McConaughey and Gyllenhaal were chasing the seven-time Tour de France champion on the challenging ride Wednesday, TMZ.com reported.
The Web site said all three were wearing skin-tight spandex and aerodynamic helmets. Armstrong, who was recently voted Sexiest Man Alive by People magazine, reportedly stayed with McConaughey and Gyllenhaal throughout the workout.
Reps told TMZ that Gyllenhaal and McConaughey are not training for anything -- they just enjoy riding.
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