Sex differences in single muscle fiber power in older adults.
OBJECTIVE
This study was conducted to determine whether differences in power at the single muscle fiber level contribute to sex differences in whole muscle power production in the elderly.


METHODS
A total of 16 sedentary older persons (10 women, 6 men), mean age 72 yr, had percutaneous needle biopsy of musculus vastus lateralis. Chemically skinned single muscle fibers were activated with Ca for maximal isometric force measurement (Po). The slack test was performed to determine maximal unloaded shortening velocity (Vo). Force-velocity and power curves were generated via a series of isotonic contractions, allowing measurement of peak power and specific power. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to determine myosin heavy chain composition of single muscle fibers. Whole muscle strength, velocity, and power were measured for knee extension and double leg press.


RESULTS
Men had greater whole muscle strength, power, and velocity compared with women. Studied were 274 type I and 33 type IIa single fibers. No significant sex differences were found for fiber size, Po, specific force, Vo, power, or specific power in type I or IIa fibers.


CONCLUSIONS
Single muscle fiber quality in older women is equivalent to that in older men and can not explain the differences seen in whole muscle strength, power, or function.
Krivickas
Lisa S
LS
Department of Physical Medicine and Rehabilitation, Spaulding Rehabilitation Hospital and Harvard Medical School, Boston, MA 02114, USA. lkrivickas@partners.org
Fielding
Roger A
RA
Murray
Angelique
A
Callahan
Damien
D
Johansson
Andreas
A
Dorer
David J
DJ
Frontera
Walter R
WR
D000368:Aged; D000369:Aged, 80 and over; D044963:Biopsy, Fine-Needle; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D007551:Isotonic Contraction; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D012737:Sex Factors