Table 2.

Relative surface areas and volumes of free SR in the I band and in TAs of type IIX–IIB fibers

Age	Sex	A	B	C	D
		Free SR V/V μm3/μm3	Free SR Surface/V μm2/μm3	Calculated V/V in TA centrea μm3/μm3	Calculated S/V in TA centrea μm2/μm3
2–3 months	Male	0.08 ± 0.02	6.4 ± 1.4	–	–
7–8 months	Male	0.09 ± 0.02	5.7 ± 1.0	–	–
12 months	Male	0.10 ± 0.04	5.2 ± 1.0	–	–
24–29 months	Male	–	–	0.52	31.3

Mean ± SD. Morphometric data are from five fibers each in three male mice

^aCalculated on the basis of an average tubule diameter of 67 nm and an average tubule center to center distance of 82 nm