Industrial ERG

CHAPTER 7 BIOMECHANICS

LEARNING OBJECTIVES

At the end of this module, students will be able to describe the basic elements of biomechanics as they relate to the human body.

INTRODUCTION

Biomechanics is the study of forces on the human body (Marras, 2006). Every task from the gait of the physically handicapped, the lifting of a load by a factory worker, to the performance of an athlete can be described in terms of the specific movements and loading, both muscular and structural (Winter, 1990). The specific movements and loading on the musculoskeletal system change from case to case.

BACKGROUND

Why Is Biomechanics Important to Ergonomics?

Biomechanics is a useful tool in situations where lifting, pushing, or pulling is performed with or without a load. In certain body postures, the body's own weight creates postural stress. The goal of occupational biomechanics is to quantitatively describe the musculoskeletal loading that occurs during work, in order to derive the degree of risk associated with the task.

The Body Described as Levers

A major assumption of occupational biomechanics is that the body behaves according to the laws of Newtonian mechanics. Movement in the body is produced by a system of levers. The levers work together to produce coordinated action, some by actual movement (dynamic) and others by stabilization (static).

BIOMECHANICS OF THE HUMAN BODY

Parts of a Lever System

The following explanation of levers, as applied to the human body, is a simplified version to help all students understand the correlation between body posture and forces. Figures  7.1  and  7.2  explain the basic parts of a lever system.

Figure 7.1  Parts of a free body diagram

Figure 7.2  Parts of a lever system as displayed on the human body

Free body and force diagrams used.

· Fulcrum: pivot about which a Lever turns. In the human body, this is typically the joint center.

· Lever: rigid bar that turns about an axis of rotation or Fulcrum. In the human body, this is typically a combination of body parts, a limb for example (what anthropometry calls the bodies link lengths).

· Lever arm: distance from the fulcrum to either the load or the effort.

· Load:  applied force. In the human body, this can be the weight of the object in the hand, as well as the limb weight.

· Effort:  resistance force. In the human body, this is the force generated by the musculoskeletal system that is applied to cause movement against load and/or stabilize a joint.

Classes of Levers in the Human Body

The class of lever is determined by the relative position of the load or applied force, fulcrum, and effort force, as shown in  Figure 7.3 .

· In a first-class lever, the load and effort are at opposing ends of the lever and the fulcrum is located in the center, for example, a child's see saw. When the effort is applied, the load moves in the opposite direction.

· The skull is a human example

· In a second-class lever, the fulcrum and effort are at opposing ends of the lever and the load is located in the center, for example, a wheelbarrow. When the effort is applied, the load moves in the same direction.

· The foot is a human example

· In a third-class lever, the fulcrum and load are at opposing ends of the lever and the effort is located in the center, for example, a staple remover. When the effort is applied, the load moves in the same direction.

· The bent arm is a human example.

Figure 7.3  This figure displays the relationship between simple levers and the human body during static loading

BIOMECHANICS MADE SIMPLE

Biomechanical loads or stresses on the body are not defined purely by the magnitude of the weight or applied load. The position of the weight relative to the fulcrum (or point rotation of the joint) and direction of force defines the muscular effort required by the body. For example, as shown in  Figure 7.4 , holding a 40 lb dumbbell does not produce 40 lb of force on the elbow, nor does the bicep respond with 40 lb of force. It does, however, create a tendency for the system to rotate, and those rotational forces are called moments.

Figure 7.4  Holding a 40-lb weight results in 480 in.-lb of clockwise rotational force, the moment, at the elbow. The body must respond counterclockwise effort caused by the muscles contractile force

Let's Take a Moment

The measure of a force's tendency to cause a body to rotate about a specific point, that is, rotational force is a moment. A moment is a vector quantity having both direction and magnitude. A moment is defined as the product of the applied force and the perpendicular distance (lever arm) through which the force is applied. The perpendicular distance between the fulcrum and the effort is the muscle effort lever arm; the perpendicular distance between the fulcrum and the load is the load lever arm.

Moments are commonly expressed in Newton-meters (N m). A Newton is a unit of force that takes mass into consideration. One Newton is equal to 0.225 lb. Moments can also be expressed as inch-pounds or foot-pounds.

