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Neuromuscular changes of the aged human hamstrings.

Eric A Kirk1, Kevin J Gilmore1, Charles L Rice1,2,3

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Older men have weaker hamstrings and slower motor unit firing rates. Age-related declines in motor unit discharge rates were more pronounced in the medial hamstrings compared to the lateral hamstrings.

Keywords:
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Area of Science:

  • Neuromuscular physiology
  • Aging research
  • Human movement science

Background:

  • Hamstring muscle function is crucial for posture and locomotion throughout life.
  • Neuromuscular properties of hamstrings have not been extensively studied in the context of adult aging.

Purpose of the Study:

  • To assess and compare age-related effects on hamstring contractile function and motor unit (MU) discharge rates.
  • To investigate differences in neuromuscular properties between young and old men.

Main Methods:

  • Recruited 11 young (26±4 yr) and 10 old (80±5 yr) men.
  • Measured maximal voluntary isometric contractions (MVC), stimulated contractile properties, and electromyography (EMG) of biceps femoris (BF) and semimembranosus-semitendinosus (SS) muscles.
  • Recorded motor unit (MU) discharge rates during submaximal to MVC efforts.

Main Results:

  • Old men exhibited ~50% lower MVC torque and ~50% lower evoked twitch amplitude, with >150% longer twitch duration compared to young men.
  • Motor unit (MU) discharge rates were up to 45% lower in old men across contraction intensities (25%, 50%, 100% MVC).
  • Medial hamstrings (SS) showed significantly lower MU discharge rates in older men compared to lateral hamstrings (BF).

Conclusions:

  • Men in their 8th to 10th decades show substantially reduced strength and MU discharge rates in hamstrings.
  • Aging affects neuromuscular properties of limb muscles unequally, with greater impact on medial hamstrings.
  • Compartmental differences within functionally similar muscles may exist regarding aging effects.