Swimming is different from many other sports since it's performed in a different element. Therefore, different kinds of forces have to be considered compared to ground-based sports. There are two types of components to consider. Increasing propulsive force and reducing resistive force.
Strength training can play an essential role in both increasing propulsive forces and reducing resistive forces. Simply said, the more force a swimmer can apply in a propulsive action, the faster they will travel. People who are strong can, by definition, apply more force in a given movement. Elite swimmers generally have superior stroke lengths compared to non-elite swimmers, while stroke rates are similar. Greater stroke length is achieved by:
Increasing propulsive forces without or to a greater extent than resistance forces.
Reducing resistance forces without or to a greater extent than propulsive forces.
Strength Training to Improve Propulsion
The ability to apply large forces and the ability to move at high speeds (velocities) are different yet highly interdependent motor abilities. Optimal power output refers to the optimal balance of force and velocity of movement, which is complicated by the fact that the forces are applied to a fluid surface. When applied to a solid surface (such as the ground or blocks), an increase in power output in a particular task will result in an increase in propulsion and improvement in performance because the resultant force created at the surface is predictable. Thus, an increase in power output when pushing off during the start and turning will improve performance in these skills.
However, the effects of applying greater force or movement velocities during the stroke are dependant on the change in the propulsive drag. If increasing the force and/or movement speed that the athlete applies to the water results with the limb 'slipping' through water without increasing the athletes' grip' of the water, it will not contribute to propulsion. In this case, the extra work performed will reduce the athlete's efficiency and may ultimately inhibit performance. Hence, it is the athlete's technical ability to apply additional power output in a manner that increases propulsive drag that determines the additional propulsion that is created. Therefore, the athlete must work on technical aspects in the water, so the neuromuscular improvements made on land can be transferred to the water. It may, therefore, be argued that in a sport-specific capacity, an athlete cannot be too powerful or too strong, but it is the technique of the athlete that is limiting performance, specifically the ability to access this extra physical capacity they obtain and transfer it to the water.
Strength Training to Reduce Resistance Forces
As mentioned previously, strength training may also play a role in minimizing the resistance forces that slow the athlete down. Strength training can mitigate the negative effect of wave and form drag in two ways:
Increasing the power output produced when starting and turning, thus maximizing the use of the out-of-water and underwater phases of a race.
Improving postural control of the body during swimming to assist the athlete in achieving the optimal alignment of the body throughout the stroke.
How strength training can increase power output has already been discussed, so I won't elaborate more on that.
It has long been recognized that athletes need to maintain a 'streamlined' position in the water in order to minimize wave and form drag. However, optimal 'streamlining' does not involve simply holding the body in an extended position on the surface of the water and is dependent on the individual stroke characteristics of the athlete. Essentially, the athlete must place their body in the position that allows the limbs to maximize the propulsive force they can generate while minimizing resistive drag.
Obtaining the optimum 'streamlined' position is primarily a matter of technique to be dealt with during swim training rather than strength training. However, strength training can play an essential role in helping the athlete develop the underlying physical capability, such as core stability and body awareness, to perform the movements required and, therefore, assists in the pursuit of optimal swimming technique. It is, therefore, vital that the strength coach has an understanding of the individual swimming style and constraints of each athlete.
Strength training and Specificity
A relationship exists between specificity and overload. There is often a desire to perform sport 'specific' exercises in the gym, which mimic actions seen in the pool. This can bring a level of comfort to the participant because mimicking actions in the pool with resistance can lead to the assumption that what they are doing will directly impact their performance in the pool in a positive way. There is, of course, one issue to consider with this approach:
If we assume that exercises need to be 'specific' to swimming and that this is the most crucial aspect of training. Then why not just have swimmers perform a maximal effort race in their main event every training session then go home? After all, this would be the most sport-specific form of training they could complete.
This is, of course, not how swimmers train. They complete sets in the pool which overload particular energy systems, such as the aerobic or anaerobic systems, causing them to adapt, and then as competition grows closer, use these new, improved energy systems to perform better in their given race. They train in a less specific way to encourage greater overload.
Sport-specific exercises may look similar to swimming actions, but they will provide little overload when compared to other typical strength training exercises. We want to use exercises that allow us to train the similar muscle groups used in swimming but that provide opportunities to stress the body to adapt adequately.
Strength training for swimming is not about replicating the work with is being completed in the pool. When elite swimmers are completing 20 hours per week of swimming, it is unlikely that trying to mimic what is being done in the pool for an extra 2-3 hours per week will make a significant performance improvement.
The main propulsive muscles used during a swimming race are the latissimus dorsi, pectoralis major, and quadriceps. We, therefore, want to use exercises that allow us to sufficiently overload these muscles. Remember, the aim of strength training is not to replicate what is done in the pool but to develop a stronger swimmer who can then use their new strength when swimming. Three of the more effective exercise for overloading the propulsive swimming muscles are the chin-up, bench press and back squat.
As a swimmer moves through the water, the drag slows the forward movement. To overcome drag, the athlete must produce equal force to maintain speed and more force to increase speed. In swimming, speed increases when the athlete applies more force against the water. However, as velocity increases, drag increases proportionally to the square of velocity(D ~ V²). To pull with increased force, one must increase maximum strength. Not only must the athlete increase maximum strength but the athlete exerts almost the same force on all stroke for the duration of the race, because all swimming events have some endurance component, of course, some more than others.
Strength training can also be used to improve postural components to reduce resistive forces in the water, such as drag.