The bike to run transition (T2) will be your hardest transition not only in terms of fatigue but also biomechanically.  The fatigue has set in of course from the swimming and biking that precede the run.  Often time’s athletes will see a quick rise in heart rate, perceived effort, breathing, etc due to this change form bike to run.  The biomechanical aspect that makes T2 so difficult comes from the fact your body now has to adjust itself physiologically and mechanically to running after cycling. Although the two disciplines have similar lower body motions the muscle fibers, joints, tendons, etc. are quite dissimilar.  The feelings in your legs when you begin the run are often described as running with “tree stumps”.

To help make the transition from bike to run easier and more comfortable there are few things to keep in mind:

Practice the Bike - Run transition, especially as the racing season draws closer.  Practice should be done 
       1 x every 1-2 weeks for a up to 1-2 months prior to the first race, in the form of a BRick workout with
        the bike at least 30 min in length prior to a run of at least 10 minutes.

Pedal in your small chain ring at a higher cadence the last ¼ mile of the bike ride to help redistricted blood 
      flow which will lessen that feel of running on “tree stumps”. 

Once off your bike jog through the transition area to rack your bike this helps to make some initial biomechanical adaptations before you actually start your run.

As you begin the run portion of the race that “stumpy” feeling can really be evident so don’t force your stride. Keep your stride shortened the first portion of the run.  This portion will feel awkward but after ½-1 mile you will be able to settle in to your normal stride.  

These tips will make a big difference you making T2 as seamless as possible.