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http://isb.ri.ccf.org/biomch-l/archives/biomch-l-1995-08/00051.html
McMahon says for isometric contraction
  Peak Stress =       2      kg/cm^2
              =  20,000      kg/m^2
              = 200,000      N/m^2
              =      20      N/cm^2
              =     200      KPascals
              =       2      atmospheres
              =       0.001  the yield stress of soft steel

  But other more reliable sources quote peak numbers more like
                     30      N/cm^2
  And less reliable sources quote numbers up to
                     40      N/cm^2


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http://robotics.eecs.berkeley.edu/~ronf/MFI/TABLES/tables.html
  Peak strain rate   =   5/s
This would imply (peak stress and peak strain rate don't occur
simultaneously):
   Peak power per kg  =   (1/3 peak strain rate)
                        * (1/3 peak stress)
                        * 1/(density of muscle)
                      =   (5/s)/3   *   (3.5 * 10^5 N/m^2)/3  *  (1 m^3/1000 kg)
                      =   200 Nm/s  per kg
                      =   200 watts per kg

But actually the peak power is about 50 watts per kg.  I don't know if 
this
50 weatts is peak power is during a contraction or averaged over a
contraction cycle.


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http://www.insidetri.com/train/tips/articles/1639.0.html
Lance Armstrong, as a whole, muscle, fat, bones and all
produces about  6-7 watts/kg in the Tour de France.

For comparison I, Andy at his best, in 1980 could climb about 800 feet
in 16 minutes on my bike. That's
         (250m)*(1000N)/((90 kg)*(1000s)   =  almost 3 watts/kg.

http://answers.google.com/answers/threadview?id=44907
Another comparison,
2 stroke gas engines are typically  700 watts/kg.
That's 15 times better than pure muscle and 100 times better than 
Lance
Armstrong's
body viewed as a motor.  But Ultralight plane engines can have even 
twice
that power to weight ratio (up to 1500 watts/kg).