2.4 Levers A lever is one of the simplest mechanical devices. A lever consists of a beam or stick or rod. However, a lever by itself serves no purpose. It must have something on which to pivot. This pivot is called a fulcrum. A lever helps to do one for two things: lift weight with less effort; or accelerate an object. Terminology of levers in the body: • Fulcrum – the pivot point of the lever. • Moment arm – distance from the line of action of a force to the fulcrum. • Muscle force – force generated by biochemical activity (in the human body). • Resistive force – force external to the body which acts to oppose the muscle force. • Mechanical advantage – Ratio of (moment arm of resistive force)/moment arm of Muscle force). There …show more content…
Points further from this fulcrum move faster than points closer to the fulcrum. The power into and out of the lever must be the same. Power is the product of force and velocity, so forces applied to points closer to the pivot must be greater than when applied to points farther away. If a and b are distances from the fulcrum to points A and B and if force FA applied to A is the input force and FB exerted at B is the output, the ratio of the velocities of points A and B is given by a/b, so the ratio of the output force to the input force, or mechanical advantage, is given by It shows that if the distance a from the fulcrum to where the input force is applied (point A) is greater than the distance b from fulcrum to where the output force is applied (point B), then the lever amplifies the input force. If the distance from the fulcrum to the input force is less than from the fulcrum to the output force, then the lever reduces the input …show more content…
Two individuals with equal muscle force generation capacity could be able to lift quite different maximal loads due to the insertion point of the tendon being further away than the insertion point for the other individual because the muscle force would act through a longer moment arm, thereby generating more torque around the joint. It is important to take note of the trade-off which is involved. As mentioned earlier (in section 2.4.4) the mechanical advantage gained from lengthening the force moment will lead to a reduction in the length ratio of the moment arms. While this may have a positive effect on torque generation, maximal velocity will be reduced. This is not only true from a purely “lever mechanics” point of view. If the tendon insertion is further away, the muscle also needs to contract over a longer distance. This means a given amount of muscle shortening results in less rotation of body segments about a joint, which translates into less movement speed. If one understands this principle it is relatively easy to see that individual differences in body anatomy can have various advantages or disadvantages under different circumstances. For slower activities that require greater torque around joints (greater strength), having tendons that insert further from the axis of rotation will be more beneficial, while the opposite hold true for activities requiring greater