Spike-triggered averaging (STA) has been used experimentally to identify multiple features of neuronal output including post-synaptic potentials, whole muscle facilitation, and motor unit output. Using only the most basic assumptions, we develop equations that predict the effects of neuronal firing rate and synchronization on STA. We use exact equations to show that, if the measured neuronal output is multidimensional, then the direction of STA is unaffected by firing rate in the absence of synchronization. We then extend these equations into a framework for analyzing synchronization, and show that under reasonable conditions the analysis of synchronization can be mapped to linear algebra. We consider whether the effects of synchronization can be removed, and if there are neuronal output configurations for which STA is insensitive to synchronization. We conclude that STA can give insight into how neuronal synchronization is organized with respect to neuronal output.