Intracortical Inhibition Assessed with Paired-Pulse Transcranial Magnetic Stimulation is Modulated during Shortening and Lengthening Contractions in Young and Old Adults.

Abstract

The modulation of intracortical inhibition is thought to be impaired in older adults, which may contribute to their reduced fine motor control, particularly during lengthening muscle contractions. To quantify the magnitude of intracortical inhibition and movement performance during postural, shortening and lengthening contractions of a hand muscle in young and old adults. In 18 young (23.2 ± 4.2) and 16 old (70.6 ± 6.5) subjects, paired-pulse transcranial magnetic stimulation (TMS) was used to assess short-interval intracortical inhibition (SICI) and long-interval intracortical inhibition (LICI) during a movement task involving the first dorsal interosseous muscle. The task required a constant load (50 g) to be slowly lifted and lowered using the index finger while single- or paired-pulse TMS was delivered during the shortening or lengthening contraction. Relative to postural contractions, SICI during shortening contractions was reduced by 29% in young subjects (P < 0.0001) and 43% in old subjects (P < 0.0001), whereas SICI during lengthening contractions was reduced by 11% in young subjects (P = 0.0004) and 33% in old subjects (P < 0.0001). Furthermore, SICI was significantly less in older adults during lengthening contractions (P-values < 0.01). For LICI, inhibition was not influenced by contraction type in old subjects, but was increased by 11% during shortening contractions (P < 0.0001) and by 9% during lengthening contractions in young subjects (P = 0.0008). In addition, old subjects showed significantly less LICI than young subjects in each movement phase (both P-values < 0.05). Shortening and lengthening contractions with a constant load are associated with a modulation of GABAergic inhibition that is altered by healthy ageing.