In message , DJ Kim
writes
Although I do spinning, weight lifting, cycling during off-season in
preparation of the upcoming ski season, I noticed that those exercises
do not actually help the muscle that is extensively used in skiing -
the one that flexes the legs. I don't know the name.

The one in the back of your thigh, that flexes the knee? It's
apparently called biceps (two-headed) or biceps femoris to distinguish
it from its better-known namesake in your arm.

Which weight training, or exercise, helps the muscles that flexes the
legs?

Running gives it a good workout, especially if you change pace and
direction frequently (shuttle runs etc). So does any sport where you
dodge about, even tennis or badminton. Cross-country running is better
than flat ground or a machine.
Cyclists should also take some upright exercise to avoid spinal mobility
problems: I do regular sessions standing in the pub raising my wrist,
but that wasn't enough.
--
Sue ]