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This Report examines two key issues emerging when one deals with high-resolution data for particle tracks in bed-load transport. The first relates to how a particle motion/rest state is defined; using different definitions of motion changes the mean values computed for particle hop length and duration, that are key properties for phenomenological analysis of bed-load transport and quantitative estimation of the associated rates. The second issue refers to the experimental bias due to considering a finite observation area within the channel bed. A conceptual presentation of the problem is here complemented by an analysis of experimental data, from both the literature and new experiments with weak bed-load transport. Four definitions of motion were employed, together with two different ways (correcting or not the bias due to the observation area) of computing mean values from measured hop data. The results demonstrate that using different definitions and operational methods highly changes the mean values of hop-related quantities. This may explain the scatter among literature data and challenges future Lagrangian investigation of bed-load transport.