As we age, it is typical to see a decline in strength due to a loss of muscle mass, or atrophy, but recent evidence suggests our nervous system may also play a role in this via a decline in motor cortex excitability. With a continual decline in strength comes increasingly more functional impairments in mobility and increased risk of injury in the very old. The question is, how can we can manage these neural changes to diminish the effect they have on strength and function in older individuals? In a recent study published in the Journal of Geriatric Physical Therapy by Oki and colleagues, this question is explored further.
Oki and colleagues proposed that the use of transcranial direct current stimulation (tDCS) of the motor cortex may have positive effects on muscle strength and voluntary activation in the very old population. Their study involved 11 participants (>80 years old) completing isometric strength testing of the non-dominant elbow flexors on 3 separate occasions. The first session was to establish a baseline, the second was performed following tDCS or imitation tDCS for 20 minutes, and the last was to compare for any significant difference. In addition to measuring strength, each session included examination of electrical activity within the muscles (EMG) as well as voluntary activation (via electrical stimulation).
Surprisingly, the results of this study did not confirm researcher’s hypotheses. There was no significant difference between anodal tDCS and imitation tDCS on strength, electrical activity (EMG) or voluntary activation. The thought was that by increasing voluntary activation, there would be a resultant increase in strength and electrical activity (EMG amplitude) in the muscle. Instead, what they found was that there weren’t really any limitations in elbow flexor voluntary activation. In fact, on average participants showed 99.3% voluntary activation levels of the elbow flexors. With there already being such high voluntary activation, the use of an outside mechanism to further increase that number was unrealistic. With that being said, it is possible that you may see more significant effects of anodal tDCS in musculature with lower voluntary activation levles, but this is just one of several limitations of this study. All in all, the most surprising outcome of this study was how efficiently 80+ individuals were able to recruit their muscles when asked to!