The Effects of Floor Isometric Trunk Extension Exercise on Muscle Thickness and Activation Vary Between Different Combinations of Duration and Repetition Number.

Context: Intermittent floor trunk extensions are popular exercises in group fitness programs. The aim of this study was to investigate whether fewer repetitions of longer isometric trunk extension efforts compared with more repetitions of shorter isometric contractions have different acute effects on muscle thickness and activation as well as perceived exertion.

Design: This study followed a cross-sectional design.

Methods: Twenty healthy young males performed floor prone trunk extension exercises using 3 different exercise protocols of repetition and duration: 10 × 5 seconds (D10 × 5), 2 × 25 seconds (D2 × 25), and 5 × 10 seconds (D5 × 10). Ultrasound multifidus thickness and rate of perceived exertion on a 10-point scale were measured immediately after each protocol. Electromyographic activation from the erector spinae, multifidus, and gluteus maximum during each protocol was measured using bipolar surface electrodes.

Results: The longer duration (D2 × 25) protocol showed a significant greater rate of perceived exertion (6.22 [0.73]) and rest multifidus thickness change (median: 8.04%) compared with the other protocols (P < .05). Within each protocol, root mean square of all muscles increased from trial to trial in the D2 × 25 and D5 × 10 (P < .05), but not during the D10 × 5 protocol (P > .05). The maximum root mean square was achieved in the shorter duration (D10 × 5) protocol compared with the other ones (P < .05).

Conclusion: If trunk extension exercises on the floor are used in a training setting, then using exercises with long duration and fewer repetitions may elicit a greater metabolic response.