What determines the specific pattern of activation of primary motor cortex (M1) neurons in the context of a given motor task? We present a systems level physiological model describing the transformation from the neural activity in M1, through the muscle control signal, into joint torques and down to endpoint forces and movements. The redundancy of the system is resolved by biologically plausible optimization criteria. The model explains neural activity at both the population, and single neuron, levels. Due to model's relative simplicity and analytic tractability, it provides intuition as to the most salient features of the system, as well as a possible causal explanation of how these determine the overall behavior. Moreover, it explains a large number of recent observations, including, the temporal patterns of single-neuron and population firing rates during isometric and movement tasks, narrow tuning curves, non-cosine tuning curves, changes of preferred directions during a task and changes of preferred directions due to different experimental conditions.