When you walk into a gym there are two main types of weightlifting machines. Free weights, which have no help from machines to assist you and weightlifting machines, such as Nautilus, Cybex, and Hammer Strength which control the motion of your lift. Machines are used primarily by novices and elderly as well as those who are trying to work a specific muscle very strongly. Machines help ease the constant load of the weight, thus making it seem easier to lift a certain weight for a certain exercise.
Free weights Machines
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There are both advantages and disadvantages to both styles of weightlifting. Free weights work better if you are trying to bulk up on muscle quickly since your muscles are under a constant load of weight. Your muscles have to work to move the weight from its resting position and they also have to work to bring it back. There is no help except if you cheat, as do most weightlifters. By cheating, you are using other insignificant muscles to help lift the weight, thus removing the constant load away from the muscles that are supposed to be worked.
Weightlifting machines actually take pressure off of your muscles because you can spread the load over a larger muscle group. If you are a novice weightlifter it is smart to initially use the weightlifting machines so that you do not pull a muscle or seriously injure yourself by mishandling a free weight or using improper technique to do a lift. Weightlifting machines are also useful to perform exercises that can not be done using free weights, such as the Ground Base Jammer, which is a favorite amongst football players. This machines helps build the pectoral and shoulder muscles, a must for lineman and blockers.
Ground Base Jammer
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A machine is a device that transfers a force letting us do Work with less effort. When you use a weightlifting machine you are applying a force to that machine in order to move the apparatus. There is also a force being exerted by the machine. A person can often times lift more weight using a weightlifting machine as opposed to free weights because of the machines Actual Mechanical Advantage. Every machine has a different Actual Mechanical Advantage. Actual Mechanical Advantage is calculated by dividing the amount of force produced by the machine by the amount of force you have to apply to the machine.
AMA = Fresistance / Feffort
In other words mechanical advantage can be found by dividing the weight of the object being lifted (Force weight = mass x acceleration) by the amount of force that you have to apply to lift it. The Actual Mechanical Advantage of a machine is calculated taking into account the resistance force of friction.
If a weightlifting machine is an Ideal machine, the amount of work put into lifting the weight is equal to the amount of work done by the machine because there is no friction. Unfortunately, because there is always someamount of friction existing when two objects rub together, ideal machines exist nowhere but in theory. There is a formula to calculate the Ideal Mechanical Advantage of a machine (ignoring frictional forces).
IMA = d effort / d resistance
Which means that you can calculate the Ideal Mechanical Advantage of a machine by dividing the distance you have to apply the force for by the distance the resistance moved.
All machines have a certain level of Efficiency which is equal to the amount of Work produced by the machine divided by the amount of Work that you have to put into the machine.
Efficiency = Wout / Win
Some examples of simple machines that help you to do Work while weightlifting are levers, wheel & axle, pulleys, and incline planes. Most weightlifting machines (especially novice machines such as Cybex) use pulleys to decrease the amount of effort you have to put in to raise a certain weight. When using pulleys, the IMA can be found by adding the number of strings that the weight is hanging from and that are supporting the weight!
For example, if the weight is hanging from two cords the IMA = 2:1 which means that you only have to apply half the amount of force that you are trying to lift. If you were lifting a 100 lb weight using a machine that had 2 cords attached to the weight you would actually only feel the effect of lifting a 50 lb weight.
There are some machines in which there is only the use of one pulley and two cords, but a majority of the weightlifting machines today use many pulleys and cords as well as different angles. By having a wide variety of angles and pulleys, the user can see for himself which position is more comfortable and which load feels best.
An example of a machine with many pulleys. Some machines have as many as 12 pulleys for one exercise!
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However, with the use of machines comes another force. That force is the force of tension. Tension arises from the use of ropes or cables to carry a force. (In this case, tension forms in the cables that a weightlifter uses in machines such as cable crossovers, pushdowns, etc.) When a lifter applies a force to the bar to pull the weight down, he is not directly applying a force to the weight stack. Instead, he is exerting a force on the cable which transmits that force onto the load of weight. The force experienced by the weight stack from the cable is called the tension force.
However, in an ideal world these cables would have no mass, perfectly transmitting the force applied to the stack of weight. If the cable was in fact massless, it would always experience two opposite but equal tension forces.
Because most machines uses pulleys, we must address the following issue.
Pulleys do not change the magnitude of the force of the cable. They only change the direction!
We will assume that our cables are massless. From the figure below you can see that the pulley changes the direction. Notice the forces of -T and T. When a cable is used in a pulley, the cable must still experience two opposite but equal forces. It may look like it is experiencing the forces in the same direction, but with the pulley it makes it possible. The pulley changes the direction of the force. When analyzing a pulley situation with a force tension, you must look at the shape of the rope to determine direction. The positive direction of the rope is pointing upward on the left side and pointing downward on the right side. Now you can see that the rope does actually experience two equal but opposite forces.