DO PARASPINAL MUSCLE IMPROVEMENTS MEDIATE THE EFFECTS OF PHYSIOTHERAPEUTIC SCOLIOSIS‑SPECIFIC EXERCISE ON THE COBB ANGLE IN ADOLESCENTS WITH IDIOPATHIC SCOLIOSIS? A MEDIATION ANALYSIS OF A RANDOMIZED CONTROLLED TRIAL

Background
Physiotherapeutic scoliosis-specific exercises (PSSE) are known to reduce Cobb angle
progression in adolescents with idiopathic scoliosis (AIS) and improve paraspinal muscle
function. However, it remains unclear whether muscle improvements are part of the
mechanism driving spinal correction.
Study Design
A post-hoc mediation analysis of a 24-week assessor- and statistician-blinded randomized
controlled trial.
Objective (s)
To examine whether improvements across distinct domains of paraspinal muscle function
[metabolic (oxygen recovery asymmetry), neuromuscular (activation symmetry), and
biomechanical (tone and stiffness asymmetry)] mediate the effect of PSSE on the Cobb
angle and, exploratorily, on the angle of trunk rotation (ATR).
Methods
We randomized 102 adolescents with right thoracic or thoracolumbar AIS (Cobb angles:10–
45°, Risser 0–3) to either a 24‑week PSSE program plus standard care or standard care
alone (n=51 per group). The primary outcome was the change in Cobb angle (ΔCobb). Four
potential mediators were analyzed simultaneously: changes in oxygen recovery time
asymmetry measured by near-infrared spectroscopy at T9 (ΔTrAsy_T9), surface EMG
symmetry index at the apex (ΔsEMGSI_apex), and absolute muscle asymmetry ratio for
tone at the apex (ΔAMAR_tone_apex) and stiffness at the upper end-vertebra
(ΔAMAR_stiffness_UEV) assessed by handheld myotonometry. The levels were chosen
based on the largest treatment effect observed for these mediators. The Mediation analysis
of the change in ATR (ΔATR) was exploratory. Parallel mediation analyses (PROCESS
model 4, 5,000 bias‑corrected bootstrap samples) estimated specific and total indirect
effects for the mediators, adjusting for sex, Risser stage, and baseline values (Cobb angle
for the primary analysis; ATR and Cobb angle for the ATR analysis).
Results
At 24 weeks, PSSE yielded significantly better improvements in mean Cobb angle (−2.8°,
p<0.001) and mean ATR (−3.2°, p<0.001) than controls. PSSE significantly improved all
four muscle mediators (ΔTrAsy_T9: b=8.69; ΔsEMGSI_apex: b=1.57; ΔAMAR_tone_apex:
b=0.26; ΔAMAR_stiffness_UEV: b=0.62, p<0.001). However, among candidate mediators,
only ΔAMAR_tone_apex showed a significant indirect effect on Cobb angle reduction
(b=−1.16, 95%CI [−2.00, −0.50]), accounting for 34% of the direct effect. No other individual
or total indirect effects were significant for either the Cobb angle or ATR. The direct effects
of PSSE on both Cobb angles and ATR remained significant after accounting for mediators
(ΔCobb:3.40, ΔATR:4.33, both p<0.001), revealing a pattern consistent with a suppression
effect.
Conclusion(s)
PSSE significantly improves Cobb angle, ATR, and paraspinal muscle function, but these
muscle adaptations only partly explain its therapeutic effect. Only reduced muscle tone
asymmetry at the apex partially mediated the Cobb angle reduction. Most PSSE’s benefits
operate through other complex mechanisms (including possibly proprioception, rotational
breathing…) not measured here.
Clinical significance
Clinicians should continue to employ a holistic PSSE approach as its benefit extends
beyond simple muscle strengthening or balancing.