title: Sex differences with aging in the fatigability of dynamic contractions.
abstract: This study determined the sex difference with aging in fatigability of the elbow flexor muscles during a dynamic fatiguing task, and explored the associated mechanisms. We compared fatigability of the elbow flexor muscles in 18 young (20.2 ± 1 years: 9 men) and 36 old adults (73.5 ± 1 years: 16 men) during and in recovery from repeated dynamic contractions (~60°/s) with a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque until failure. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (an increase in the superimposed twitch, SIT) and the peak rate of muscle relaxation. Time to failure was briefer for the men than the women (6.1 ± 2.1 vs. 9.7 ± 5.5 min, respectively; P=0.02) with no difference between young and old adults (7.2 ± 2.9 vs. 8.4 ± 5.2 min, respectively, P=0.45) and no interaction (P>0.05). The relative decline in peak relaxation rate with fatigability was similar for young and old adults (P=0.11), but greater for men than women (P=0.046). Supraspinal fatigue increased for all groups and was associated with the time to failure (P<0.05). Regression analysis however, indicated that the time to failure was best predicted by the peak relaxation rate (baseline values and slowing with fatigability) (r(2)=0.55). Rate-limiting contractile mechanisms (e.g. excitation-contraction coupling) were responsible for the increased fatigability of the elbow flexors of men compared with women for a dynamic fatiguing task of slow angular velocity, and this sex difference was maintained with aging. The age difference in fatigability for the dynamic task was diminished for both sexes relative to what is typically observed with isometric fatiguing contractions.
authors.affiliation: Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, United States; Department of Kinesiology and Integrative Physiology, Michigan Technological University, Houghton, MI, United States.
authors.forename: Tejin
authors.initials: T
authors.lastname: Yoon
authors.affiliation: Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, United States; Creighton University School of Medicine, Omaha, NE, United States.
authors.forename: Ryan
authors.initials: R
authors.lastname: Doyel
authors.affiliation: Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, United States; Uniformed Services University of the Health Sciences, F. Edward Hebert School of Medicine, Bethesda, MD, United States.
authors.forename: Claire
authors.initials: C
authors.lastname: Widule
authors.affiliation: Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, United States. Electronic address: Sandra.Hunter@marquette.edu.
authors.forename: Sandra K
authors.initials: SK
authors.lastname: Hunter
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D004551:Elbow Joint / Q000502:physiology; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D009044:Motor Cortex / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology; D012727:Sex Characteristics; D055815:Young Adult
keywords: Aging; Contractile properties; Gender; Muscle fatigue; Supraspinal fatigue; Women