Journal of Electromyography and Kinesiology 11 (2001) 39–51 www.elsevier.com/locate/jelekin Anticipatory postural adjustments associated with lateral and rotational perturbations during standing Alexander S. Aruin a, b,* , Tetsuo Ota c , Mark L. Latash c a Rehabilitation Foundation, Inc., Wheaton, IL 60189, USA b Department of Physical Medicine and Rehabilitation, Rush-Presbyterian, St. Luke’s Medical Center, Chicago, IL 60612, USA c Department of Kinesiology, Pennsylvania State University, University Park, PA 16802, USA Received 15 May 2000; received in revised form 30 July 2000; accepted 5 September 2000 Abstract We studied the role of different leg and trunk muscle groups in the generation of anticipatory postural adjustments (APAs) prior to lateral and rotational perturbations associated with predictable and self-triggered postural perturbations during standing. Postural perturbations were induced by a variety of manipulations including catching and releasing a load with the right hand extended either in front of the body or to the right side, performing bilateral fast shoulder movements in different directions, and applying brief force pulses with a hand against the wall. Perturbations in a frontal plane (“lateral perturbations”) were associated with significant asymmetries in APAs seen in the right and left distal (soleus and tibialis anterior) muscles; these asymmetries dependent on the direction of the perturbation. Rotational perturbations about the vertical axis of the body generated by fast movements of the two shoulders in the opposite directions were also associated with direction-dependent asymmetries in the APAs in soleus muscles. However, rotational perturbations generated by an off-body-midline force pulse application were accompanied by direction- dependent asymmetries in proximal muscle groups, but not in the distal muscles. We conclude that muscles controlling the ankle joint play an important role in the compensation of lateral and rotational perturbations. The abundance of muscles participating in maintaining vertical posture allows the control system to use different task-dependent strategies during the generation of APAs in anticipation of rotational perturbation.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Posture; Anticipatory postural adjustments; EMG; Human 1. Introduction Perturbations of vertical posture of a standing individ- ual are frequently associated with self-initiated actions. In particular, fast arm movements, load manipulations such as catching and releasing a load, and interactions with external objects may all be the sources of inertial and reactive forces perturbing the vertical posture. A number of mechanisms serve to maintain the equilib- rium. Among them are changes in the background activity of postural muscles seen prior to the perturbation that are commonly addressed as anticipatory postural adjustments (APAs) [1–4]. APAs are believed to be gen- * Corresponding author. Current address: Department of Physical Therapy, University of Illinois at Chicago, 1919 W. Taylor Street M/C 898, Chicago, IL 60612, USA. Tel.: + 1-312-355-0902; fax: + 1-312- 996-4583. E-mail address: aaruin@uic.edu (A.S. Aruin). 1050-6411/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII:S1050-6411(00)00034-1 erated in a feed-forward fashion based on predictions of an upcoming perturbation associated with a planned action; their purpose is to counteract the effects of the perturbation on vertical posture. A number of factors are likely to play major roles in the process of APA gener- ation. Among them are the magnitude and direction of an expected perturbation, properties of a voluntary action associated with the perturbation, body configuration prior to the action, and features of the postural task, in particular, postural stability [5–10]. In studies of APAs during fast arm movements, repro- ducible patterns have been reported for proximal postural muscle pairs such as rectus abdominis–erector spinae (RA–ES) and rectus femoris–biceps femoris (RF–BF) [11–14]. Changes in the background activity of the tib- ialis anterior–soleus (TA–SOL) pair were considerably more variable across subjects. These observations allowed Aruin and Latash [14] to suggest that the role of the distal muscles, controlling the ankle joint, was