RotoBaller's Nick Mariano looks at some fantasy baseball bats for week thirteen that are hitting either extremely hot or cold in the power department.

Welcome back to this investigative piece where we examine players who have seen some notable changes in their power profiles -- for better or for worse -- in 2017.

As usual, you don’t need me to tell you that Miguel Sano or Aaron Judge are strong or that Jarrod Dyson and Billy Hamilton are toward the bottom in average exit velocity. The next time we meet up here will have seen the season’s halfway point cross over into our rear-view mirrors. Now, bear with me as I’m in a hotel room in the middle of nowhere and need to drive eight hours tomorrow, so we’ll just have three names in each section compared to the usual four.

Identifying top power risers and fallers for each week can help you spot the best pickups before your competition. We'll do the hard work for you, looking at the underlying metrics that influence a hitter's power: fly-ball, pull, hard-hit rates and exit velocity.  Consider buying these week 13 power risers and selling these week 13 power fallers.

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Power Risers

Curtis Granderson - (OF, NYM):

The Grandy Man has popped off for five homers with two doubles, a triple, 12 runs scored and nine RBI in his last eight games thanks to a resurgent power stroke. Over the last two weeks, Granderson’s fly-ball rate sits at 57.6 percent with a 60.6 percent pull rate and 48.5 percent hard-hit rate, with all three of those metrics being roughly 10-15 percent above his marks from the first two and a half months of 2017. While I was admittedly ready to write off the 36-year-old, he’s shown a strong affinity for pop as recently as last season with 30 HRs and manager Terry Collins has made it clear that Curtis is a favorite of his. Granderson's best seasons came when his power stats were extremely tilted toward a pull-happy power swing, so this return to form is beyond encouraging.

Carlos Gomez- (OF, TEX):

While Gomez has gone 0-for-4 with four strikeouts since missing Tuesday’s game, the speedy 31-year-old had popped six homers in a nine-game span since being removed from the disabled list on June 16. There is zero chance that he can sustain a 75 percent HR/FB rate, but he is lifting the ball 44 percent of the time with a wild 61.1 percent hard-hit rate over this window. His batted balls are also being pulled at a 56 percent clip, so while the raw power totals will regress, the potency for surges like this is verifiably housed in CarGoGo’s bat. Mix in the fact that he can swipe his fair share of bases and was a strong first-round pick just three seasons ago, and I'm in.

David Peralta - (OF, ARI):

Before Peralta went 0-for-4 on Thursday, the 29-year-old had hit safely in 15 of his last 17 games to the tune of a .385 average with three doubles, a triple, four homers, 16 runs scored and 12 RBI. While he still doesn’t lift the ball with any real regularity (just a 25.4 percent fly ball rate over that stretch) and is using all fields with near equality (33.9 percent pull rate), his hard-hit rate has shot up to 40.7 percent with a mere 11.9 percent soft-contact rate accompanying it. With the word leaking that Arizona's humidor is not to be in 2017, perhaps Peralta can end up flirting with a 20-homer, 100-run season. The run total is lovely since he bats second in this insanely powerful lineup, though his mere 24 RBI on the year would greatly benefit from the return of A.J. Pollock hitting ahead of him. Get well soon, Mr. Pollock.

Power Fallers

Dansby Swanson - (SS, ATL):

Swanson went 0-for-3 with two strikeouts on Thursday, making him hitless in his last four games. That’s lousy enough, but you might be surprised to learn he’s actually batting .292 in June. The thing is, we’re here to discuss a much bigger problem than his batting average. The 23-year-old hasn’t logged an extra-base hit since June 13, with a measly 15.4 percent hard-hit rate and 12.8 percent fly ball rate behind the letdown. Labelling him as a "sell" isn't exactly meant to say "trade him", so much as "you really can just let him go". I know he's young and will have his bumps and bruises -- he had just 127 games of seasoning in the Minors before being called up to the front lines in Atlanta last season -- but this is just plain ugly.

 J.D. Martinez - (OF, DET):

Martinez’s first 30 games of 2017 saw him crush 11 homers with 25 RBI and a .301 batting average thanks to an extreme 59.5 percent hard-hit rate, 41.9 percent pull rate and identical fly ball rate. His swing was simply made for power. Since then, his pull and fly ball rates have both fallen to 33.3 percent, which isn’t terrible by any means, but it meant that his hard-hit rate falling 23 percentage points carried some extra oomph. That’s not even a bad mark, but it isn’t a world-beating elite rate like in his first month of the season. He will still produce above-average numbers for sure, and may even have another one or two major power spikes in him for '17, but if you can sell on his overall numbers right now then you should be able to come out a winner.

Matt Kemp - (OF, ATL):

Through June 12, Kemp had launched 11 homers with a .232 ISO, .384 BABIP and .327/.362/.559 slash line backed by a 33.5 percent fly ball rate, 34.1 percent pull rate and 36.5 percent hard-hit rate. None of those metrics jump off the page, but bunch them all up and you get a sturdy bat with 25-30 homer potential. Interestingly, his rates have only fallen by roughly 3-5 percent each -- but again, swirl it all together and you get a problem. It’s also just a 12-game sample size here, but hitting at a .205 clip with three extra-base hits is an issue no matter what style of gift wrap you put on it. It isn't as though he doesn't have the track record to buy into a rebound any day now, but if I could sell off Kemp to someone who truly buys into his 25-HR pop and an average in the .300s then my mouse will be dancing all the way to the accept button.

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