Measuring force in the water is difficult, let alone the contribution of force in swimming success.
Unfortunately, only a limited number of reviews identify resisted, or drag forces, in swimming.
Unfortunately, only a limited number of reviews identify resisted, or drag forces, in swimming.
The drag force is the force component parallel to and in the same direction as the relative fluid force. Drag force D is calculated by: Df = 1/2CDrv2A, Cd is the drag coefficient, and r is the density of fluid, v is the velocity of the object and a is the frontal surface area of the object.
The resistive forces the swimmer interacts with in the water are form, wave, and frictional drag. These are influenced by the swimmer's velocity, boundary layer, shape, size, and the frontal surface area. In swimming, the resistive forces are termed the active and passive drag.
- Active drag is the water resistance associated with the dynamic swimming motion.
- Passive drag is the water resistance that a human body experiences in a fixed or unchanging posture.
Komogorov (1992) determined active drag varies between individuals and seems to related to swim technique and anthropometry. For swimming, drag can be active or passive. However, we will only discuss active drag.
Active Drag and Swim Performance
The two most commonly identified factors for swimming speed are propulsion and drag. The ability to reduce the active drag encountered allows propulsive forces to be efficiently applied, increasing swimming velocities. Elite swimmers are more able to reduce active drag than nonelite swimmers (Kolmogorov 1992). This efficiency allows elite swimmers to minimize wasted kinetic energy.
However, Hollander (1985) did not find a significant correlation in active drag and swim velocity at a constant velocity.
Mechanical Power Output in Swimming
Some suggest swimming performance is defined by the relationship between the useful mechanical power output, active drag, the hydrodynamic force coefficient (drag coefficient) and the maximal free swim velocity. The mechanical power is the power delivered to overcome drag. Swimming power is evaluated as the product of swimming drag (D) and velocity (v):
The ratio between the useful power and the wasted kinetic energy is defined as the propelling efficiency of a swimmer:
hP = Pd/PO
hP is the propelling efficiency and Pd is the useful power.
Techniques of Drag Assessment
Measurements of active and passive drag have been attempted through the years. However, there is much controversy on the techniques. The energetics approach, numerical solutions and experimental techniques have been developed and use to estimate or measure drag forces in swimming.
The energetics approach is also coined theoretical calculations, investigates the relationship between
Di Prampero (1974) identified a linear relationship between drag and maximal oxygen consumption at constant swim velocities, which led to this technique of determining drag as a function of VO2net. Clarys stated “extrapolated the linear regression between VO2net and the added propulsion and added drag to VO2net = 0.” At a constant mean velocity, the mean propulsive force exerted by the swimmer will be equal and opposite to the active drag produced.
Similar values of active drag were found when comparing propelling efficiency values as a percentage. In this studies the drag values were similar, but the authors assumed propelling efficiency did not change where active drag was calculated. It is likely propelling efficiency will change, even at a constant speed, when external loads are applied, as is this case in this approach. Also, small changes in VO2net values due to small deviations in propelling efficiency will be amplified by the extrapolations that are a basis for these studies. Van de Vaart believes this form of testing overestimates active drag. Also, a snorkel is used to measure VO2net, altering the frontal surface area which likely modifies the active drag.
These models use the computational modeling of the water flow surround the swimming to determine
Experimental techniques attempt to accurately determine the resistive forces encountered by a swimmer. This often uses the measuring active drag system (MAD-system), and the indirect techniques of collecting active drag values, the pertubation method (VPM), and the assisted towing method (ATM).
Measuring active drag system
Hallander (1986) developed this system which measures the drag force generated by a swimmer, which enables the calculation of the propulsive force production during the trial. The assumption is made that the mean propulsive force would be equal to the mean active drag values when the swim velocity is constant.
The MAD-system, requires the swimmer to push off from fixed pads underneath the water. Originally this technique different but constant velocities. The swimmer's legs were restricted by a small buoy. The depths of the pads were able to be adjusted for the swimmer’s height as well as the distance between pads. For each trial, the registered output signal of the force transducer was transmitted telemetrically to determine mean force. The average propulsive force was calculated by integration from the force registrations at a constant swim velocity. The swim velocity was determined from the sample frequency and the pad distance (between the second and final pad).
Toussaint (2011) represents the calculation for drag as Da=Kv2, where Da represents total active drag, K is the constant (incorporating the density, coefficient of drag, and frontal surface area), and v equals swimming speed.
Although this technique has been used extensively, it has much criticism. For one, it limits the swimmer's natural stroke mechanics, and it can only be used at a constant velocity. Also, this technique should only be compared against itself or if a swim velocity is the same between techniques. It also uses pads and doesn't allow the swimmer to be in contact with the water. This may alter the normal hand trajectories. Having the pads requires more coordination to constant the pads and may slow down effort. Also, the use of a pull buoy alters swimming biomechanics.
