Acute Effects of Isometric Trunk Extension Exercise on Muscle Tone, Stiffness, and Flexion-Relaxation Ratio in Healthy Adults

Background Low-intensity trunk extension exercises have been reported to influence muscle recruitment and viscoelastic properties, but the relationships among these variables remain unclear. Objectives To examine immediate changes and explore correlations among muscle tone, stiffness, and FR ratio before and after trunk extension exercise. Methods Twenty healthy participants performed a single session of isometric trunk extension exercise in prone position (six 20-s contractions, 3-min rest intervals). Muscle tone (Hz) and stiffness (N/m) were measured using a myotonometer, and FR ratios were calculated from surface electromyography (EMG) at thoracic (T-level) and lumbar (L-level) erector spinae. Pre-post differences were analyzed using paired t-tests, and correlations between FR ratio and viscoelastic variables were examined using Pearson coefficients (α = 0.05). Results After exercise, muscle stiffness significantly increased (p = 0.020), while muscle tone showed no significant change (p = 0.120). The FR ratio at the T-level significantly increased (p < 0.001), whereas the L-level FR ratio showed no difference (p = 0.430). The T-level FR ratio was negatively correlated with both muscle tone and stiffness (r = -0.58 to -0.60, p < 0.05), while no significant correlation was found at the L-level. Conclusions A single session trunk extension exercise induced a significant increase in thoracic FR ratio and muscle stiffness, suggesting concurrent changes in neuromuscular activation and viscoelastic adaptation. The negative relationship between T-level FR ratio and stiffness indicates that greater recruitment efficiency is associated with lower mechanical resistance. These findings highlight the need to monitor both EMG-based FR ratio and mechanical muscle properties when designing spinal stabilization exercises.