Isokinetic assessment of trunk flexor and extensor performance in asymptomatic adult women (20–55 years): reference values and effects of age and angular velocity

Trunk muscle strength is crucial for spinal stability and injury prevention. Existing studies generally focus on clinical or male individuals with a limitation in reference values for asymptomatic women. This study aim was to establish sample-based normative values for trunk muscle performance in asymptomatic women and examine the effects of age and angular velocity using isokinetic dynamometry. 150 asymptomatic women (20–55 y) were assessed for trunk muscle performance using isokinetic dynamometry and electromyographic (EMG) analysis. Linear mixed models (LMM) were used to analyze trunk muscle performance data, treating age as a continuous predictor and angular velocity as a repeated-measures factor, while accounting for individual variability through random effects. EMG amplitudes were normalized to peak dynamic activation within each testing condition, with extensor activity of 52 % during extension and flexors ranging from 43 % to 45 % during flexion. LMM showed minimal age effects, while angular velocity was associated with variations in several outcomes, including torque, power, and total work. Sample-based normative values for trunk muscles were established at 60, 90, and 120°/s. Within this convenience sample of asymptomatic women aged 20–55 years, age effects were small, supporting the use of a single dataset. Angular velocity conditions accounted for a greater proportion of the observed variability in trunk isokinetic outcomes than age-related differences. Age showed limited influence on strength, power, and EMG activity within the investigated age range. Trunk muscles exhibited a force–velocity pattern, with torque decreasing and power increasing as angular velocity increased.