I have a genetics degree and do recall a bit of population genetics.
To literally answer the question, "Do African-Americans have extra energy in their muscles? " requires a little clarification.
Muscles primarily use a chemical, ATP, as the preferred fuel source. There is no reason I’ve ever seen to indicate that African Americans have more ATP in their muscles than others. If that is the measure of “extra energy” then I’d say the answer to the OP’s question is simply No.
Muscles do have a couple different types of muscles fibers, fast-twitch and slow-twitch. Production of these fiber types falls under genetic control.
Fast twitch fibers contract faster (no surprise there) and with more force, but tire more quickly. A particular individual genetically blessed with a high proportion of fast-twitch fibers may, through further training and conditioning, be able to particularly excel at sporting events favoring bursts of muscle contraction such as sprinting in track and field or weightlifting.
Slow twitch fibers are more of the slow-and-steady wins the race type of fiber. They don’t contract as fast or as forcefully but can keep going like the Energizer Bunny. Individuals blessed with favorable amounts of slow twitch fibers may, through training and conditioning, be able to particularly excel at events requiring prolonged endurance such as running a marathon or boxing.
In as much as the slave trade to the Americans primarily drew slaves from western Africa it is possible, in theory, that the relative prevalence of slow and fast twitch fiber genes from that source population was different than that from elsewhere in Africa or Europe. Different environments may have favored different gene variants.
So maybe African-Americans have more of one muscle fiber type, on average as a population as a whole, than do populations with ancestry elsewhere. Maybe. Not proven. If having more of one or the other type of muscle fiber is a measure of “extra energy” then I’d say it’s possible, but unproven.
Finally, there is one way that the relatively short time scale (short in evolutionary terms) of the slave trade could have produced dramatic swings in gene frequency other than directed breeding - by random chance and a founder effect in a relatively small isolated population. As an example, if the modern Afro-Caribbean population on a particular Caribbean Island were descended from a small group of slaves then there could be significant deviation from Hardy Weinberg equilibrium for genes affecting particular types of muscle twitch fibers when compared to the larger population of descendants of slaves throughout the broader Americas.