While this method is simplistic, the general relation of forces acting and the body's reaction to them is sound. For the purposes of this chapter, a moment or rotational force is the product of weight and distance.

(D) Distance (or moment arm or lever arm) is the measurement from the point of rotation perpendicular to the direction of the applied weight or load. In  Figure 7.5 , the distance from the elbow to the object being held in the hand is applied force lever arm. Distance is expressed in inches (in.).

Figure 7.5  Moment (or force) = weight × distance

(W) Weight is the force generated by the gravitational attraction of the earth on the mass of an object. Weight is expressed in pound-force (lb) as opposed to pound-mass (lbm). In the example from  Figure 7.5 , a third-class lever can be seen in the arm with the point of rotation or fulcrum at the elbow.

Holding a weight in the hand creates a moment around the elbow, tending to make it extend. Muscles spanning the elbow create the opposite moment by contracting, so that the elbow is able to support the weight; the greater the weight in the hands, the larger the moment at the elbow.

Holding a 40-lb object in the hand produces a 480 in. lb (.) of rotational force at the elbow ( Figure 7.5 ). The rotational force created by the effort must be equal in magnitude to a 480 in. lb to stop rotation, but applied in the opposite or counterclockwise direction ( Figure 7.6 ).

Figure 7.6  To stabilize a joint the resultant or muscle force (Mm) must equal the applied force (Ma)

This 480 in. lb applied moment from the load results in the muscle responding with 960 lb of effort ( Figure 7.7 ).

Figure 7.7  Resultant or muscle force is significantly higher than applied force

As inferred in this example, the longer the distance between the object and the point of rotation, the greater the rotational force or moment. When lifting an object, the point of rotation in the torso is the L5/S1 spinal unit. The lever arm or distance is measured from the L5/S1 vertebra to the object in the hands, reference  Figure 7.8 .

Figure 7.8  The spine as a third-class lever system (Adapted with permission The Ergonomics Image Gallery)

If rotational force or moment = weight × distance, how much rotational force or moment is generated on the L5/S1 spinal unit when the 40 lb weight is lifted?

It Depends on the Distance!

· Holding 40 lb, 20 in. from the L5/S1 results in 800 in. lb of rotational force.

· Holding 40 lb, 15 in. from the L5/S1 results in 600 in. lb of rotational force.

· Holding 40 lb, 10 in. from the L5/S1 results in 400 in. lb of rotational force.

Clearly, a practical application to understanding the forces on the body is demonstrated. Reducing the distance from the load to the point of rotation reduces the applied rotational forces on the spine. In other words, when lifting, the load should be held as close to the body as possible.

The muscles in our body need to generate the same amount of rotational force, in the opposite direction, to keep the body from rotating. The lever arm in the spine, the distance from the fulcrum to the muscle attachment, is small. Therefore, the muscle must generate a force greater than that which is held in the hands.

Third-Class Lever

The musculoskeletal system can be represented by a lever system. Three types are present in the human body. Most of the joint rotations in our body behave as a third-class lever. A third-class lever has the benefit of good range of motion, speed, and power. However, large muscle forces are required to move even a small amount of weight.

In a third-class lever, the point of rotation or fulcrum is located at one end of the system. The applied load acts on the other end of the system and the force (muscle force) acts between the two (Figure 7.5).

A third-class lever system puts the body at a biomechanical disadvantage because the muscles have to generate considerably more rotational force than the rotational force generated by the load. This is because the distance from the point of rotation to the muscle action (muscle lever arm) is smaller than the distance from the point of rotation to load (applied lever arm).

The same lever system is found in the spine; the point of rotation or fulcrum is the L5/S1 spinal unit. The distance from the muscle attachment point to the L5/S1 is approximately 1 in. (see Figure 7.9). Given our greatest effort, we can never bring the load any closer to our spine than the depth of our torso.

Figure 7.9  The spine as a third-class lever system displaying that the further the load is from body the higher the forces on the spine (The Ergonomics Image Gallery)

To keep the weight of the object from bending the torso forward, the muscles need to generate a moment equal in magnitude to the moment of the applied weight (i.e., the load).

