This paper is more than 30 years old but it is a beauty. Farley et al talk about human preferred hopping frequency and how it relates to metabolic cost and how fast muscles are contracting, and also how our instructions can influence how well people hop (and the mechanisms that drive this). Important to consider hopping instruction when assessing submaximal hopping in the clinic. 

Here's a link to the paper

Farley, C.T., Blickhan, R., Saito, J. and Taylor, C.R., 1991. Hopping frequency in humans: a test of how springs set stride frequency in bouncing gaits. Journal of applied physiology, 71(6), pp.2127-2132.