Appropriate trunk muscle activation is important for maintaining trunk stiffness during activities
of daily living and sports. While numerous studies have compared different trunk muscle
activation techniques, fewer studies have investigated the effects of different verbal instructions
on trunk muscle activity. The aim of this study was to compare the effects of different verbal
instructions on trunk muscle activity. The objectives were to: (1) compare trunk muscle
electromyographic (EMG) signal amplitudes during voluntary exertion under different verbal
instructions; (2) compare muscle coactivation between different verbal instructions; and (3)
compare EMG amplitudes and muscle coactivation between two body positions (standing and
sitting) for each verbal instruction.
The study compared the activity of the m. erector spinae (ES), mm. multifidi (Mf), m. transversus
abdominis in combination with the m. obliquus internus abdominis (TrA + IO), and the m. obliquus
externus abdominis (EO). In addition, the muscle activation ratios EO / (TrA + IO) and Mf / (TrA +
IO) were compared during voluntary exertion under different verbal instructions in two positions:
standing while holding weights with the arms extended forward and sitting. A cross-sectional
study was conducted involving 24 participants (12 men and 12 women), with a mean age of 23.6
± 2.8 years and a mean body mass index of 22.8 ± 2.7 kg/m². Muscle activity was measured using
surface electromyography and expressed as a percentage (%) of maximal voluntary isometric
contraction (MVIC). Five verbal instructions were used: »Maximally tighten your abdominal
muscles« (»Tighten«); »Maximally pull your navel toward your spine« (»Navel«); »Someone is going
to punch you in the abdomen; resist maximally« (»Boxing«); »Place your hands on your hips and
maximally push your fingers apart« (»Fingers«); and »Cough and maximally maintain the pressure
you feel in your trunk when coughing« (»Cough«). One-way and two-way analyses of variance were
used to analyze differences between verbal instructions and body positions.
No statistically significant differences in muscle activity were found between verbal instructions.
Statistically significant differences in muscle activity between body positions (standing and
sitting) were observed for all muscles except TrA + IO under the »Tighten«, »Navel«, and »Boxing«
instructions. For all remaining muscle–instruction combinations, muscle activity was
significantly greater when standing with the arms extended forward while holding weights than
when sitting. In the sex-specific analysis, a statistically significant interaction between verbal
instruction and sex was found for EO activity in the sitting position. This interaction was
associated with the »Cough« instruction: EO activity under this instruction was the lowest of all
instructions in men and the highest in women.
As no statistically significant differences in trunk muscle activity were found between verbal
instructions, different instructions may be used during exercises aimed at strengthening or
activating the trunk muscles. However, greater attention should be paid to body position, as
muscle activity was generally greater in standing than in sitting.
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