Figure 7.9 illustrates that holding 40 lb, 20 in. from the L5/S1 results in 800 in. lb moment. To calculate the forces on the spine:

Figure 7.9 illustrates that the muscles generate 800 lb. of force to statically hold a 40-lb weight and that the muscles of the spine are at a biomechanical disadvantage. To increase the biomechanical advantage of the spine, keep the weight located close to the body, as this decreases the lever arm (D) of the applied force.

When bending to pick up a load, the weight of the torso adds considerably to the applied force especially if lifting in an awkward posture. The torso contains approximately 60% of your body weight. A 200-lb person carries 120 lb in their torso.

The moment or rotational force is calculated the same way (i.e., ). The distance in this example is the distance from the L5/S1 spinal unit to the center of gravity of the torso. Note these calculations are simplified to illustrate the ratio of applied force to muscle force.

The moment of the body . or . This value is added to the applied moment (Ma) or . To solve for forces acting on the spine when bending forward to lift:

where

Keeping the spine in a neutral posture reduces the muscle contraction forces and thus the forces on the spinal unit. In this example, the muscles generate 1520 lb of force to statically hold a 40-lb weight because of the added force caused by the weight of the torso when leaning forward. Clearly, the muscles are at a biomechanical disadvantage.

To increase the biomechanical advantage of the spine, keep the weight located close to the body, as this decreases the lever arm of the applied force (Ma), and keep the ears over the shoulders and shoulders over the hips to limit the contribution of the torso to the applied forces. Figure 7.10 shows an extraordinarily difficult posture to maintain for lifting.

Figure 7.10  Awkward lifting posture results in high spinal loading

Second-Class Lever

A second-class lever has the fulcrum on one end, the muscle force or effort on the other, and the load between the two. A wheel barrel or nutcracker is an example of a second-class lever (see Figure 7.3). In the body, the foot is a second-class lever; the ball of the foot acts as the fulcrum or point of rotation. The load is applied through the tibia (or lower leg bone) and the muscle force is applied through the gastronomies or calf muscle.

The muscle has the biomechanical advantage in this lever system due to the length of the lever arm between the point of rotation and the muscle force. Relatively little muscle force is necessary to move a heavy weight (refer to Figure 7.11).

Figure 7.11  In the body, your foot has the mechanical advantage; this is an example of a second-class lever

How much muscle force does it take to move a 200-lb person? The distance from the ball of the foot (fulcrum) to the tibia is assumed to be 6.5 in. (for a 6-ft man of average build who weighs 200 lb).

To calculate the applied moment:

To move the body, the calf muscle needs to generate a moment equal in magnitude to the moment of the applied weight but in the opposite direction; the body weight is pulling the foot downward, while the calf is lifting the foot up.

The muscles in a second-class lever system have the mechanical advantage. In this example, the muscles generate 162.5 lb of force to move a 200-lb person (see Figure 7.12).

Figure 7.12  Second-class lever in the foot give us a biomechanical advantage, our muscles contract with far less force than is required to move the body

First-Class Lever

First-class levers are those that have a fulcrum in the middle of the system, an applied load on one end, and an opposing (muscle) force on the opposite end of the system. Sea saws, crowbars, and scissors are examples of a first-class lever (see Figures 7.3 and 7.13). This type of lever results in balanced movement, good force, and range of motion. Our skull is an example of a first-class lever in the body.

Figure 7.13  A first-class lever can be found in the skull

The applied force is the center of gravity of the skull located in the proximity of the chin, the fulcrum is the joint between the skull and the spine, and the muscle force is actuated by the muscles on the back of the head or neck that tilt the head backward.

How much muscle force does it take to hold my head in a neutral posture? Using the same 200 lb male, the distance from the center of the skull (fulcrum) to the chin (assumed center of gravity of the head) is 4 in. In addition, the average adult head weighs between 15 and 21 lb, for this example, we will use 18 lb.

To calculate the moment created by the applied weight (the skull) reference Figure 7.13:

To stabilize the head in a neutral posture, neck muscle needs to generate a moment equal in magnitude to the moment of the applied weight.

