According to Spanish newspaper ABC (h/t Francesc Gimeno of Sport), the 31-year-old is working on reducing his muscle mass in order to keep the electric pace that helped define much of his career, and "it was proposed that he loses one kilogram each year" after he turned 30 and has thus far shed two since January last year.
Per ABC: "He currently weighs 78 kilograms. And it’s his idea to reduce his muscle mass a little more to continue at the top of the game for as long as he possibly can. For that reason, he has had to also change his demanding personal training problem."
Ronaldo said the following: "You can count on the fingers of one hand the number of players that have been at the top as long as I have, for six, seven, eight, nine or 10 years. I feel privileged to be at this level."
Indeed, for Ronaldo to continue at the top well into his 30s, he's right to recognise the need to manage his body more carefully—particularly as he has announced he plans to play on until at least 2022, per Marca (via Dermot Corrigan of ESPN FC).
In November, Sky Sports' Guillem Balague reported the forward was no longer at his best physically due to the toll taken by his "gruelling physical regime" and having played between 30 and 60 games every season since he was 17.
The Portuguese is still scoring plenty of goals for Los Blancos, as Squawka Football revealed:
Squawka Football @Squawka
Cristiano Ronaldo has now scored 34 goals in 32 matches for Real Madrid in 2015/16. https://t.co/5yqkQtMwMw2016-2-21 15:39:01
In the 2-0 win over Roma in the Champions League earlier in February, Ronaldo opened the scoring with an exceptional effort:
BT Sport Football @btsportfootball
A moment of magic from @Cristiano fires @realmadriden ahead ✨ #UCL #RomaReal https://t.co/w7ri2CyxnX2016-2-17 21:04:07
However, such a strike—which was so prevalent in his days at Manchester United and during the early years in Madrid—is now something of a rarity, relatively speaking, as these days he more often than not finds the net from close range after finishing a delivery from one of his team-mates.
Further, he is finding it more difficult to make consistent contributions—often racking up several goals in one game rather than scoring week after week as he did in the past. Per WhoScored.com, he recently endured what, by his standards, can be considered a goal drought:
Cristiano Ronaldo: Has now failed to score in 3 consecutive La Liga matches for the first time since February 2011 http://t.co/iYXeiUIM9C2015-2-14 18:59:59
Ronaldo's development from a pacey winger into a physically strong but slightly slower forward has been visibly evident in recent seasons.
If he can avoid losing any further pace—or perhaps even regain a yard or two after losing his muscle mass—and adapt his body to the limitations age will inevitably bring, he should be able to not only prolong his career but return to the consistent level of performance he once enjoyed.