The objective of this paper was to perform a comprehensive review of psychophysically determined maximum acceptable pushing and pulling forces. Factors affecting pushing and pulling forces are identified and discussed. Recent studies show a significant decrease (compared to previous studies) in maximum acceptable forces for males but not for females when pushing and pulling on a treadmill. A comparison of pushing and pulling forces measured using a high inertia cart with those measured on a treadmill shows that the pushing and pulling forces using high inertia cart are higher for males but are about the same for females. It is concluded that the recommendations of

At present it is not clear whether pushing or pulling should be favored. Similarly, it is not clear what handle heights would be optimal for pushing and pulling. Epidemiological studies are needed to determine relationships between psychophysically determined maximum acceptable pushing and pulling forces and risk of musculoskeletal injuries, in particular to low back and shoulders.

Pushing/pulling tasks are common in industries and services such as shipping and receiving, moving, warehousing, garbage collection, agriculture, farming, fire fighting, construction, airlines, gardening and nursing (

Pushing and pulling of carts and objects exposes workers to two types of hazards: (i) stresses to the musculoskeletal system from applied hand force, and (ii) accidents due to slipping or tripping (

The objective of this paper was to summarize the psychophysical literature on pushing/pulling of carts and to make recommendations for acceptable pushing/pulling forces based on psychophysical studies.

Psychophysics is a branch of psychology studying relationships between sensations and their physical stimuli. According to psychophysical theory, the perceived strength of a sensation (^{n}

Advantages of the psychophysical approach include: (i) ability to realistically simulate industrial work, (ii) allows study of both intermittent as well as repetitive tasks, (iii) psychophysically determined maximum acceptable weights (MAWs) and forces (MAFs) are based on integrated response of the body from the worker (

Pushing/pulling is characterized by exertion of hand force in a horizontal direction – away from the body for pushing and toward the body for pulling. Often, the direction of exerted force is not strictly horizontal and likely includes a vertical component, depending upon the vertical height of the hands during the push/pull. In general, the vertical component for pushing is downward (

Pushing/pulling forces are characterized by (i) initial force required to start the movement of an object, (ii) sustained force – a lower force required to sustain the movement – and (iii) stopping force required to stop the movement of an object. Most of the published literature in ergonomics deals with initial and sustained forces for pushing and pulling.

The results are inconsistent when comparing maximum pushing strength with maximum pulling strength.

Psychophysical studies on maximum acceptable forces for pushing and pulling of carts have found either no statistically significant differences between pushing and pulling maximum acceptable forces or reported that pushing resulted in higher maximum acceptable forces (

Friction affects an individual’s ability to push/pull an object and subsequent risk of musculoskeletal disorders (MSDS) (_{req}) to move the object. The magnitude of the required horizontal force needed to move an object across a surface is defined as the product of the coefficient of static friction (μS) multiplied by the normal force (force exerted perpendicular to the surface) between the object and the supporting surface. For wheeled objects, the force required for movement is determined by the friction between the wheel and axle and the rolling resistance between the wheel and the floor (e.g. carts typically require greater pushing/pulling force on thick carpet than on smooth concrete). From a dynamics standpoint, the speed of push/pull as well as the size and type of wheel may also affect the required horizontal force needed to move an object.

Foot traction affects a person’s ability to generate muscle force needed to push/pull an object, as well as the duration of force exertion and the body posture necessary to maintain body balance (

It has been suggested that ramps should be less than 3.5% grade (2°) (

In general, the harder the rolling wheels of a cart and the harder the surface over which the cart rolls, the less pulling/pushing force will be required to move the cart (

Swiveling of wheels can affect the force required to move a cart as well as stop a cart. A cart with all four swiveling casters requires more force to turn (

Maintenance of the wheels and wheel bearings affect the amount of pushing/pulling force required to move a cart.

Uneven floor surfaces can significantly increase the force required to push/pull a cart.

For a given cart and floor surface, as the weight of the cart increases the force required to push/pull a cart increases linearly (

From the above discussions it is clear that handle height is an important parameter in cart design. Handle height affects (i) force exerted on the cart to initiate and sustain movement, (ii) maximum voluntary strength, (iii) compressive and shear loading of spinal discs, and (iv) stresses to the shoulder joints. One would expect that handle height would also have an impact on localized muscle fatigue (shoulders and low back) as well as whole body fatigue (energy expenditure) when pushing/pulling tasks are performed frequently and/or over a large distance. Unfortunately, at this time there are insufficient conclusive data to recommend handle heights that would result in lower strength requirements and lower stresses to low back and shoulder as well as minimum localized and whole body fatigue.

In order to use their body weight to assist in pushing and pulling objects, individuals tend to lean forward to push and backward to pull. Trunk posture affects forces in trunk muscles (back and abdominal), and compressive and shear forces on spinal discs and stresses to shoulder joints. It is not clear what posture(s) would be optimal to minimize compressive and shear forces on spinal discs as well as stresses to shoulder joints.

