Motor unit discharge behaviour in human muscles throughout force gradation: a systematic-review and meta-analysis with meta-regression.

The analysis of motor unit (MU) discharge behaviour provides an effective way of assembling information about the generation and control of movement. In this systematic-review and meta-analysis we identified and summarized the literature investigating MU discharge rate and discharge rate variability (CoV-ISI) during voluntary isometric contractions at various force levels. Databases were searched up to January 7, 2025, and a total of 262 studies were included. The meta-means of MU discharge rate and CoV-ISI were estimated and compared across human muscles. The influence of contraction intensity on MU discharge behaviour was assessed through linear meta-regressions. At low-to-moderate forces (<60% MVC), the first dorsal interosseous, biceps brachii (BB) and forearm extensors (FE) had the highest discharge rate, while the soleus had the lowest. At high force levels (>60% MVC), the tibialis anterior (TA) had the highest mean discharge rate compared to all other muscles, with the soleus maintaining the lowest. Regarding CoV-ISI results at low forces (<30% MVC), the TA had the lowest CoV-ISI values, except in comparison to the vastii. Additionally, the vastii had lower CoV-ISI values than the FE, gastrocnemius medialis, and soleus. Contraction intensity was positively associated with the mean discharge rates in all muscles investigated, although some muscles showed steeper slopes than others. Similar results were observed for CoV-ISI meta-regressions, with muscle-specific differences in slope. These findings suggest potential variations in neural control strategies across muscles during force gradation, such as differences in the relative contribution of rate coding to facilitate increasing force demands.