This study investigated possible motor unit (MU) firing patterns underlying changes in biceps brachii (BB) surface electromyographic (SEMG) activity in 96 participants who performed isometric actions of the elbow flexors at 40%, 60%, 80%, and 100% of maximum voluntary contraction (MVC). We also conducted a modeling investigation to determine the extent to which a model would fit the experimental results. Experimentally, there was a linear increase (277%; p<0.01) in root-mean-square (RMS) amplitude with increasing force. The mean power frequency (MNF) remained stable from 40% to 80% of MVC, but there was a decrease (8.2%; p<0.01) between 80% and 100% of MVC. A modeling approach was taken wherein well-known recruitment and rate-coding schemes activated MUs whose basic building block was the muscle fibre action potential. Two conditions were investigated: (1) an increase in firing rate (rate-coding) and (2) synchronization. The levels of rate-coding and synchronization were selected to produce a linear RMS-force relationship as observed in the experimental data. Then, the impact of these two strategies on changes in MNF was assessed. The MNF remained stable from 40% to 80% of maximum excitation for both the rate-coding and synchronization conditions. There was a decrease in MNF between 80% and 100% of maximum excitation for both modeling conditions, similar to that observed for the experimental data. Thus, at these high forces at which experimental data are technically difficult to obtain, the model supports the idea that both rate-coding and synchronization are responsible for the changes observed in surface EMG amplitude and frequency characteristics.