Swim all strokes, not just freestyle. Triathletes, I have found, tend to have the mindset that because they only race freestyle, they should only train freestyle. Unfortunately, they are not considering the big picture. If you overwork muscles in one direction and don't equally strengthen the opposing muscles then it will result in muscle imbalances. Why is that a problem, you might ask? Well for starters it can make your joints more prone to injury and thereby damaging the cartilage. Movement occurs when one muscle contracts while the opposing muscle lengthens. If one muscle is stronger or tighter it can pull joints out of alignment as it overpowers the opposite muscle. Muscle imbalances can therefore create joint dysfunction and can lead to postural changes and injury. This is what can happen to the shoulder joints of those swimmers who only swim freestyle. The range of motion, flexibility and reaction time of the muscles can also become compromised with the overall performance of an athlete being adversely affected. Swimming all four strokes gives a greater all body workout. Swimming provides low impact exercise on the joints unlike running, so for cross-training purposes swimming is the way to go. Backstroke is the great companion to freestyle as it turns the shoulders in the opposite direction, helping to minimise the chance of muscle imbalances. Breaststroke kick helps to work the inner thighs and buttocks and butterfly helps to build lower back and core strength.
Have good technique. I have often heard triathletes say they don't need to swim like a pool swimmer and they don't need high elbows. However, I have noticed time and again good pool swimmers out swimming triathletes in the open water. Triathletes do themselves a disservice by disregarding good technique. Good technique is based on science, bio-mechanics, which when applied gives you the maximum forward motion using the least amount of energy, producing the least amount of resistance and helps in preventing injury.
Work on your kick. This is often neglected in a triathlete's training program. Their reasoning, understandably, is because they need their legs for the bike and run, they will save their legs on the swim. On short distance triathlons, elite athletes six beat kick the entire distance! If you ever hope to be right up there, then you need to kick. Even for long distance triathlons there will be times when six beat kicking is advantageous, especially at the start of the race to position yourself in the lead pack or to make a break away or find clear water. Who will want to come near you if you're kicking up a storm right in their face? This will give them no chance of swimming over the top of you. Also, if your feet are near the surface, your body alignment in the water will be better. Kick work in the pool is a great way to not only strengthen the legs but also the buttock muscles. Weak gluteus medius muscles can cause injuries to runners and decrease their stride length. This weakness can be the primary cause for shin splints and Achilles tendinitis. Kick work should not just be freestyle but all four strokes as well as lateral, vertical and resistance kicking. This will give you a great workout for legs, buttocks, lower back, thighs and core. The stronger your legs are in the pool, the better they will manage the cycle and run.
Speed work. If you want to find clear water in a pack swim, then not only do you need fast kicking but you need to know how to sprint. There can be many times during the swimming leg of a race where a short burst of speed can be useful. No matter the distance you're swimming, if you're able to sprint at given times it will be to your advantage. Sprint work also helps in developing quick reflexes. Being able to change tempo however requires practice and incorporating speed play sets into your training is one way of achieving this. Short burst sprints, known as SBS's are also great to add to your regime. Dive start, sprint to the 15m line, jump out and walk back and repeat, many times! Have someone time these for you with the aim of beating your time every sprint.
Train like a swimmer. By this I mean doing your turns at each end of the pool, streamlining off the wall, climbing out over the end of the pool (not using the ladder!), doing dive starts and finishing right to the wall on every lap. It also means spending more time training in the water. Doing turns at each end of the pool increases the stress on your cardio. Pool cardio is fantastic for an endurance runner so the more you strengthen your cardio in swimming the better. A great swimming set to strive for, if you're not there yet, is 60 x 50m's on the 60 seconds with jump outs and dives starts. This set helps to build cardio, speed and strength. Think about it, you would have performed 60 push-ups to get out of the pool every 50m that work arm and chest muscles and 60 dives that work leg and butt muscles. Not to mention the numerous muscles and cardio used to swim at speed.