Mechanomyography (MMG) provides a measure of muscle mechanical changes during contractions. The purpose of this study was to quantify alterations in MMG signals during fatigue at two muscle lengths. Comparisons with electromyographic (EMG) recordings were made. A group of 13 subjects performed isometric dorsiflexions (50% of maximum for 60 s) at 40 degrees of plantarflexion (long, l(l)) and 5 degrees of dorsiflexion (short, l(s)). The mean power frequency of the EMG (f(EMG)) and MMG (f(MMG)) signals and the mean rectified MMG (rMMG) and EMG (rEMG) were determined over each 1-s period, normalized to the respective maximal value, regressed against time, and the resulting slopes (units = %max. s(-1)) were analyzed. The slopes were larger (P = 0.007) at l(l) compared to l(s) [mean l(l) 0.50 (SD 0.26), mean l(s) 0.27 (SD 0.16)], however there were no differences (P = 0.24) between mean fMMG slopes [l(l) -0.10 (SD 0.16), l(s) -0.16 (SD 0.11)]. Similarly, slopes were larger (P = 0.001) at l(l) versus l(s) [l(l) 0.26 (SD 0.13), l(s) 0.08 (SD 0.15)] and there were no differences (P=0.89) between mean fEMG slopes [l(l) -0.15 (SD 0.14), l(s) -0.14 (SD 0.12)]. At 5 s following the exercise to fatigue mean MVC (units = %max) were not significantly different between l(l) and l(s) [P = 0.08; l(l) 78.8 (SD 9.1), l(s) 85.2 (SD 6. 0)]. These results showed that during fatiguing contractions, MMG and EMG amplitudes increased while frequency characteristics decreased at both muscle lengths. The change in and was greater at l(l) but no differences in fMMG or fEMG slopes occurred between lengths. These results would suggest that larger increases in motor unit recruitment occur with time during fatigue at l(l) compared to l(s).