Six-time world champion Marc Marquez admitted he felt "a lack of strength" as he struggled in practice for the Spanish Grand Prix on the same Jerez track where a crash ended his 2020 campaign after just one race.
The 28-year-old, who underwent four surgeries on his right arm and missed the opening two races of the 2021 season, finished down in 16th place on Friday.
"In the first session I was able to ride as I wanted, when I wanted to do a quick lap I was able to do it," said the Honda star who was seventh on his return to the sport in the Portuguese Grand Prix at Portimao earlier this month.
"But in the afternoon, I felt something in the back part of the triceps where I was operated on.
"I felt a lack of strength in this area, my position on the bike wasn't the same. It's something I didn't understand, because it was the first time.
"I'll try not to stress and see if I feel better tomorrow. In Portimao, it got worse from Friday to Sunday, so I will try to maintain a constant physical condition throughout the weekend. The idea is to try to finish the race in better shape."
Italy's Francesco Bagnaia was the fastest man on the track on Friday on his Ducati ahead of France's Fabio Quartararo, the Yamaha rider who leads the championship after winning two of three rounds so far.
Bagnaia, second in the standings, was 0.178sec ahead of Quartararo while Spain's Aleix Espargaro, on an Aprilia, was third at 0.437sec off the pace.
Franco Morbidelli, of Yamaha-SRT, was fourth fastest with Yamaha's Maverick Vinales, the winner of the season-opener in Qatar, in fifth.
Practice times on Friday:
1. Francesco Bagnaia (ITA/Ducati) 1min 37.209sec, 2. Fabio Quartararo (FRA/Yamaha) 1min 37.387sec, 3. Aleix Espargaro (ESP/Aprilia) 1min 37.646sec, 4. Franco Morbidelli (ITA/Yamaha-SRT) 1min 37.704sec, 5. Maverick Vinales (ESP/Yamaha) 1min 37.726sec, 6. Takaaki Nakagami (JPN/Honda-LCR) 1min 37.775sec, 7. Miguel Oliveira (POR/KTM) 1min 37.816sec, 8. Alex Rins (ESP/Suzuki) 1min 37.847sec, 9. Johann Zarco (FRA/Ducati-Pramac) 1min 37.888sec, 10. Brad Binder (RSA/KTM) 1min 37.896sec
16. Marc Marquez (ESP/Honda) 1min 38.291sec
21. Valentino Rossi (ITA/Yamaha-SRT) 1min 38.698sec