Levers often are referred to as machines that operate according to the principles of torques. People use them in daily life to make every- day tasks easier, either by enabling the over- coming of a resistance that is greater than the force directly applied (for example, in lifting an object with a crowbar) or increasing the speed and range of motion through which an object can be moved (for example, hitting a tennis ball with a racket or a baseball with a bat; figure 5.16).
The human body contains a complicated arrangement of levers that work together to optimize the mechanical advantage to the body during skilled movement. Mechanical advantage can be thought of as the measure of force or velocity multiplication that results from using a lever system (or machine) to do
E5649/Brewer/fig 05.16/548502/mh-R1 b
Point a travels farther than point b in the same time, therefore it travels faster
a
a
Figure 5.16 Speed-multiplying levers in striking
Table 5.4 Lever Systems in the Human Body
Classification Definition Benefits Example
First-class lever Pivot is between the required muscular force and the resistive load (as in a see-saw).
Can act as a lever, providing both mechanical advantage and speed advantage depending on the position of the fulcrum.
Extension of the elbow with the wrist pronated.
Second-class lever Muscle force and resistive force are on the same side of the fulcrum, and the resistive mass is between the resistive force and the pivot (as in a wheelbarrow).
Acts at a mechanical advantage in terms of force. It magnifies the effect of a small force, and less effort is required to move a large force.
Plantarflexion of the ankle.
Third-class lever Muscle force and resistive force are on the same side of the fulcrum, and the resistive force is between the resistive mass and the pivot (as in a pair of tweezers).
Acts at a force disadvantage and a speed advantage. It enables speed and range of motion at the expense of force.
Flexion of the elbow. the work. It is a measure of increasing work
done against the energy cost (or indeed system capabilities) for performing that work.7
Muscles produce pulling forces to the rigid lever skeletal system. Torques are rotational, or twisting, forces that occur as an object is rotated about an axis or pivot. Torques (or moments) are proportional to the distance between the pivot multiplied by the force applied at the end of the lever (moment equals force times distance). In the human body, the bones can be considered the rigid lever arms of the lever system. Different bones have different lengths and different leverage potentials, although because the relative positioning of bones to each other will change during a movement, the relative lengths and lever arms may also change. Joints are the pivots, muscles produce the force that moves the lever arms and resis- tive loads are determined by the work required to be done. This work can be ground reaction force, an external weight, gravity or a collision with an opponent, and it must include the system mass (e.g., the weight of the body or body segment that is being moved) as well as any external load or resistive force.
A change in length of the force arm alters the magnitude of force required to overcome a resistance. Although this point may seem
logical when discussing a crowbar (for exam- ple), it initially takes some thought when applied to human movement. Because the length of a bone doesn’t change, why isn’t the moment always the same throughout a movement? This problem can be explained by relating back to the example of the biceps curl. (Note that this movement is not often used in functional sport training, but it is highly useful in explaining physio-mechanical concepts.) As with most major limb movements, this action is an example of a third-class lever (table 5.4).
In the biceps curl, as the dumbbell is lifted, the length of the moment arm (the distance between the pivot and the insertion of the biceps brachii tendon to the ulna) changes. During the lift, the muscular force (torque) that is required to lift the weight changes, and the external work that is performed changes. The conclusion is that a relationship exists between the angle of a joint and the torque that the muscles can produce (figure 5.17). When teaching techniques, coaches should emphasize joint positions that optimize not only the force–velocity potential of the muscle fibres but also the ability to produce muscular torques in the lever systems.
Because the muscles are required to pro- duce relatively large forces to gain the speed
advantage offered by the lever, actions involv- ing these lever systems have been demon- strated to be associated with higher instances of injury to athletes. Understanding lever sys- tems also helps in making informed decisions about exercise selections and the potential implications for performance enhancement. To illustrate the concept, consider a comparison between the good morning and the stiff-legged (or Romanian) deadlift, both of which are commonly prescribed exercises for strength- ening the hamstrings, particularly in terms of generating eccentric strength.
As figure 5.18 illustrates, the movement of the trunk in these exercises is similar, and, without the load being present, the demands placed on the hamstrings to resist trunk flexion and cause trunk extension would be similar. Indeed, without considering the external load, the moment arm of muscular force would involve a third-class lever system in both move- ments. The resistance moment arm, however, causes the exercises to be significantly different.
Placing the load on the shoulders in the good morning movement significantly increases the resistance lever arm. This aspect has two potentially negative effects on the athlete. First, in a lever system that is at a force disadvan- tage, the hamstring muscle must work harder to achieve the trunk extension as the load is raised. In this instance, with a large resistance arm and small muscle force arm, the body is at an extremely disadvantaged position mechan- ically. Although this point may be considered an advantage in terms of training, coaches need to recognize that the exercise may rapidly overload the hamstrings and cause the athlete to use other muscles in the lumbar spine (for example, the erector spinae, which are assistors in this trunk extension movement) as prime movers. This itself is an injury risk.
Of greater potential concern, however, are the shear stresses that the lumbar vertebrae E5649/Brewer/fig 05.17/542810/HR/R1 Joint angle Tor que f act or
Figure 5.17 The relationship between joint angle
and torque production.
E5649/Brewer/fig 05.18/548504/mh-R2 Moment arm of resistive force
Moment arm of muscle force Moment arm of
muscle force
Moment arm of resistive force
Figure 5.18 The moment arm of resistive force in the (a) good morning and (b) stiff-legged (or Romanian)
deadlift movements.
are subject to in a movement such as this that has such a big (long) resistance movement area. Thus the stiff-legged deadlift, with a much smaller resistance moment arm, is a much safer and arguably more effective exer- cise for developing force-generation potential in the hamstrings. Another consideration for choosing between the two exercises is a more practical one. If the athlete fails the lift and cannot return the load to a standing position during the stiff-legged deadlift, the athlete can simply drop the bar. Conversely, during a good morning, the bar cannot be easily dumped unless it is rolled over the neck.