Velocity Perturbation Method
The VPM method is based on the assumption that a swimmer is capable of producing an equal amount of useful mechanical power output and that the simmer will swim at a constant velocity. This technique is seen as a progression from the energetics approach in estimating active drag, without the use of maximal oxygen consumption. In the VPM, a swimmer must produce two equal maximal efforts. This is typically used over 25 m, but has been used in other distances. The first swim is freely, without any attachments, and the second is swum with a hydrodynamic body attached to the swimmer, creating extra resistance. The maximal mean velocity when swimming with the hydrodynamic body was compared with the maximal mean free swimming velocity, which along with the a known addition resistance is used to calculate active free drag for free swimming:
Db is the additional resistance from the pertubation buoy and vb and v are the swimming velocities with and without the hydrodynamic body.
This method is frequently criticized, as it is an indirect measurement of active drag, and may overestimate active drag. It is also assumed participants use the same velocity level. This is difficult due to the added device and it is found that the maximal error due to stroke cycle fluctuations is around 6 - 8%. The other error is that it is difficult for non-elite swimmers to swim with this added device.
To allow different skill levels, different hydrodynamic bodies were developed. However, no matter the size, a hydrodynamic body will still alter swimming skill. Xin-Feng (2007) created an apparatus which stayed in a steady position and minimizes the floating movement of the hydrodynamic body. During Xin-Feng's study they measured the variation in tension of the tread when the gliding block was moved by the swimmer. The results revealed that the tension of the thread fluctuates, revealing the additional resistance does not have a constant value as assumed.
Despite the limitations of the VPM, there are benefits. For one, the VPM method is easily set-up at a pool, allowing coaches and athletes use. It also doesn't need adaptations for different strokes.
Assisted Towing Method
The assisted towing method (ATM) technique is essentially the reverse of the VPM method, by assisting, instead of resisting the swimmer. The ATM is also based on the equal power assumption and the constant velocity assumptions. However, as outlined by Xin-Feng (2007), a swimmer will not be swimming at a constant velocity at any point throughout a maximal effort due to the intrastroke fluctuations in the swimming. Such fluctuations are a result of the intrastroke forces that are generated during a natural arm cycle. kicking also leads to fluctuations.
Mason (2011) compared constant active drag values with fluctuating active drag values. During these trials, the mean velocity maximal free swims (individual swimming without any attachments) and the mean velocity of the towed swims (towed from the hip using the dynamometer) are used to calculate for drag with respect to the drag force required to tow the swimmer. This helped create the VPM equation:
Da=Fbv2v1^2/v2^3 - v1^3
Fb is the force required to tow the athlete at the increased speed as measured from the force platform, v2 is the increased tow velocity, and v1 is the maximal free swimming velocity. Similar to the VPM approach, when using this system with a constant velocity, the dynamometer is set to 5% faster than the swimmer's mean maximum free swim velocity with a high force selection to allow for a near constant tow. To allow the swimmer's intrastroke fluctuations, the force setting on the dynamoeter is reduced and the velocity setting is increased to 120% of the swimmer's maximum free swim velocity. Along with these changes in set force and velocity settings, a paramter on the towing dynamometer is
P is propulsion, m is the passive drag force of the swimmers (as a substitute for the mass of a swimmer), v is the velocity profile, and Da is the active drag.
Although ATM is promising, validation is needed. Although a small sample, a recent study revealed very good reliability value for within-subject mean active drag values (interclass correlation of 0.91, at a confidence limit of 95% and a likely range of 0.58 and 0.98). Future studies must investigate into whether the velocity/force profiles obtained during this technique mimic real stroke mechanics. However, until accurate measurements of basic kinematics while a swimmer is submerged in water, research with the method is under scrutiny. Also, the use of this system for other strokes is not well established due to the intracyclic variations between strokes.
So Does Great Force Production Equal Faster Swimming?
The simple answer is no. Elite male sprinters have a peak force of 50 – 80 pounds and on average 20
Instead, it seems variation in force production plays a larger role in swimming speed. This isn't to say, if you improve your swimming force production, then you won't be a better swimmer, as you may improve your force production as you decrease your variation in force production.
Also, we must consider the timing and displacement of force. Sometimes, a swimmer will create force, but in a wasted manner, ie when their hand is not facing perpendicular to their body or as their arm is out of the water. This is a wasted increase in force production.
Overall, a careless increase in force production doesn't necessarily increase swimming velocity. However, increasing force production in a vacuum will increase swimming velocity.
Now the question is how can you improve this force production with altering the rest of their biomechanics...
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