HAMILTON, JL*; RITTER, DA; GOSLOW, GE; Brown University, Providence, RI: Muscle activity patterns during swimming in Thick-billed murres (Uria lomvia)

Birds in the Family Alcidae use their wings for propulsion in air and also in water, a medium that is 800x denser and 70x more viscous. As part of a larger project investigating how Alcids effectively locomote through these very different media, here we describe the kinematics of horizontal swimming in conjunction with electromyographic (EMG) activity of the primary depressor and elevator muscles of the wing, the pectoralis and supracoracoideus (SC). We recorded high-speed video in lateral and ventral views (250 Hz) with markers fixed to the cranial and caudal ends of the sternum, wing tip and wrist joint of two Thick-billed murres (Uria lomvia). EMGs were recorded simultaneously from three architecturally distinct regions of the pectoralis and the SC. The anterior region of the pectoralis possesses fascicles that originate on the furcula and course laterally to insert on the deltopectoral crest (DPC) of the humerus; fascicles of the middle region originate on the sternal keel and course anterolaterally to the DPC; fascicles of the thoracobrachialis (TB) originate on the lateral posterior third of the sternal plate and course anteriorly to the DPC. At wingbeat frequencies averaging 3.1 Hz, the duration of muscle activity in all regions of the pectoralis is similar, averaging 78 ms. The onset of anterior pectoralis activity is during late upstroke, corresponding with the main period of wing protraction and pronation. The middle and TB regions of the pectoralis are activated later than the anterior pectoralis and remain active throughout downstroke, as the wing is depressed and retracted. The onset of SC activity is at late downstroke and continues throughout upstroke, overlapping with the activity of the anterior pectoralis. The total duration of SC activity is 150 ms, roughly twice that of the pectoralis.