This is an excerpt from Strength Training for All Body Types by Lee Boyce & Melody L. Schoenfeld.
The body is a simple machine, meaning something that can transmit or modify force in order to perform work (i.e., move an object). Much like a crane lifting steel girders or a pair of tweezers pulling out a splinter, the human body can be seen as a system of levers. The forces we put on those levers stress our bodies in different ways, and those stresses can lead to results such as muscle hypertrophy, strength increases, or even injury.
Characteristics of Levers
All levers have two basic parts:
- The fulcrum, which is the pivot point of a lever. In the human body, the fulcrum will generally be a joint. In a biceps curl, the fulcrum is the elbow. In a squat, there are multiple fulcrums—the knee, the hip, and the ankle. The more complex the lift, the more levers come into play. When multiple levers are working together, they increase the leverage that can be produced. This is why a lifter can generally move a lot more weight in a squat, which uses multiple fulcrums, than in a biceps curl, which uses only one.
- A rigid arm that connects to the fulcrum in some capacity. The arm should not break or bend, or else the lever won’t work very well. In the human body, the arm will likely be a bone or series of bones. In a leg extension, the arm is the shin, or more technically, the tibia and fibula. In a deadlift, the arm is the collective bones and joints of the spine, held together as a unit by the contracting spinal muscles.
The load is the object that is to be moved (for the purposes of this book, a barbell, dumbbell, or other type of weight), and the force is what is applied to the lever to move the load (i.e., muscle action).
Mechanical advantage defines how effectively the application of a force on a simple machine will move an object. The mechanical advantage of a lever system can be determined by the following equation:
Mechanical advantage = length of effort arm/length of resistance arm
The higher this ratio, the better the mechanical advantage. A mechanical advantage of 4, for instance, means it would take one quarter of the effort to lift the load than if you were to do it without a lever.
So, the farther a load is from the fulcrum, the more effort it will take to move that load. This is why you will generally need much lighter weights to do straight-arm exercises such as a lateral raise than bent-arm exercises such as a biceps curl. It is also why it may be more challenging for an athlete with longer arms to perform a lateral raise with the same weight as an athlete with shorter arms.
While a load farther away from the fulcrum will take more effort to move, that load will also move faster and for a longer distance than will a load that is closer to the fulcrum. When speed is the goal, a lower mechanical advantage would generally be desirable. It is for this reason that a longer-armed person might be a better thrower than someone with short arms. The longer the distance of the object to be thrown (the load) from the shoulder and elbow (which would be the fulcrums in this case), the faster the travel over a longer range for the object.
Classes of Levers
There are three basic classes of levers.
First-class levers are set up like a seesaw, with the fulcrum between the effort and the load. This is the least common type of lever in the human body. The head is an excellent example of a first-class lever, where the fulcrum is the atlantooccipital joint, the load is the anterior portion of the skull, and the force is applied by the muscles that extend the neck (figure 4.2).
There aren’t very many first-class lever exercises in the weight room. The most obvious example would be neck extensions. There are a few others, though. A triceps extension or dip is a good example of a first-class lever exercise—the elbow, as the fulcrum, would lie between the load (the weight) and the effort (the triceps).
In a second-class lever, the load is between the effort and the fulcrum. A wheelbarrow is the most common example of this type of lever—the fulcrum is the wheel, the load is whatever is inside the wheelbarrow, and the effort is the handles of the wheelbarrow. The mechanical advantage will always be greater than 1, because the effort arm will always be farther from the fulcrum than the load arm. Second-class levers always produce more force but at the expense of range of motion and speed.
One example of a second-class lever in the human body is a calf raise. In this case, the ball of the foot is the fulcrum, the load is all the load-bearing forces of the body plus whatever weight is being used, and the effort is applied to the heel of the foot by the calf muscles.
A push-up is a kind of second-class lever, but in this case, the lever itself (i.e., the body) is the load, which makes it an interesting example (figure 4.3). The fulcrum is at the toes (or knees, depending on how the push-up is being done), and the effort is the hands pushing into the floor. A push-up from the knees has a shorter distance to the fulcrum than a push-up from the toes, so less of a load is shifted to the hands and less effort is required to execute the movement.
Third-class levers are the most common type of lever in the body. The load is on one end of the lever arm, the fulcrum is on the other, and the effort is between the two. In a third-class lever, the load arm will always be longer than the effort arm, so the mechanical advantage will always be less than 1. This is great for increasing the speed and the distance the load travels, but not as much effort will be produced. A low mechanical advantage means low efficiency—third-class levers are therefore the most inefficient type of lever. The load will always be farther from the fulcrum, so we need to generate more force in order to move those loads.
There are many examples of third-class levers in the human body. A biceps curl is a very simple example of a third-class lever (figure 4.4). The biceps brachii attaches to the tubercle of the radius and the deep fascia of the forearm. In the biceps curl, the elbow is the fulcrum, and the load is whatever is on the opposite side of the biceps attachment point (e.g., a dumbbell, barbell, wrist weight, band).
As you can see, knowing the attachment points of muscles is very useful for determining the location of the force within the human body. One formula to determine effort in a third-class lever is as follows:
Effort = load × (distance from fulcrum to load/distance from fulcrum to effort)
This means that the farther the force is from the fulcrum, the easier it will be to move the load. So, in the case of the biceps curl, if the biceps were to attach farther away from the elbow joint, it wouldn’t need to work as hard to lift the load in question. Similarly, if the load were in the middle of the forearm, it would be far easier to curl the weight.