ANTAGONIST FORCES ARE UNDERESTIMATED WHEN ASSESSED USING EMG-TORQUE RELATIONSHIP

Introduction Antagonist force production cannot be calculated directly in vivo and is usually estimated by using the relationship between surface EMG activity and exerted torque of the antagonist muscles, while they act as agonists (Maganaris et al., 1998; Mademli et al., 2004; Kellis et al., 1997). An implicit assumption of this method is that during an isometric contraction, the relationship between the level of EMG activity and the mechanical output is the same when a muscle is acting either as an agonist or as an antagonist. If this were the case, one would expect changes in EMG to match changes in architecture (Hodges et al., 2003) in both conditions. Therefore, the aim of this study was to investigate whether the behaviour of Gastrocnemius Medialis muscle (GM) - as assessed by its architecture - would be identical during isometric agonist and antagonist contractions at equal level of EMG activity. Methods Torque, architecture (pennation angle and fiber length) and EMG activity of GM were obtained while 8 young healthy males performed ramp isometric contractions in dorsi-flexion (DF) and plantar-flexion (PF) up to maximal voluntary contractions (MVC). Results At a given EMG activity, GM fiber length was significantly shorter while performing a sub-maximal PF contraction than during DF MVC (-18%; p < 0.001). Similarly, pennation angle during sub-maximal PF was significantly larger than during DF MVC (+28 %; p = 0.014). Discussion At a given level of GM EMG activity,architectural parameters were significantly different when the muscle acted as agonist or antagonist: fiber length was longer, and pennation angle was smaller during antagonist than during agonist isometric contractions. These findings indicate that GM elicits a higher mechanical output while acting as an antagonist when EMG activity is matched. This means that estimation of antagonistic force using the common method based on the EMG/net torque relationship yields underestimated values. References Hodges PW, Pengel LH, Herbert RD, Gandevia SC. (2003). Muscle Nerve;27(6):682-692 Kellis E, Baltzopoulos V. (1997). Eur J Appl Physiol Occup Physiol;76(3):253- 259 Mademli L, Arampatzis A, Morey-Klapsing G, Bruggemann GP. (2004). J Electromyogr Kinesiol;14(5):591-597 Maganaris CN, Baltzopoulos V, Sargeant AJ. (1998). Exp Physiol;83(6):843-855