg ${}^{-1}$ : Newton-meter per kg. W.kg ${}^{-1}$ : Watts per kg. Significance level: NS $=$ not significant $p<$ 0.05 $=$ significant $p<$ 0.01 $=$ very significant $p<$ 0.001 $=$ highly significant.

The main results of the present study show weaker flexor trunk muscles in the AIS population than in asymptomatic adolescents. The PM at the lower speeds (60 and 90 ${}^{\circ}$ /s), AIS flexors were up to 17% weaker than in the asymptomatic participants. On the other hand, no significant difference was found for the extensors. However, the MP which was evaluated with respect to the higher speed (120 ${}^{\circ}$ /s) indicated that the girls with AIS were down 33 and 31% in the flexors and extensors, respectively, compared to the healthy cohort. These results confirm those reported by Skrzek et al. [26] on a population of 35 AIS adolescents without brace treatment, indicating a significant difference for PM mostly on flexors ( $p=$ 0.003) and extensors ( $p=$ 0.042), whereas in term of MP, only flexors were weaker at ( $p<$ 0.001). Importantly, the AIS cohort in Skrzek’s was not treated with brace and yet the results were similar, possibly implicating the disorder rather than the treatment in the observed deficiency. Notably, Fayolle-Minon and Calmels [28] contradicted any impact of brace on trunk muscles. A possible explanation for the higher reduction in power compared to strength relates to the actual definition of power which is the rate at which work is being expanded. While two isokinetic curves may be of identical PM, the value of the Work (the integral of the curve) may differ [29]. As a result the MP relating to the curve with the smaller Work will respectively be lower. This distinction was highlighted in a previous paper which has underlined the role of Work as a potentially sharper identifier of gains/losses than the PM [30]. It is thus suggested that in girls with AIS, the Work during individual contractions may be more compromised than their ability to reach maximal strength as expressed by the PM.

Regarding ratios, no significant difference was found in terms of PM or MP. The respective values differ in the scientific literature: Bernard et al. [25] found similar ratios (0.81–0.94) in a group of adolescents with low back pain, but lower for their control group (0.75–0.95). Skrzek et al. [26] found average ratios of 0.76 in AIS and control groups. Barczyk et al. [22] found ratios of between 0.65 and 0.69 in a population of young girls (10–11 years old) with sagittal postural disorders while and the ratios in the control group were 0.62–0.74. However, these findings should be considered with caution, because of the variability of the populations analyzed and the different isokinetic protocols.

Regarding the effect of the confounding variables on the isokinetic parameters, we showed that weight and BMI have a negative impact: the higher were weight and BMI of AIS adolescents, the lower was the isokinetic performances. This influence was mostly apparent relative to the extensors, but also on flexors. To avoid overestimation bias from gravity parameters, correlation analyses of these variables have been also computed adding gravity correction. The results were the same: weight and BMI were negatively correlated with the isokinetic performances. To our knowledge, no others publication have mentioned any relation between weight or BMI on isokinetic results, except Bernard et al. [25] in low back pain adolescents but they found a positive impact of BMI on isokinetic performances while our findings suggested the opposite. Elements like impedance measurement [31, 32] or DXA scan [33] could be useful to be more precise on weight and BMI analyses.

In terms of the radiographic data, we did not find any correlation between pelvic spinal parameters and isokinetic performance, in line with Abelin-Genevois et al. [34] who have reported no impact of pelvic parameters on scoliosis occurrence. The most surprising concerns the severity and the types of scoliosis: no significant difference was found on muscle strength for any degree nor any type of scoliosis. These results support a recent study [35] which demonstrated no relationship between isokinetic performances and scoliosis parameters in adult population. We can assume a lack of power for the different Cobb ${}_{\text{grp10}}$ , and a too high Cobb ${}_{30}$ cutoff; indeed only 37% of AIS patients had a scoliosis of more than 30 ${}^{\circ}$ . The sensitivity of the isokinetic measures could be insufficient for detecting any impact of scoliotic deformation; or muscular weakness could be a symptom of scoliosis more than a consequence. It is also important to note that evaluations were pursued one year after the end of treatment; the efficiency of brace treatment being possibly similar across all ranges of severity of scoliosis.

Regarding vertebral rotation, we have focused on computer 3D reconstruction of X-rays with EOS system, in order to obtain rotational values in an axial view. We found no correlation between the level of rotation and muscular performances. We chose to select one parameter (axial rotation) for detecting an impact in the axial plane but we cannot deny the fact that other parameters could have been more efficient, like torsional index [34]. It could also mean that the deformity of the scoliosis was not severe enough to have an impact on these criteria of muscular analysis, meaning that eccentricity of the apical vertebra does not have a repercussion on muscular dynamism.

There is quite a number of limitations to the present study, mandating care in straightforward application of the results and their clinical significance.

First, we studied adolescent females therefore generalization of our results to the adolescent males must be considered with caution. Second is the timing protocol of evaluations, only one year after the end of brace treatment, without any evaluation before the brace treatment. Although this may become the subject of a future study there were several reasons for our primary choice not to provide evaluation before treatment. Comparison with analyses before treatment would be difficult to interpret, because of the important impact of growth on muscular development, while some children are too young to be evaluated isokinetically before treatment (the installation is requiring minimal size). We did not have a control group of non-treated AIS. However non-treated participants are usually those who do not seek medical attention, because of moderate or non-existent discomfort, and so we do not see them in medical consultation in first place.

