The triceps surae is composed of the mono-articular soleus and the bi-articular gastrocnemeii. Mean maximal motor unit discharge rates (MUDRs) reported for the soleus (~16Hz) are lower than other limb muscles tested (Dalton et al., 2009). Because of differences in fibre-type and functional anatomy it is important to determine maximal MUDRs in the two heads of gastrocnemeii, as compared with the soleus, to fully understand the interplay of these three muscles for plantar flexion. The purpose of the study was to record maximal MUDRs of the medial (MG) and lateral gastrocnemeii (LG) in 9 recreationally active, young men (age 24.2 ± 1.6y; 81.3 ± 8.1kg; 180.3 ± 5.3cm). During 3 separate visits to the lab, participants performed a series of 6-8, 7s maximal voluntary isometric contractions (MVC) of the plantar flexors with a knee joint angle of 90 degrees, with 5 minutes of rest between contractions. Maximum voluntary activation of the plantar flexors using the interpolated twitch technique was calculated. Custom tungsten microelectrodes were inserted individually into the belly of the lateral and medial gastrocnemeii and gently manipulated during the contractions to sample from as many distinct motor units throughout the muscle as possible. Action potential trains were analyzed offline to calculate discharge rates for each identified motor unit. All subjects were capable of high % of voluntary activation (\u3e96%) and achieved a mean maximal plantar flexor torque of 194.6 ± 57.1Nm. A total of 198 and 117 motor unit action potential trains were identified in the MG and LG, respectively. The mean maximal motor unit discharge rates were 22.7 ± 8.6Hz and 22.4 ± 8.1 Hz (Range: 5.5 - 64Hz) in the MG and LG, respectively, and were not significantly different from one another, p \u3e 0.05. The coefficient of variation of discharge frequencies in the identified trains were 14.6 ± 1.4% and 14.3 ± 2.2% in the MG and LG respectively, and were not significantly different, p \u3e 0.05. Maximal MUDRs in both heads of the gastrocnemeii are greater than in the soleus, but are not different from one another. Despite their similar roles in plantar flexion, the amount and degree of habitual activation (phasic vs. tonic) or their functional role (flexor vs. extensor) may account for motor unit discharge rate differences between the gastrocnemeii and soleus during plantar flexion actions
