title: Age-related weakness of proximal muscle studied with motor cortical mapping: a TMS study.
abstract: Aging-related weakness is due in part to degeneration within the central nervous system. However, it is unknown how changes to the representation of corticospinal output in the primary motor cortex (M1) relate to such weakness. Transcranial magnetic stimulation (TMS) is a noninvasive method of cortical stimulation that can map representation of corticospinal output devoted to a muscle. Using TMS, we examined age-related alterations in maps devoted to biceps brachii muscle to determine whether they predicted its age-induced weakness. Forty-seven right-handed subjects participated: 20 young (22.6 ± 0.90 years) and 27 old (74.96 ± 1.35 years). We measured strength as force of elbow flexion and electromyographic activation of biceps brachii during maximum voluntary contraction. Mapping variables included: 1) center of gravity or weighted mean location of corticospinal output, 2) size of map, 3) volume or excitation of corticospinal output, and 4) response density or corticospinal excitation per unit area. Center of gravity was more anterior in old than in young (p<0.001), though there was no significant difference in strength between the age groups. Map size, volume, and response density showed no significant difference between groups. Regardless of age, center of gravity significantly predicted strength (β = -0.34, p = 0.005), while volume adjacent to the core of map predicted voluntary activation of biceps (β = 0.32, p = 0.008). Overall, the anterior shift of the map in older adults may reflect an adaptive change that allowed for the maintenance of strength. Laterally located center of gravity and higher excitation in the region adjacent to the core in weaker individuals could reflect compensatory recruitment of synergistic muscles. Thus, our study substantiates the role of M1 in adapting to aging-related weakness and subtending strength and muscle activation across age groups. Mapping from M1 may offer foundation for an examination of mechanisms that preserve strength in elderly.
authors.affiliation: Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio, United States of America ; Department of Physical Medicine and Rehabilitation, Neurological Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Ela B
authors.initials: EB
authors.lastname: Plow
authors.affiliation: Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Nicole
authors.initials: N
authors.lastname: Varnerin
authors.affiliation: Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: David A
authors.initials: DA
authors.lastname: Cunningham
authors.affiliation: Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Daniel
authors.initials: D
authors.lastname: Janini
authors.affiliation: Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Corin
authors.initials: C
authors.lastname: Bonnett
authors.affiliation: Center for Neurological Restoration, Department of Neurosurgery, Neurological Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Alexandria
authors.initials: A
authors.lastname: Wyant
authors.affiliation: Department of Physical Medicine and Rehabilitation, Neurological Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Juliet
authors.initials: J
authors.lastname: Hou
authors.affiliation: Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Vlodek
authors.initials: V
authors.lastname: Siemionow
authors.affiliation: Quantitative Health Sciences, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Xiao-Feng
authors.initials: XF
authors.lastname: Wang
authors.affiliation: Center for Neurological Restoration, Department of Neurosurgery, Neurological Institute, Cleveland Clinic, Cleveland, Ohio, United States of America.
authors.forename: Andre G
authors.initials: AG
authors.lastname: Machado
authors.affiliation: Human Performance and Engineering Laboratory, Kessler Foundation Research Center, West Orange, New Jersey, United States of America.
authors.forename: Guang H
authors.initials: GH
authors.lastname: Yue
mesh_terms: D000368:Aged; D000375:Aging / Q000502:physiology*; D001931:Brain Mapping; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology / Q000503:physiopathology*; D009119:Muscle Contraction / Q000502:physiology; D018908:Muscle Weakness / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult