title: Intramuscular phosphagen status and the relationship to muscle performance across the age spectrum.
abstract: OBJECTIVE
  To examine age-related differences in intramuscular concentrations of adenosine triphosphate (ATP), free creatine (FCr), phosphocreatine (PCr) and total creatine (TCr) and if these differences were related to muscle performance.
  
  
  METHODS
  Forty-two healthy, non-sedentary, males between 20 and 76 years provided muscle samples to determine [ATP], [FCr], [PCr], and [TCr]. Maximal strength and endurance were assessed and correlated with intramuscular variables.
  
  
  RESULTS
  Intramuscular [ATP] decreased by 13.5% (p = 0.013) in the older cohort (18.0 ± 0.6 mmol/kg dry wt) vs. the young cohort (20.8 ± 0.9 mmol/kg dry wt) and was significantly correlated to age (r = -0.38, p = 0.008). No other differences were observed between age groups for intramuscular [PCr], [FCr], [TCr], or [PCr]:[TCr] (p > 0.05). The older cohort consumed significantly less (p < 0.05) dietary protein when compared to the young cohort. Bivariate correlations were found for intramuscular [ATP] and lower body 1RM (r = 0.24, p = 0.066), leg press volume and free creatine (r = 0.325, p = 0.036) and leg press repetitions and free creatine (r = 0.373, p = 0.015). Partial correlations controlling for age eliminated the relationship between [ATP] and 1RM while intramuscular free creatine and leg press repetitions remained significant (p < 0.05) and leg press volume approached significance (p = 0.095).
  
  
  CONCLUSIONS
  These results expand upon previous observations indicative of age-related reductions in intramuscular [ATP] and dietary protein intake. The lack of change in other intramuscular PCr system markers are suggestive of dysfunctions at the mitochondrial level while the impact of neuromuscular changes, lean mass cross-sectional area and differences in physical activity are also important.
authors: Department of Exercise Science, School of Sport, Recreation and Exercise Sciences, Lindenwood University, St. Charles, MO, 63301, USA. ckerksick@lindenwood.edu.
authors: Chad M
authors: http://orcid.org/0000-0003-0458-7294
authors: CM
authors: Kerksick
authors: School of Kinesiology, Auburn University, Auburn, AL, 36849, USA.
authors: Michael D
authors: MD
authors: Roberts
authors: Clinical Biochemistry Laboratory, School of Medicine and Applied Sciences, Central Queensland University, Rockhampton, QLD, 4702, Australia.
authors: Vincent J
authors: VJ
authors: Dalbo
authors: Department of Exercise Science, High Point University, High Point, NC, 27262, USA.
authors: Kyle L
authors: KL
authors: Sunderland
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000328:Adult; D000367:Age Factors; D000368:Aged; D003401:Creatine / Q000378:metabolism*; D004044:Dietary Proteins / Q000378:metabolism; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008928:Mitochondria / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000378:metabolism*; D010725:Phosphocreatine / Q000378:metabolism*; D055815:Young Adult
chemical_list: D004044:Dietary Proteins; D010725:Phosphocreatine; D000255:Adenosine Triphosphate; D003401:Creatine
keywords: ATP; Aging; Muscle strength; Phosphocreatine