In a first-class lever system, neither the muscles nor the load have the mechanical advantage as long as the head is held in a neutral posture. From this example, Figure 7.14, it is easy to see that the further the head leans forward, the greater the contribution of the applied moment and thus the greater the muscle force required to maintain the posture.

Figure 7.14  The loading tray has a straight handle that results in a deviated write posture, change the angle of the handle would result in a neutral posture and less stress on the wrists

Back Injury Prevention Rules of Thumb

· If the load is not close, the pressure is gross.

· If the back is bent, one will not prevent.

· If muscles are slack, you will hurt your back.

SUMMARY

The goal of occupational biomechanics is to describe the degree of postural stresses on the body. A basic understanding of the degree of postural stress a person experiences is beneficial to the ergonomic practitioner for recognizing where workplace, tool, or task enhancements will best benefit the worker and the employer.

CASE STUDY

Recommend searching www.youtube.com for “firing of the M198 Howitzer” to gain a better understanding of the highly repetitive and physical nature of this drill coupled with the heavy weight and high number of rounds fired. More detailed information can also be found in the Case Study Chapter 16.

The United States Marine Corps Artillery Instructional Battery (AIB) stationed at The Basic School Marine Corps Base Quantico, VA, was experiencing a high rate of injuries and our ergonomics team (Dr Lee Ostrom, Dr Cheryl Wilhelmsen and Theresa Stack) was asked to perform an analysis of the “Call for Fire” operation to help determine why these injuries were occurring. We designed and carried out an ergonomics study to determine causes of these injuries. The study was carried out using data collection methods that would collect anthropometric, psychophysical, biomechanical, and human error data.

The “Call for Fire” exercise takes place over 3 days. During the first day, the AIB sets up the M198 Howitzers (see Figure 7.15). The ammunition is also delivered during day 1. Approximately 1100 rounds of 155 mm ammunition, fuses, and sufficient powder are brought to the site and staged behind each gun. Days 2 and 3 are when the actual firing of the guns occurs; each gun fires between 350 and 400 rounds of ammunition. The rounds weigh between 95 and 105 lb each depending on whether they contain high explosive or white phosphorus.

Figure 7.15  Biomechanical modeling of step 5 – moving the round from the pallet to the loading tray

The Marines move the rounds multiple times through the course of the 2-day shoots. Biomechanical modeling was used to determine if moving the rounds produces an increased risk of injury for the combination of force, posture, frequency, and duration.

Three-Dimensional Static Strength Prediction Program 6.0.2 (Michigan, 1999) was used to model the task as well as to compare the round weight, posture, frequency, and duration to the weight handling limits in MIL-STD 1472F (Defense, 1999).

The procedure for moving the round is as follows and can also be found on the Internet with the search term:

1. The fire instructions are radioed to the recorder.

2. The recorder announces the fire order.

3. The powder person adjusts the powder.

4. The type of round is verified.

5. A round is removed from the ready board and laid into the loading tray (Figure 7.11).

6. The two loaders pick up the round.

7. The rammer places the ram at the back of the round.

8. The breach is opened.

9. The loaders bring the round up to the breach, Figure 7.10.

10. The rammer pushes the round in the breach.

11. The loader on the right side of the loading tray releases his grip.

12. The other loader steps back and to the left of the gun.

13. The powder man brings up the powder and hands it to the A-gunner.

14. The A-gunner verifies the amount of powder and places it in the breach.

15. The A-gunner closes the breach and places a primer in the priming hole.

16. The lanyard is attached and then pulled.

17. The gun fires.

18. The A-gunner opens the breach and swabs the breach and breach plug.

Multiple steps were analyzed for the full study; as shown in  Table 7.1 , it was found that steps 5 and 6 produced the highest forces on the spine as well as the wrist.

Table 7.1  Loading Tray and Example of Changing the Angle on the Tray to Reduce the Force on the Wrist

Body Segment	Compressive Forces	Result
L5/S1	1762 lb ±124 lb	Spinal compression fall above the action limit of 770 lb and is therefore considered above hazard threshold for that body segment
	Population Capable of Performing the Task (%)
Wrist	23	Above hazard threshold
Elbow	75	Marginal
Shoulder	50	Above hazard threshold
Torso	77	Below hazard threshold
Hip	62	Marginal
Knee	97	Below hazard threshold
Ankle	84	Below hazard threshold

To verify the results and provide an alternative evaluation, the Department of Defense Design Criteria Stand for Human Engineering (Defense, 1999; Section 5.9.11.3) was used (see  Tables 7.2  and  7.3 ).

