Introduction. Exploration of human motor unit (MU) characteristics in clinical and research settings is commonly applied using intramuscular electromyography (iEMG) [1]. This typically involves the insertion of a concentric needle electrode into the muscle during voluntary contractions, and although minimally invasive, may cause discomfort and/or intramuscular damage. Afferent feedback from the muscle as a direct cause of needle insertion may influence MU characteristics such as firing rate (FR) and recruitment [2], thereby influencing the parameters it is designed to investigate. However, this has not yet been fully explored. Methodological advances in non-invasive high-density (HD) EMG allow the direct tracking of MUs across successive contractions in humans, inclusive of simultaneous recording with intramuscular needles, and presents an opportunity to investigate the true effects of needle insertion.

Aim. The aim of this study was to track the function of individual MU characteristics across subsequent contractions, with and without intramuscular needle insertion.

Method. Following familiarisation, six individuals (females n = 3) performed two dorsiflexor trapezoid contractions at 25% of maximum force (3s ascent, 12s hold, 3s descent). HD-EMG was recorded from the tibialis anterior (TA) with a 64ch surface electrode, and signals were decomposed into individual MU potentials and their corresponding spike trains [3]. Prior to the second contraction, a 26-gauge iEMG needle electrode was inserted into the TA, and individual MUs from HD-EMG were tracked with and without needle insertion. Multi-level regression models were used to determine the effect of needle insertion on MUFR at recruitment, decruitment and during the sustained phase, MU recruitment threshold ratio (recruitment:derecuitment), and the coefficient of variation in force. Statistical significance was accepted at p<0.05.

Results. The mean number of recorded and tracked across MUs contractions was 27 ± 13. MUFR at recruitment was 11.33 Hz without the needle and did not differ with needle insertion (9.96 Hz, p = 0.259). During the steady phase at 25% MVC, MUFR was 15.40Hz without needle and did not differ significantly with needle insertion (13.02 Hz, p = 0.175). MUFR at de-recruitment was 7.90 Hz without the needle and did not change significantly with the needle inserted (7.24 Hz, p = 0.198). The recruitment ratio was not altered between the contraction types (p = 0.627). The coefficient of variation in force was not impaired with needle insertion (p = .895).

Conclusion. The current pilot data reveal minimal effects of intramuscular needle insertion on a range of TA MU characteristics at 25% of maximum force and highlight the applicability of combining EMG techniques. However, further exploration is required in a variety of muscle sizes and locations, at different contraction intensities to allow definitive conclusions to be determined.