Mirror Neurons in Older Participants (MNOP)

Mirror Neurons in Older Participants (Project MNOP): Acute Effects of Action Observation on Muscle Strength/Weakness and Neurophysiological Factors in Older Adults

A critical problem facing aging adults is muscle weakness. Whereas scientists have traditionally attributed the loss of muscle strength with aging to muscle atrophy, emerging evidence suggests that impairments in the neuromuscular system's ability to voluntarily generate force plays a more central role than previously appreciated. One area that has not yet been investigated includes the role that observing another's actions - thereby activating mirror neurons - plays in muscle force generation. Therefore, the purpose of this study is to examine the acute effects of action observation on muscular strength, voluntary muscle activation, and cortical excitability and inhibition in older adults.

A critical problem facing aging adults is muscle weakness. Whereas scientists have traditionally attributed the loss of muscle strength with aging to atrophic effects, emerging evidence suggests that impairments in the neuromuscular system's ability to voluntarily generate force plays a more central role than previously appreciated. One area that has not yet been investigated includes the role that observing another's actions - thereby activating mirror neurons - plays in muscle force generation. Therefore, the purpose of this study is to examine the acute effects of action observation on muscular strength, voluntary activation, and cortical excitability and inhibition in older adults. Following a thorough familiarization visit, twenty-five men and women ≥60 years of age will complete three action observation sessions in a randomized, counterbalanced manner: 1) observation of very strong hand/wrist contractions, 2) observation of very weak hand/wrist contractions, and 3) a control condition. Maximal voluntary contractions (MVCs) of the wrist flexors will be performed before and after observation sessions. Percent voluntary activation will be determined via the interpolated twitch technique. Single-pulse transcranial magnetic stimulation (TMS) and electromyographic (EMG) recordings from the flexor carpi radialis and first dorsal interosseous will be used to quantify cortical excitability and inhibition, via motor evoked potential amplitude and silent period duration, respectively. The hypothesis of this study is that observation of strong muscle contractions will acutely increase muscle strength, and such changes will be facilitated by enhanced corticospinal excitability and decreased inhibition. In contrast, it is hypothesized that observation of very weak contractions will cause no such efforts or even acute muscle weakness. Collectively, we propose that manipulation of mirror neurons is a worthwhile strategy for clinicians hoping to induce neuromuscular adaptations in older adults, particularly in settings where movement of a joint is painful or infeasible (e.g., bedrest or immobilization).

Strength, voluntary activation, and cortical responses to three conditions will be measured: 1) action observation of very strong, forceful contractions of the hand and wrist flexors 2) action observation of very weak, feeble contractions of the hand and wrist flexors 3) no action observation. Experimental conditions will be randomized and counterbalanced.

Isometric muscle strength of the non-dominant hand and wrist flexors will be assessed during maximal voluntary contractions (MVCs).

Percent voluntary activation will be quantified during the MVCs to determine each participant's ability to maximally activate their wrist flexor muscles voluntarily.

Transcranial magnetic stimulation will be used to quantify corticospinal excitability throughout the study.

Inclusion Criteria: Healthy men and women ≥60 years of age Exclusion Criteria: Neuromuscular disease (e.g. Parkinson's, MS, ALS) Metabolic disease (e.g. diabetes, thyroid disorder, metabolic syndrome) Arthritis in the upper limbs (hands, arms, shoulders) Trouble using or controlling one's muscles History of cancer History of stroke History of heart attack Use of an assistive walking device or other mobility aids Physician mandated contraindication to exercise within the last 6 months Epilepsy or history of convulsions/seizures History of fainting or syncope History of head trauma that was diagnosed as concussion or was associated with loss of consciousness History of hearing problems or tinnitus Cochlear implants Implanted metal in the brain, skull, or elsewhere in the body Implanted neurotransmitter Cardiac pacemaker or intracardiac lines Medication infusion device Past problems with brain stimulation Past problems with MRI Use of muscle relaxants or benzodiazepines Allergy to rubbing alcohol Any other health related illnesses that would prohibit a participant from physical performance testing Lack of transportation to and from the laboratory

Harmon KK, Girts RM, Pagan JI, Rodriguez G, Stock MS. The acute effects of action observation on muscle strength/weakness and corticospinal excitability in older adults. Exp Brain Res. 2022 Jun;240(6):1801-1810. doi: 10.1007/s00221-022-06370-2. Epub 2022 Apr 29.