Using micro elecrics.
This "boasts" a 20% efficiency ratio, which is kind of high for such small systems (according to the article). Besides the flow of blood it can also use the flow of fluids caused by your movements, so in some cases it can be used to create energy out od your movement (Stillsuits anyone?)
In support of DyingLight's answer, chemical power generation using sugars seems a solid option: https://www.google.com/amp/s/www.rbth.com/science-and-tech/327650-russian-scientists-electricity-from-blood/amp
Note that this wont be powering anything like a full prostetic leg, just something small like a pacemaker.
Then for something outlandish that requires some genetic engineering: muscle powered dynamo's.
A muscle is essentially two "plates" with a fiber in the middle. This fiber has arms which can use ATP to grab extrusions on the plates and pull itself onwards. In the middle of the muscle all arms can grab the plates while at the maximum extention/contraction states not all arms can help pull, which reduces the maximum muscle strength at those positions.
But with some genetic engineering, you can put the plates on a circle that turns either a dynamo or more directly some gears. With a circle the arms will always have full access and can generate as much power as any muscle. So if you put enough of them in a row you can turn the biochemical muscle energy into the kinetic/electric energy you need.
*I have no idea of the accuracy of this article.