The load on the low back and hips and muscular activity during machine milking

Abstract We investigated the load on the lumbo-sacral and hip joints and muscle activity in the erector spinae, oblique abdominal, rectus femoris, biceps femoris and gluteus maximus muscles in ten professional milkers simulating 20 different standardized machine-milking work postures in our laboratory. A computerized biomechanical sagittal plane model was used for calculations of load moments and compressive forces. Muscle activity was recorded as time-averaged, full-wave-rectified EMGs, and normalized, ie., expressed as a percentage of an isometric test-contraction. Muscle activity was low in the erector spinae and very low in the oblique abdominal muscles. Activity in hip muscles were generally low compared to that in other activities such as lifting a 12 kg burden, descending stairs and walking. The load on the low back ranged between 64 Nm (corresponding to 1.9 times body-weight compressive force at the L5-S1 disc) and 170 Nm (3.8 times body weight). The lowest induced lumbar load occurred with a vertical level difference (pit depth) of 0.85 m or 1.0 m between milker and cow. The peak load moment about the hip was 96 Nm; the lowest, 31Nm, occuring in the posture with a difference in level of 1.0 m between milker and cow. In this posture only 15 percent of the maximum muscle strength was used to counteract the load moment. Because the load during machine milking is relatively static, the recorded muscle activity is considered to be sufficiently high to partially occlude circulation to the muscle and therefore may result in muscle fatigue and pain.