title: Impact of vaginal parity and aging on the architectural design of pelvic floor muscles.
abstract: Vaginal delivery and aging are key risk factors for pelvic floor muscle dysfunction, which is a critical component of pelvic floor disorders. However, alterations in the pelvic floor muscle intrinsic structure that lead to muscle dysfunction because of childbirth and aging remain elusive.
  
  
  
  The purpose of this study was to determine the impact of vaginal deliveries and aging on human cadaveric pelvic floor muscle architecture, which is the strongest predictor of active muscle function.
  
  
  
  Coccygeus, iliococcygeus, and pubovisceralis were obtained from younger donors who were ≤51 years old, vaginally nulliparous (n = 5) and vaginally parous (n = 6) and older donors who were >51 years old, vaginally nulliparous (n = 6) and vaginally parous (n = 6), all of whom had no history of pelvic floor disorders. Architectural parameters, which are predictive of muscle's excursion and force-generating capacity, were determined with the use of validated methods. Intramuscular collagen content was quantified by hydroxyproline assay. Main effects of parity and aging and the interactions were determined with the use of 2-way analysis of variance, with Tukey's post-hoc testing and a significance level of .05.
  
  
  
  The mean age of younger and older donors differed by approximately 40 years (P = .001) but was similar between nulliparous and parous donors within each age group (P > .9). The median parity was 2 (range, 1-3) in younger and older vaginally parous groups (P = .7). The main impact of parity was increased fiber length in the more proximal coccygeus (P = .03) and iliococcygeus (P = .04). Aging changes manifested as decreased physiologic cross-sectional area across all pelvic floor muscles (P < .05), which substantially exceeded the age-related decline in muscle mass. The physiologic cross-sectional area was lower in younger vaginally parous, compared with younger vaginally nulliparous, pelvic floor muscles; however, the differences did not reach statistical significance. Pelvic floor muscle collagen content was not altered by parity but increased dramatically with aging (P < .05).
  
  
  
  Increased fiber length in more proximal pelvic floor muscles likely represents an adaptive response to the chronically increased load placed on these muscles by the displaced apical structures, presumably as a consequence of vaginal delivery. In younger specimens, a consistent trend towards decrease in force-generating capacity of all pelvic floor muscles in the parous group suggests a potential mechanism for clinically identified pelvic floor muscle weakness in vaginally parous women. The substantial decrease in predicted muscle force production and fibrosis with aging represent likely mechanisms for the pelvic floor muscle dysfunction in older women.
authors.affiliation: Division of Urogynecology and Pelvic Reconstructive Surgery, the Department of Reproductive Medicine, University of California, San Diego, San Diego, CA. Electronic address: malperin@ucsd.edu.
authors.forename: Marianna
authors.initials: M
authors.lastname: Alperin
authors.affiliation: Department of Integrative Biology and Physiology, University of Minnesota, Minneapolis, MN.
authors.forename: Mark
authors.initials: M
authors.lastname: Cook
authors.affiliation: Exercise and Nutritional Sciences Physical Therapy, San Diego State University, San Diego CA.
authors.forename: Lori J
authors.initials: LJ
authors.lastname: Tuttle
authors.affiliation: Department of Orthopaedic Surgery, University of California, San Diego, San Diego, CA.
authors.forename: Mary C
authors.initials: MC
authors.lastname: Esparza
authors.affiliation: Department of Orthopaedic Surgery, University of California, San Diego, San Diego, CA; Department of Bioengineering, University of California, San Diego, San Diego, CA; Rehabilitation Institute of Chicago, Chicago, IL.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Lieber
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D005260:Female; D006801:Humans; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000033:anatomy & histology* / Q000502:physiology; D010298:Parity / Q000502:physiology*; D017773:Pelvic Floor / Q000033:anatomy & histology* / Q000502:physiology; D014621:Vagina / Q000033:anatomy & histology* / Q000502:physiology
keywords: aging; architecture; pelvic floor muscle; vaginal delivery