Inertia is the property of an object that causes it to resist changes to its state of motion by an application of force. A stationary object will tend to resist being moved, and a moving object will resist change in its speed or direction of movement. Comparing two objects of different masses, the object with the most mass will resist changes in its state of motion the most, and so is said to have the most inertia. So, for example, a train has much more inertia than a car, and a football linebacker has more inertia than a gymnast. Although the heavier object has the most inertia, as above, it is the mass that determines inertia, as measured in kilograms.
How Does Inertia Affect Strength Training?
In maximum strength training, inertia comes into play because the barbell or other weighted implement has a great deal of mass, and so a large inertia. In order to move a barbell, we must overcome its inertia. It takes a great deal of effort to get a barbell moving as opposed to a tennis racket, for example, owing to the higher inertia.
Newton’s first law of motion or the Law of Inertia describes this resistance of objects to a change in their state of motion. Application of this law in biomechanics is called the inertia principle. In athletic training, We manipulate inertia in many ways in order to change the execution of a movement. Thus we are applying the inertia principle.
Rotational inertia or mass moment of inertia is the resistance of a body to change its rotational motion. This is the angular or rotational equivalent of mass. Here, the word force gets replaced with torque and mass is replaced with moment of inertia. The angular version of the first law of motion states that a rotating body will maintain a constant angular velocity unless acted on by an external torque.
In most areas of human performance, this tends to have less influence than mass since mass has more influence on the motion of a human body. Humans tend to be more concerned with translational motion, which is motion along a straight line or axis, than with rotational motion, and human movements rarely rotate at high rates. However, it becomes more important in examining acrobatics, diving, gymnastics, skateboarding, etc. We do, however, manipulate the moment of inertia in many ways without being aware of it, such as in the action of choking up on a bat to help overcome it being too massive to swing properly. This action of bringing the grip up closer to the more massive end of the bat has the effect of bringing the mass closer to the axis of rotation.
Human limbs are actually designed to manipulate the moment of inertia in that they are tapered from the proximal to the distal end. Notice the way your thighs are much more massive than your lower legs, and notice that the arms are the same. This means that most of the mass of your limbs is closer to the axis of rotation, making them easier to control by keeping the moment of inertia as small as possible. If most of the weight were towards the ends of your limbs, they would be much more difficult to swing and to stop, much like swinging a heavy object that is weighted on the opposite end is much more unwieldy than swinging a well-balanced object. Objects such as hammers, which have a high moment of inertia, take advantage of the opposite, meaning that the mass on the end, and higher rotational inertial, does most of the work of hammering in a nail.
Theoretically, an athlete with longer legs should be able to generate more speed in running if the angular velocity of the hip remained the same. However, a longer leg has more mass distributed away from the hip joint so the moment of intertia is greater. Therefore, runners with longer legs may have more of a need for strength training to increae their generation of force and take advantage of the longer stride.