An electromyographic study of isokinetic axial rotation in young adults

Trunk rotation is associated with over 60% of all low back injuries. However, there are gaps in the knowledge about trunk rotation.

To study the axial rotation torque and electromyographs (EMGs) of seven trunk muscles bilaterally in static and isokinetic modes with increasing angular velocity to determine qualitative and quantitative muscle response.

An electromyographic study of seven trunk muscles bilaterally was carried out in 50 normal subjects during static and isokinetic axial rotation at varying angular velocity. The qualitative and quantitative force and EMG measures were made and analyzed phenomenologically and statistically.

Fifty normal young adults (27 men, 23 women) performed an isometric maximal voluntary contraction (MVC) from the neutral upright-seated posture to their right and left. Also, these subjects performed isokinetic axial rotation from neutral seated position to right and left, and from prerotated right and left "end of range posture" toward the neutral at 10, 20 and 40 degrees per second angular velocity. All experimental trials were made on axial rotation tester designed for the study. EMG was recorded from the erector spinae at L3 and T10 vertebral levels, latissimus dorsi, pectoralis major, rectus abdominis and external and internal obliques bilaterally. The torque and EMG in isokinetic condition were normalized against those of isometric condition. Descriptive statistics were calculated. Data were subjected to analysis of variance, and torque was regressed on EMG.

The peak isokinetic torques were significantly lower than the isometric torques (p<.01), but the EMGs of the isokinetic conditions were significantly higher than those of isometric trials (p<.01). The isokinetic axial rotation torque decreased by 1 Nm to 1.5 Nm with per degree increase in velocity of rotation, with the values for 10, 20 and 40 degrees per second angular velocities being significantly different (p<.01). There was a significant increase (p<.01) in percent EMG (up to 28%) per unit torque with increasing angular velocity of rotation. The rotation torque from prerotated position to neutral was significantly higher than that of rotation away from the neutral (p<.001). The EMG magnitude increased significantly with increasing velocity (2% to 17% at 10 degrees per second), 21% to 28% at 20 degrees per second, 30% to 36% at 40 degrees per second). Regression analysis revealed poor predictability of torque based on EMG. The latter was suggested because of the role and behavior of the ligaments and joint capsules of the spine.

The axial rotation is initiated and maintained by the contralateral external obliques, ipsilateral latissimus dorsi and internal oblique. The ipsilateral erector spinae likely play a stabilizer role. The isometric torque is greater than the isokinetic, which decreases with increasing velocity. Even with decreasing torque, EMG progressively increases, indicating a disproportionally higher stress in the spinal connective tissues potentiating injury. The data presented here suggest that, for safety, load and velocity of rotation should be kept low.