The straight leg flutter kick is highly dependent on the gluteal muscles and the hamstrings, as well as the hip flexors. Very little that one would do on land trains for hip flexion and extension with a straight leg and against resistance, and the mechanical advantage of the muscles for doing that is poor. It's a kick that requires a lot of leg strength to do with power, and it's my personal belief that the use of that kick is responsible for a lot of the focus in training on cramp removal
The frog kick is a much more efficient kick in still water, because it utilizes muscle groups that you are accustomed to using, where the muscles have good mechanical advantage across the joints. However, the glide phase of the frog kick makes it less advisable against flow or current.
In those situations, the cave diver type flutter kick, with the bent knee, is a better kick, because it provides propulsion throughout the entire kick cycle. In addition, flexing and extending the knee utilizes the hamstrings and quadriceps muscles across the knee joint, where they have excellent mechanical advantage, and these are muscles most of us have some strength in, simple from activities of daily living.
The frog kick is a much more efficient kick in still water, because it utilizes muscle groups that you are accustomed to using, where the muscles have good mechanical advantage across the joints. However, the glide phase of the frog kick makes it less advisable against flow or current.
In those situations, the cave diver type flutter kick, with the bent knee, is a better kick, because it provides propulsion throughout the entire kick cycle. In addition, flexing and extending the knee utilizes the hamstrings and quadriceps muscles across the knee joint, where they have excellent mechanical advantage, and these are muscles most of us have some strength in, simple from activities of daily living.