Physical Therapy in Sport 8 (2007) 7–13 Original research The relevance of increasing resistance on trunk muscle activity during seated axial rotation Veerle Stevens à , Erik Witvrouw, Guy Vanderstraeten, Thierry Parlevliet, Katie Bouche, Nele Mahieu, Lieven Danneels Department of Rehabilitation Sciences and Physiotherapy, Faculty of Medicine and Health Sciences, Ghent University, De Pintelaan 185, 6K3, B9000 Ghent, Belgium Received 10 May 2006; received in revised form 21 September 2006; accepted 26 September 2006 Abstract Objectives: To investigate the electromyographic (EMG) trunk muscle activity during a low-load instrumented axial rotation exercise (Tergumed) and the relevance of increasing resistance. Setting: Evaluation was done in a training centre in a university hospital. Participants: Thirty healthy subjects without musculoskeletal or neuromuscular complaints. Main outcome measures: The normalized (as a percentage of maximal voluntary isometric contractions (MVIC)) EMG activity of 14 abdominal and back muscles were investigated during dynamic trunk rotation exertions at 30%, 50% and 70% of maximum mean force (MMF). Results: During the low-load (30% MMF) rotation exercise, the internal abdominal oblique muscle reached activity levels of 30% of MVIC. All the examined back muscles and the external abdominal obliques reached activity levels of 60% of MVIC. Increasing the resistance during seated axial rotation, created significantly higher relative muscle activity levels for all trunk muscles. Conclusions: The results of the present study indicate that increasing resistance to 50% and 70% MMF during seated axial rotation in a Tergumed training device consistently created higher relative activity levels in all trunk muscles. In the vulnerable spine undergoing rehabilitation the results suggest that training at 30% MMF may be sufficient. r 2006 Elsevier Ltd. All rights reserved. Keywords: Electromyography; Rotation; Resistance 1. Introduction The majority of our everyday activities such as walking, running and most industrial occupations, require axial rotation of the spine. In most sport activities such as tennis, racquetball, squash, ice hockey, soccer, rugby, cricket, volleyball, badminton, golf, baseball and many others, axial rotation is imperative (Evans, Refshauge, Adams, & Aliprandi, 2005; Kumar, 2004). Rotation of the trunk during tennis serve and groundstroke for example is an essential part of the development of power and transfer of energy up the kinetic chain from the lower to upper extremities (Ellenbecker & Roetert, 2004). General agreement consists on the need to train the athlete in the sports- pecific movements. However, trunk rotation has been reported to be significantly associated with low back pain (Kumar, 2004). Disc-related lower back injuries are very common in elite athletes (Gerbino, & d’Hemecourt, 2002; Hickey, Fricker, & McDonald, 1997a, b; Sward, Hellstrom, Jacobsson, Nyman, & Peterson, 1991). To prevent rotational injuries during sports and daily activities, coordination and strength training of the muscles which are important during rotation gains interest. High ARTICLE IN PRESS www.elsevier.com/locate/yptsp 1466-853X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ptsp.2006.09.021 à Corresponding author. Tel.: +32 9240 29 96; fax: +32 9240 38 11. E-mail address: Veerle.Stevens@UGent.be (V. Stevens).