Table 7.2  Move One Round from Staging to Pallet and Move One Round from Pallet to Loading Tray

Round Weight (lb)	Recommended Weight Limit (MIL-STD) (lb)	Task Factors	Result
97	64	Recommended weight limit for a one-man carry is 82 lb under optimal conditions This value is discounted (reduced) by 22% due to the frequency of the exposure Assume 200 lifts in a 9 h/day (2.7 lifts/min)	Round weight exceeds recommended carrying weight, the task is therefore considered above hazard threshold

Table 7.3  Moving the Round and the Loading Tray to the Gun (Start of Task)

Round Weight (lb)	Recommended Weight Limit (MIL-STD) (lb)	Task Factors	Result
97	136	Recommended weight limit for a two-man lift (below 36 in.) is 174 lb under optimal conditions This value is discounted (reduced) by 22% due to the frequency of the exposure. Assume 200 lifts in a 9 h/day (2.7 lifts/min)	Round weight does not exceed recommended lifting weight limit; the task is therefore considered below hazard threshold

Based on the biomechanical modeling and comparison to the MIL-STD, moving the round from the pallet to loading tray produces an increased risk of injury to the wrists. Redesigning the carry tool may reduce these factors by improving not only the coupling of the hands to the tool but also the degree of hand/wrist deviation. Decreasing the frequency of moving the rounds from the staging area to the pallet, for example, by allowing the LTs to assist, would decrease the risk of injury to the low back and torso by allowing time for the soft tissues to rest and recover.

Moving the round from the pallet to the loading tray produces the greatest spinal compression due to the squatted posture and twisting motion. Elevating the loading tray improves the task characteristics. The only feasible way to reduce the risk of injury is to allow frequent task rotation.

Loading the gun with the loading tray produces a moderate risk of spinal injury. The risk factors can be greatly reduced by allowing for frequent task rotation. For example, decreasing the frequency of the exposure (using the MIL-STD) reduces the risk level from unsafe to safe. In other words, when loading the gun, it is the frequency of the exposure that is causing the risk.

The Marines received a full report and implemented solutions to reduce the physical demand on the service members.

KEY POINTS

· A person holding an object closer to the body results in lower rotational forces on the spine and thus lower muscle forces. The following is qualitative as there are many factors that affect the spinal loading.

·

·

·

· Movement in the body can be described by a system of levers.

· The levers work together to produce coordinated action, some by actual movement (dynamic), and others by stabilization (static).

· Biomechanical loads or stresses on the body are not purely defined by the magnitude of the weight.

· The longer the lever arm (the distance from the fulcrum to the applied weight), the greater the force.

REVIEW QUESTIONS

1. What are the three lever systems used to describe the human body?

2. Which lever system most represents the human spine?

3. If the load is positioned at arm's length from the body, what happens to the forces on the spine?

4. What are the ways a person can reduce the forces on their spine without changing the weight of the load?

5. Which lever system is considered a biomechanical advantage to the human and why?

EXERCISE

1. Reference appendix.

REFERENCES

1. Department of Defense. (1999). MIL-STD-1472F, Department of Defense Design Criteria Standard: Human Engineering. United States Government Printing Office.

2. Marras, W. (2006). Fundamentals and Assessment Tools for Occupational Ergonomics 2nd edn. CRC Press.

3. University of Michigan. (1999). 3D Static Strength Prediction Program.  http://www.umich.edu/∼ioe/3DSSPP/background.html .

4. Winter, D. (1990). Biomechanics and Motor Control of Human Movement, 2nd edn. Wiley-Interscience.

ADDITIONAL SOURCES

1. Chaffin, D. (2006). Occupational Biomechanics, 4th edn. Wiley-Interscience.

2. McGinnis, P. (2013). Biomechanics of Sport and Exercise, 3rd edn. Human Kinetics.

3. Peterson, D. R. (2014). Biomechanics: Principles and Practices. CRC Press.