Running for Cyclists

Bad weather and cold temperatures in wintertime often make it very hard to go out on the bike and perform the daily training. 

Specific gym sessions or indoor training on stationary bikes therefore have to be part of the usual winter program; some athletes prefer doing some MBK, cross-country ski or swimming as complementary activities. 

Many cyclists are hesitant and suspicious when it comes to running, commonly deemed responsible for sore legs, joints and tendons problems and said to utilize muscles not relevant to the cycling gesture. 

Indeed running implies both concentric muscle contractions (fibers working to shorten) in the pushing phase and eccentric muscle contractions (fibers working to stretch) in the landing phase, from the impact with the ground until the beginning of the pushing phase. 
The eccentric component increases when running downhill, whereas it reduces when running uphill. 

Cycling instead involves almost exclusively concentric muscular work, with a modest eccentric component when pedaling out of the saddle, especially in sprints and accelerations. 

Human movement (with the exception of swimming, requiring only concentric contractions) and generally any muscular activity imply both concentric and eccentric work, alternating in the cyclic nature of gesture. 

The eccentric component is mainly the one that induces the most significant increases in muscle strength: repeating exercises such as leg press or squats are most effective in the “return phase” of the movement. 

A cyclist who exclusively relies on pedaling tends to lose strength. 

I believe running can be useful to cyclists during wintertime, either to maintain a good cardio-respiratory efficiency, or to improve the strength, or to remedy muscular unbalances caused by cycling practice. 

Obviously one has to start with moderation (15-20 minutes, 3 times a week), preferring soft grounds (e.g. grass) and flat or slightly uphill courses. 

One could then gradually increase up to 40-60 minutes, 3-4 times per week.
Heavier athletes will have to carefully choose the proper courses and be wary of the progression of distances.