Another important limitation was the lack of information about rehabilitation during and after treatment. Recently Negrini et al. [36] showed that specific scoliosis exercises were better than classical rehabilitation, and others studies [37, 38, 39] provided arguments about muscular training in idiopathic scoliosis.

Concerning the technical tools used on isokinetic dynamometer, we chose not to use a gravity correction parameter. The majority of studies in adolescents [22, 24, 25, 26, 40, 41] fail to mention whether they use this tool or not, so there is no standard protocol exists. Furthermore, gravity correction is supposed to consider the weight of the body segment in motion (i.e. the trunk). However, lower limbs and hips muscles also support the flexion-extension motion of the trunk, and are not integrated in the gravity correction. Hence the values may significantly differ with or without gravity correction. Moreover, the purpose of our study was to evaluate muscle values for a clinical interpretation in daily activities, and gravity is an integral part of daily life with important spinal constraints. Another limitation about the isokinetic protocol was the endurance parameter, usually evaluated with repeated series at high speed (at least 10 repetitions or more, at 120 ${}^{\circ}$ /s). Due to a lack of data, we analyzed neither this parameter nor the fatigue index.

Finally, a major limitation of our study was the information about the physical activity in both groups. Exercise data were collected through self-report, without any precise information relating to the level or the intensity of practice. Therefore, there was a meaningful difference of practice between the 2 groups in favor of the control group, for school-related practice ( $p=$ 0.02) and especially for extra-curricular practice ( $p=$ 0.0005). Concerning school-related practice, the difference was limited between groups, on average 1.9 h/w for a maximum of 4 h/w while for extra-school practice it was 1.6–17 h/w resulting in a highly heterogenous load. In the AIS group, the duration of physical activity was evaluated for having a potential impact on isokinetic performances, but this was not the case. The lack of information on the level and the intensity of practice could explain these results. Negrini et al. [24] analyzed spinal muscle strength with exercise in an adolescent group with low back pain and found better isokinetic performances in the group of athletic adolescents compared to those who practiced leisure exercise and those who were inactive. Thus, the level and intensity of practice could be an important influencing factor.

5. Conclusion

In adolescent girls with AIS, one year after the end of brace treatment, there is a decline in strength and power, in the trunk flexors and extensors, compared to a control group of asymptomatic adolescents. Within this AIS population, weight and BMI seem to have a negative impact on muscular performances. Radiological characteristics of the scoliosis do not seem to have any influence on muscle strength. This work underlines the important role of muscular evaluation, and will allow targeting the rehabilitation of AIS patients during and following brace treatment.

Author contributions

CONCEPTION: Jean-Claude Bernard and Grégoire Le Blay.

PERFORMANCE OF WORK: Lénaïc Minjollet, Liza Sakoun, Anne Pujol and Gautier De Chelle.

INTERPRETATION OR ANALYSIS OF DATA: Lénaïc Minjollet and Jean-Claude Bernard.

PREPARATION OF THE MANUSCRIPT: Lénaïc Minjollet and Jean-Claude Bernard.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Lénaïc Minjollet, Jean-Claude Bernard and Kariman Abelin-Genevois.

SUPERVISION: Jean-Claude Bernard.

Ethical considerations

The study was performed in compliance with the Helsinki Declaration and with French law. As a retrospective analytic study, it was accepted by the local ethics committee from French Red-Cross. All parents received a written letter with information about the study and provided consent for participation.

Funding

None.

Footnotes

Acknowledgments

The authors wish to warmly thanks Mrs Rachel Bard-Pondarré and the Association de Recherche Médicale du Centre des Massues for the support and help in designing the manuscript; Sahara Graf and Stéphane Verdun from Lille Catholic hospitals, Biostatistics Department – Delegation for Clinical Research and Innovation, Lille Catholic University, Lille, France, for their help with statistical analyses; Monique Mendelson (PhD) for her help with the manuscript writing.

Conflict of interest

The authors declare that there is no conflicts of interest regarding the publication of this paper.

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Are trunk muscles weaker in adolescent females with adolescent idiopathic scoliosis compared with their healthy counterparts?

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.2 Study design

2.3 Evaluation procedure

2.4 Outcome criteria

2.5 Statistical analysis

3. Results

3.1 Characteristics of participants

Table 1 Comparison of participants by descriptive characteristics

Table 2 Comparison of groups by isokinetic parameters

3.2.2 MP values

3.2.3 R F/E values

3.3 AIS values vs control values

3.4 Relationship between the isokinetic and scoliosis parameters

4. Discussion

Table 3 Correlation analysis between isokinetic values and AIS quantitative variables

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Comparison of participants by descriptive characteristics

Table 2
Comparison of groups by isokinetic parameters

3.2.3 $R_{\textit{F/E}}$ values

Table 3
Correlation analysis between isokinetic values and AIS quantitative variables