How Phillies No. 2 prospect Aidan Miller is developing his 5th tool: baserunning

Aidan Miller’s base-stealing prowess wasn’t as eye-popping as his other attributes out of high school. When the Phillies selected him in the 2023 MLB Draft, it was his power that made him a first-round selection.

But as Baserunning Coordinator Gary Cathcart watched Miller in action for the first time at draft camp, he saw untapped potential. Cathcart told then-Director of Player Development Preston Mattingly he thought Miller could blossom into a player who regularly stole 20 bases a season, which came as a shock to the now-Philadelphia general manager.

Miller has quickly proven Cathcart right, even surpassing the coach’s early expectations. The Phillies' No. 2 prospect and MLB Pipeline’s No. 22-ranked player has tallied 24 stolen bases through 52 games with Reading in 2025. The total has already outdone his 2024 total in 50 fewer contests.

Cathcart said Miller’s sprint speed is near the top of the organization based on their metrics despite his sturdy 6-foot-1, 205-pound build. He’s become nearly automatic against right-handed pitchers with a 91.6% success rate on 24 attempts, building off the offseason lessons from Cathcart and Tampa-based strength trainer Denny Locascio. With his raw power and newly unlocked baserunning ability, Miller is emerging as one of baseball’s most dynamic prospects.

“If you want to be one of the best players in the game, you have to perfect all aspects of the game. It's all the little things, and baserunning is included in that,” Miller said. “I want to be a five-tool player.”

Cathcart saw Miller’s base-stealing future from the beginning. But even he said he would’ve thought Miller’s current totals — after swiping 23 bags across 102 games in his first professional season — would’ve been more of an ambitious goal than a realistic one. Miller understands most people know him for skillsets outside of his baserunning.

It’s for good reason.

He recorded a 1.135 OPS in his junior year at J.W. Mitchell High School (FL) and holds a 60 power grade. Miller’s also stuck at shortstop through his first two years of professional baseball despite many believing he’d be forced to move to third base due to his size.

It’s widely agreed upon among industry experts that Miller would’ve gone much higher in the draft if not for a broken hamate bone in his senior season. Philadelphia believed it got a steal.

Miller admittedly focused largely on hitting and fielding as a prep player, glossing over improving on the base paths. After his first full professional season in 2024, he knew he wanted to get to work.

“Baserunning is for everybody, base stealing is a skill that just a few guys can be really good at. They have the God-given ability to do that,” Cathcart said. “I saw (Miller) was potentially one of those guys.”

Cathcart, a 13-year Minor League manager, thought Miller’s speed was underrated but still saw finer details in his base stealing that could use polishing. The coach was already pleased with his lead techniques and instincts, though he targeted quicker first steps.

They worked together throughout the offseason and continued in spring training as Miller was the youngest player in the Phillies’ Major League camp. Miller said he learned more points on consistency from prolific speedsters like Trea Turner and Johan Rojas.

Fast forward a few months, and he’s made stealing bases look nearly automatic. On June 11 against New Hampshire, Miller reached on a walk in the bottom of the ninth with the Fightin Phils down one. He stole on the first pitch, immediately moving into scoring position with no outs. The week prior, he stole six bases in six games in Harrisburg. To no fault of his own, he was stranded at second against the Fisher Cats. But Miller nearly flipped the game on its head in the blink of an eye with his recently advanced ability.

The moment mirrored Miller’s simple yet effective mindset: get on first base so he can steal and score. Miller believes the strategy translates to seeing more pitches and ultimately setting the tone, especially as he’s moved into the leadoff spot.

While boosting his talents with Cathcart, his work with Locascio has been paramount in transforming his physical traits. Locascio, the co-founder of Impact Fitness, trains top talent in the area with NFL wide receivers George Pickens, Diontae Johnson and Auden Tate at the top of the list. Miller has worked sparingly with the other athletes. Most are in private sessions, but at times they’ve worked in groups together. Locascio said there are often discussions around the facility about what it takes to be a professional athlete for an extended period, nutrition and a flurry of other topics Miller digests for more knowledge.

He began working at Impact six years ago in high school, joining his older brother and 2020 second-round pick, Jackson. Locascio said both brothers are built more like football players than baseball ones, with powerful lower bodies. While increasing his power, the build creates challenges and requires more maintenance.

“Many look at someone like (Miller), who's a little bit more muscular, and they may say, ‘Man, he might not have the full flexibility range of motion,’” Locascio said. “Even though he's big and strong, we want to make sure he still has flexibility.”

To retain proper playing shape, Miller sits down with Locascio at the beginning of each offseason to discuss what he wants to work on. What went well? What didn’t? Following the 2024 season, baserunning was at the top of Miller’s list.

From there, the two planned out what the offseason would look like on a week-by-week schedule. On Mondays, Wednesdays and Fridays, Miller worked on his speed and agility at the facility. Two days a week, he was at the field strictly working on baserunning. Workouts usually were twice a day, weight lifting in the mornings and hitting or fielding in the afternoons.

Each training session with Locascio begins with speed or agility. Miller uses a jump rope to warm up his Achilles, ankles and calves. Then, he works with a mixture of speed ladders, cone drills, hurdles and rolling (also known as falling) starts as a continued warm-up. Squats and deadlifts are also a priority.

They later work into more explosive practices to emulate baserunning movements, opening the hips to the right and pulling his left leg over his right. Miller works off his own timing before Losciano simulates a pitcher’s motion or a catcher’s throw to second.

Rinse and repeat, all offseason long.

Miller has also worked on his nutrition with Locascio. He tests multiple snacks, protein bars, and electrolyte enhancers during offseason workouts to see what helps him retain strength through a three-hour game without upsetting his stomach. Peanut butter and jelly has become a staple.

Along with his physical improvements, Miller continuously develops the mental portion of base stealing. Each Monday off day, Miller and his teammates receive a report and video of each opposing pitcher for the week’s opponent. Miller devours the information with the help of Fightin Phils’ first base coach Mycal Jones. Miller credits Jones with assisting in finding the minute tendencies of a pitcher, helping calculate when off-speed pitches will be thrown and in which counts.

Cathcart’s offseason work, and in-season from time to time, honed in on working within the constraints of the pitch clock. He alerts base runners to continue breathing while a pitcher holds the ball for a few extra seconds, keeping the body loose. Miller pushing the envelope with a lengthy lead puts pressure on the pitcher to overuse their two allotted pick-offs.

The stolen base tally alone has forced opponents to pay attention. Miller enjoys learning the process. He understands it increases his value.

“Being a good base runner helps to be a good teammate,” Miller said. “It helps you score more runs and ultimately, helps you win more games. That's always the goal.”

As Miller approached his total from last season in the beginning of June, he texted with Cathcart about the recent success. The goal ahead of the season for Miller was to reach 30 stolen bases. He’s only six away from the number with three months to play and leads the Eastern League. He told Cathcart his goal is to finish on top.

As for the mark that will give him the crown, Miller and Cathcart have a change of plans.

Heck with 30. Miller’s shooting for 50.