Received November 15, 2005
Revised December 5, 2005
Accepted after revision December 22, 2005

¹ University of Maryland Eastern Shore
² University of Maryland
³ University of Maryland School of Medicine
⁴ Michigan State University
⁵ Colorado State University
⁶ National Institute on Aging

^* To whom correspondence should be addressed. E-mail: gfmartel{at}mail.umes.edu.

	Abstract

This study assessed age and sex effects on muscle fibre adaptations to heavy resistance strength training (ST). Twenty-two young men and women (20-30 yrs) and 18 older men and women (65-75 yrs) completed nine weeks of heavy resistance knee extension exercises with the dominant leg three days/week; the non-dominant leg served as a within-subject, untrained control. Bilateral vastus lateralis muscle biopsies were obtained before and after ST for analysis of type I, IIa, and IIx muscle fibre cross-sectional area (CSA) and fibre type distribution. One-repetition maximum (1-RM) strength was also assessed before and after ST. ST resulted in increased CSA of type I, IIa, and IIx muscle fibres in the trained leg of young men, type I and IIa fibres in young women, type IIa fibres in older men, and type IIx fibres in older women (all P<0.05). Analysis of fibre type distribution revealed a significant increase in the percentage of type I fibres (P<0.05) along with a decrease in type IIx fibres (P=0.054) after ST only in young women. There were no significant changes in muscle fibre CSA or fibre type distribution in the untrained leg for any group. All groups displayed significant increases in 1-RM (27-39%; all P<0.01). In summary, ST led to significant increases in 1-RM and type II fibre CSA in all groups; however, age and sex influence specific muscle fibre subtype responses to ST.

Key Words: Ageing, Exercise, Muscle fibre

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