I need the reference for the first or most relevant paper (just
suitable for referencing) which gives the maximal active contractile
strength of normal skeletal muscle per cross-sectional area in man or
animals (mammals). I think it is about 40-70 N/cm2, but cannot find
the publication anymore.

Thank you very much

The number I have heard most often is 35 N/cm2.  This figure is
reported in references #1 and #2 below.  The others are also primary
sources for the specific tension of skeletal muscle; the values they
report range from 39-137 N/cm2.  These references are taken from the
list provided in this more recent paper:

Buchanan TS. "Evidence that maximum muscle stress is not a constant:
differences in specific tension in elbow flexors and extensors." Med
Eng Phys.  1995 Oct;17(7):529-36.

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References:

1. AM Arkin, Absolute muscle power. The internal kinesiology of
muscle. In: Research Seminar Notes. Dept. Ortho. Surg., State Univ. of
Iowa 12D (1938), p. 123.

2. N Rechlinghausen. In: Gliedermechanik und Lahmungsprothesen,
Springer, Berlin (1920).

3. T Hertinger. In: M.H. Thurlwell, Editor, Physiology of Strength,
Thomas, Springfield, IL (1964).

4. H Fick. In: Handbuch der Anatomie und Mechanik der Gelenke, G.
Fischer, Jena (1904).

5. JV Johnson. Ergebnisse der Physiologie 2 2 (1903), p. 623.

6. M Ikai and T Fukunaga, Calculation of muscle strength per unit
cross-sectional area of human muscles by means of ultrasonic
measurements. Int. Z. Angew. Physiol. 26 (1962), pp. 26?32.

7. CB Morris, The measurement of the strength of muscle relative to
the cross-section. Res. Q. Am. Assoc. Hlth. Phys. Edu. 19 (1948), pp.
295?303.

Thank you very much for your kind help, this is more than I have
expected and I am very grateful. Vielen Dank.