The relationship between lumbar spine load and muscle activity during extensor exercises.

BACKGROUND AND PURPOSE: There have been no previous studies that quantitatively assessed the load on the spine during extensor exercises. The purpose of our study was to investigate the loading of the lumbar spine and trunk muscle activity levels while subjects performed typical trunk extensor exercises.

SUBJECTS: Thirteen male volunteers (mean age = 21.0 years, SD = 1.0, range = 19-23; mean height = 176.0 cm, SD = 6.2, range = 165-188; mean mass = 77.0 kg, SD = 7.0, range = 63-89) participated.

METHODS

The subjects performed four different back exercises. Electromyographic (EMG) activity was recorded from 14 trunk muscles. The postures that corresponded to the maximum external moment were identified and quantified using rigid body modeling combined with an EMG-driven model to determine joint loading at the L4-5 joint. The exercises were then evaluated based on the lumbar spine loading and peak muscle activity levels. A reference task of lifting 10 kg from midthigh was included for comparison.

RESULTS

The exercises involving active trunk extension produced the highest joint forces and muscle activity levels. Exercises involving leg extension with the spine held isometrically demonstrated asymmetrical activity of the trunk muscles, thereby reducing loads on the spine.

CONCLUSION AND DISCUSSION: The back extensor exercises examined provided a wide range of joint loading and muscle activity levels. Single-leg extension tasks appear to constitute a low-risk exercise for initial extensor strengthening, given the low spine load and mild extensor muscle challenge. When combined with contralateral arm extensions, the challenge and demand of the exercise were increased. The compressive loading and extensor muscle activity levels were highest for the trunk extension exercises.