I'm no sports scientist but assuming are those efforts run at an equal pace? If so my guess would be increasing fatigue, and therefore increased effort to maintain the same pace.
@ewanmac started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
I'm no sports scientist but assuming are those efforts run at an equal pace? If so my guess would be increasing fatigue, and therefore increased effort to maintain the same pace.