Key points are not available for this paper at this time.
The author would like to thank R.E. Burke for editorial advice and C.A. Pratt for suggestions regarding the form of the review; D.G. Stuart, R. Enoka, Z. Hasan, and T. Hamm for critical reading of the manuscript; M. O'Malley for preparation of figures; and J. Kraft for typing.
Gerald E. Loeb (Sun,) studied this question.