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veterinary
anatomy
nutrition
farriery
2004
Cohort Study

Myosin heavy chain profiles and body composition are different in old versus young Standardbred mares.

Authors: Lehnhard Robert A, McKeever Kenneth H, Kearns Charles F, Beekley Matthew D

Journal: Veterinary journal (London, England : 1997)

Summary

# Editorial Summary: Age-Related Changes in Muscle Fibre Type and Body Composition in Standardbred Mares Age-related shifts in skeletal muscle composition have been well documented in humans and rodents, yet equivalent research in equines remains sparse; this 2004 study sought to address that gap by comparing muscle fibre profiles and body composition between young (4–8 years) and older (20+ years) Standardbred mares. Researchers measured rump fat thickness via ultrasound, calculated percentage body fat, and analysed myosin heavy chain (MHC) isoform distribution from gluteus medius needle biopsies using gel electrophoresis in 31 unfit mares across two age groups. Surprisingly, total body fat percentage, absolute fat mass, and bodyweight showed no significant difference between groups, yet older mares paradoxically carried greater lean body mass (427.1 versus 405.7 kg)—a counterintuitive finding potentially reflecting loss of muscle quality rather than quantity. The critical discovery lay in fibre-type remodelling: aged mares demonstrated substantially lower proportions of oxidative type I fibres (7.8 versus 12.1%) and aerobic type IIA fibres (27.8 versus 36.1%), alongside a marked increase in anaerobic type IIX fibres (64.6 versus 51.8%), mirroring age-related patterns documented in other species. These findings have implications for training prescription and performance expectations in older horses, suggesting that apparent maintenance of muscle mass may mask functionally significant shifts towards less efficient, fatigue-prone fibre types—a consideration particularly relevant when rehabilitating or retraining senior animals.

Read the full abstract on PubMed

Practical Takeaways

  • Older mares maintain similar overall body weight and composition to younger mares but with altered muscle fiber distribution, which may affect exercise capacity and recovery in aged performance or broodstock horses
  • The shift toward type IIX fibers in aged horses suggests reduced oxidative capacity and potentially greater fatigue susceptibility, informing conditioning and workload management for geriatric equines
  • Increased lean body mass in older mares may reflect selective retention of muscle tissue or metabolic changes; monitor feeding and conditioning programs accordingly as muscle composition, not just weight, changes with age

Key Findings

  • Old mares (20+ years) had significantly greater lean body mass than young mares (427.1 vs 405.7 kg) despite no difference in total body weight or fat percentage
  • Aged mares showed significantly lower type I (7.8% vs 12.1%) and type IIA (27.8% vs 36.1%) myosin heavy chain fibers compared to young mares
  • Aged mares had significantly higher type IIX fibers (64.6% vs 51.8%), indicating a shift toward faster, more fatigable muscle fiber types with aging
  • Age-related myosin heavy chain changes in horses are consistent with patterns observed in other mammalian species

Conditions Studied

agingage-related changes in muscle composition