The activity of the back extending muscles during upright standing and sitting with the back curved - An electromyographical study

Problem: There is a controversial discussion about the loading of the spine by the force of the extending muscles of the back in upright sitting with the back curved as in upright standing, Experimental studies investigating this issue are still lacking. Methods: In the study presented in this paper we measured in 30 subjects the activity of the extending back muscles both in a natural upright standing posture and in sitting with identical postur e of the back as recommended in some back training programs. To this end we recorded the electromyographical surface activity of the back muscles at f our levels of the spine in both postures. The EMG activities were intraindi vidually normalized with reference to the respective activity recorded at m aximum voluntary isometric contraction (MVC), Results: At three of the four levels of the spine examined, the group averages of the EMG activity in si tting were significantly higher than in standing (p < 0.001). The maximum a ctivity enhancement evaluated was 48% at Th12. The force of the lower back muscles in sitting with a posture of the back as in natural upright standin g could be estimated to be at least greater than 30% of the force at MVC. C onclusion: The force needed to enforce in sitting a posture of the back ide ntical to the posture at natural standing entails lower back muscle fatigue in a few minutes. The results of our study agree with recent experimental findings about an increased loading of the spine in lordotic sitting.