While Francisco Rodriguez's uniform has thrice changed since his record 62-save season in 2008 as a member of the Los Angeles Angels, one thing has remained constant: His strikeout totals. Punching out 10.1 batters per nine innings that season, Rodriguez went on to record K/9 rates of 9.7, 10.5, 9.9 and 9.0 in each of his four subsequent seasons, and a healthy 10.4 K/9 combined ratio last season with Milwaukee and Baltimore. After signing him to a one-year deal on Friday, the Brewers will be thrilled if that number perpetuates into 2014.
But as Bill Petti of Fangraphs.com showed in an article last May, the average reliever's K/9 rate drops significantly with age. More specifically, the average reliever's rate at age 26 drops by about 2.0 compared to their age 31 season. Yet as we've already noted, Rodriguez hasn't followed this typical reliever trend, as his K/9 rate has actually increased from his age 26 season in 2008 (10.1) to his age 31 campaign (10.9) in 2013. What makes this more intriguing is his declining fastball velocity, which stood at 91.1 MPH last season compared to 92.2 MPH in '08. Given this, how is it that Rodriguez continues to augment his strikeout capacity with age and declining fastball velocity?
It all starts with his secondary stuff, which he commanded masterfully against left-handed batters last season.
Francisco's non-fastball pitch frequency vs. LHH, 2008
Francisco's non-fastball pitch frequency vs. LHH, 2013
Overall, lefties hit .227/.346/.336 against Rodriguez in 2008 compared to right-handers' .205/.276/.295 slash line. A big reason for lefties' high on-base percentage that year was his lack of command -- walking left-handers at an escalated 13.1% rate (and 29.9% strikeout rate). Those numbers went in opposite directions last season, as Rodriguez struck out southpaws at a 35.2% rate while lowering his walk rate to 7.6% against them. Against his 'soft' stuff (i.e. non-fastballs), these increases were much more extreme -- punching out lefties at a 58.1% clip compared to 38.6% in 2008.
The reason for this sharp increase can be seen in the images above; Rodriguez was more efficient at commanding his changeup and curveball away from lefties last season (51.8%) in comparison to 2008 (36.7%). Not only did this improved command positively affect his strikeout rate, but it also helped him increase is ground-ball rate to 52.9% (juxtaposed to 44.4% in 2008) and boost his called-strike rate to 41.4% (opposed to 32.3% in 2008). In two-strike counts, lefties chased at a whopping 64% of his soft offerings and struck out at a 78.1% rate (fifth highest among relievers with 45 innings).
While sample size should be considered when comparing his 2008 and 2013 campaigns (he registered 68.1 innings in 2008 and only 46.2 last season), this is somewhat of an unsound argument against his strikeout increase moving forward, as I'm certain Ron Roenicke and Milwaukee's managerial staff won't call on Rodriguez more than 70 times next season -- even if he's lights out. They'll need to preserve his aging arm if they're anywhere near contention in September.
Considering everything, we can definitively say that Rodriguez is still every bit the strikeout artist he was in his most dominant years -- even with his waning velocity. And that's a credit to his improved secondary stuff against lefties, who gave him trouble even at his best in 2008.