The contractile properties of the M. supracoracoideus In the pigeon and starling: a case for long-axis rotation of the humerus

摘要: Wing upstroke in birds capable of powered flight is kinematically the most complicated phase wingbeat cycle. The M. supracoracoideus (SC), generally considered to be primary elevator wing, a muscle with highly derived but stereotyped morphology modern flying birds. contractile portion SC arises from ventral sternum, its tendon insertion courses above glenohumeral joint insert on dorsal surface humerus. To clarify role during wing upstroke, we studied and mechanical properties European starlings (Sturnus vulgaris) pigeons (Columba livia), two contrasting styles. We made situ measurements isometric forces humeral elevation rotation and, addition, measured extent unrestrained excursion stimulation nerve. also generated passive active length-force curves for each species. Stimulation at angles elevation/depression protraction/retraction coincident downstroke-upstroke transition mid-upstroke produced substantially higher long-axis than elevation. When humerus was allowed move (rotate/elevate) stimulation, observed about longitudinal axis up 70-80 degrees , elevations only 40-60 horizontal (as lateral view). In experiments, mean (+/-s.d.) maximal tetanic 6.5+/-1.2 N (N=4) 39.4+/-6.2 (N=6), unexpectedly high approximately 10 times body weight. working range both species corresponds ascending limb (but not plateau) curve. potential greatest force transition, time when depressed below rotated forward maximally. As shortens counterrotate elevate early shorter lengths declines relatively rapid rate. These findings reveal that impart high-velocity axis, which rapidly elevates distal wing. This twisting responsible positioning forearm hand so their subsequent extension orients outstretched parasagittal plane appropriate downstroke. propose that, variable levels co-contraction pectoralis interact provide level kinematic control shoulder would possible were antagonists work independently. lack morphologically Late Jurassic Early Cretaceous precluded recovery stroke undoubtedly limited these forms. Subsequent evolution imparting large rotational an important step ability supinate (circumflex) manus movement fundamental reducing air resistance stroke.