title: Muscle function in monozygotic female twin pairs discordant for hormone replacement therapy.
abstract: BACKGROUND
  Postmenopausal monozygotic twin pairs discordant for hormone replacement therapy (HRT) provide an advantageous study design controlling for genetic background for elucidating the relationships between aging, sex hormone levels, muscle strength, contractile capacity, and fatigability.
  
  
  METHODS
  Thirteen postmenopausal monozygotic twin pairs discordant for HRT were measured for maximal voluntary torque (MVC) and twitch characteristics using electrical stimulation before and after intermittent dynamic plantarflexor exercise until exhaustion.
  
  
  RESULTS
  Peak twitch torque was 32% higher in HRT users than in their non-HRT, genetically identical sisters (P = 0.002), but MVC did not differ. There were no differences in the activation level or twitch time characteristics between the co-twins. Fatigue caused decreases in MVC (P = 0.001), twitch torque (P = 0.001), time to peak (P = 0.013), and half-relaxation time (P = 0.001) similarly in HRT users and non-HRT users.
  
  
  CONCLUSIONS
  In early postmenopausal women, involuntary but not voluntary force-generating mechanisms of the plantarflexors are augmented by the use of HRT.
authors: Neuromuscular Research Center, Department of Biology of Physical Activity, P.O. Box 35 (VIV227), University of Jyväskylä, FI-40014 Jyväskylä, Finland. taija.finni@jyu.fi
authors: Taija
authors: T
authors: Finni
authors: Marika
authors: M
authors: Noorkoiv
authors: Eija
authors: E
authors: Pöllänen
authors: Paula H A
authors: PH
authors: Ronkainen
authors: Markku
authors: M
authors: Alén
authors: Jaakko
authors: J
authors: Kaprio
authors: Vuokko
authors: V
authors: Kovanen
authors: Sarianna
authors: S
authors: Sipilä
mesh_terms: D004576:Electromyography / Q000379:methods; D004958:Estradiol / Q000097:blood / Q000627:therapeutic use; D015914:Estrogen Replacement Therapy / Q000379:methods; D005260:Female; D006801:Humans; D008875:Middle Aged; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018763:Muscle Fatigue / Q000187:drug effects / Q000502:physiology*; D053580:Muscle Strength / Q000187:drug effects / Q000502:physiology; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D014430:Twins, Monozygotic / Q000235:genetics
chemical_list: D004958:Estradiol