Foot placement influences stability (balance) of the body. It provides leverage for generating pushing and pulling forces and it has been suggested that workers feet should be staggered rather than planted side by side (

Several studies have reported that both initial and sustained maximum acceptable pushing and pulling forces decrease with an increase in frequency of exertion (

Several studies using a psychophysical approach have shown that both the initial and sustained forces decrease with an increase in pushing/pulling distance (

There have been a few studies on static and isokinetic pushing/pulling strengths (

^{−1}). Among the three variables, pulling speed was found to be the most critical. The mean dynamic strength was 360, 250, and 180 N and the peak strength was 600, 425 and 320 N at 0.7, 1 and 1.1 m s^{−1}, respectively. The strengths decreased with an increase in handle height from 100% of maximum at 40% shoulder height to 83% of maximum at 70% of shoulder height and were the highest at an angle of 25° from the horizontal plane. The handles at 50% and 60% of shoulder height and at an angle of 25° were perceived as being more comfortable than those at other heights and angles (

As far as maximum acceptable pushing and pulling forces are concerned, Snook, Ciriello and their colleagues at the Liberty Mutual Research Institute have conducted most of the studies reported in the literature (_{2}) and heart rate (HR) were also measured. Subjects were given control of force; all other task variables, such as distance moved, task frequency, hand height, etc., were controlled. Pushing and pulling tasks were simulated on a specially controlled treadmill. The treadmill was powered by the subject as he or she pushed or pulled against a stationary bar. The subject controlled the resistance of the treadmill belt by varying the amount of electric current. A load cell on the stationary bar measured the horizontal force being exerted. Subjects were second-shift workers from a local industry.

In 1978, Snook first reported a comprehensive database for maximum acceptable pushing and pulling forces by integrating the results from his previous studies. Later, _{2max}).

In another study,

For males,

Guidelines for maximum acceptable pushing and pulling forces were developed by integrating studies conducted over a 21-year span and published in 1991. One concern is that the physical capabilities of male and female industrial populations may have changed since the data were published in 1991. Four different studies (

From the above discussions it appears that pushing/pulling on a cart versus on a treadmill has little effect on maximum pushing and pulling forces acceptable to females. Further, there has been practically no change in pushing and pulling physical capabilities of females since those data were published in 1991. Overall, the 1991 guidelines still provide an accurate estimate of maximum acceptable forces for the selected combinations of distance and frequency of push/pull for female industrial workers (

For male workers, data suggest that maximum acceptable forces for pushing and pulling a cart are significantly higher (21%) than those determined using the MPB treadmill. This would suggest that an adjustment to maximum acceptable pushing and pulling forces published in 1991 is needed. However, this increase in maximum acceptable forces is countered by a comparable decrease (18%) in male pushing and pulling physical capability on treadmill due to secular changes (

The 1991 publication provided the most comprehensive guidelines for the maximum acceptable two-handed pushing and pulling forces by revising the maximum acceptable initial and sustained pushing and pulling forces published earlier (_{2}) recommended by

Gender has a significant effect on both the initial and sustained pushing and pulling forces. In general, the maximum acceptable pushing or pulling forces were lower for females relative to males.

Both initial and sustained pushing and pulling forces for both males and females decrease significantly with an increase in pushing/pulling frequency.

Both initial and sustained pushing and pulling forces for both males and females decrease significantly with an increase in pushing/pulling distance.

Handle height does not appear to have a profound effect on pushing and pulling initial and sustained forces. For pushing optimum height for males is 95 cm and for females 135 cm, both for initial and sustained forces. For males, the optimum height for pulling is 64 cm both for initial and sustained forces. For females, the optimum heights for pulling are 57 cm for initial force and 135 cm for sustained force. The worst heights for males are 64 cm for pushing and 144 cm for pulling. The worst height for females is 57 cm for pushing.

In general, maximum acceptable pushing forces were a little higher than those for pulling.

The above observations are consistent with those of

Use of maximum acceptable forces data reported by

To develop these regression equations we stratified maximum forces acceptable to 75% of workers by type of task (pushing v. pulling), type of force (initial v. sustained) and by gender (female v. male). We then plotted maximum acceptable forces against (i) frequency of exertion, (ii) distance of pushing or pulling and (iii) handle height for pushing or pulling while blocking two of the three independent variables. For example, we plotted initial maximum pushing force acceptable to 75% females against frequency of exertion for each unique combination of distance and handle height. A visual inspection of these graphs showed the following relationships between maximum acceptable forces and the three independent variables (frequency of exertion, distance and handle height):

Both initial and sustained pushing and pulling forces showed a logarithmic relationship with frequency of exertion. Subsequent plots of natural log transformations of frequency of exertion showed quadratic relationships with initial pushing and pulling forces both for females and males. Similarly, plots for sustained pushing and pulling forces showed interactions with distance of pushing and pulling.

Plots of initial and sustained pushing and pulling forces against distance of pushing or pulling showed logarithmic relationships. The only exception was the plots for initial pulling forces for males showed nearly linear relationships with pulling distance.

Plots of initial pushing forces and sustained pushing and pulling forces both for males and females showed quadratic relationships with handle height. However, initial pulling forces both for males and females showed linear relationships with handle height.

Using the above-described relationships, frequency of exertion, distance and handle height were transformed. Separate multiple linear regression equations were fitted for each combination of gender (male or female), task (pushing or pulling), and type of force (initial or sustained). The resulting equations are given below and the correlation coefficients (^{2}) and residual standard errors (S.E.) are provided in

Initial Push Force Acceptable to 75% of Female Workers:

Sustained Push Force Acceptable to 75% of Female Workers:

Initial Pull Force Acceptable to 75% of Female Workers:

Sustained Pull Force Acceptable to 75% of Female Workers:

Initial Push Force Acceptable to 75% of Male Workers:

Sustained Push Force Acceptable to 75% of Male Workers:

Initial Pull Force Acceptable to 75% of Male Workers:

Sustained Pull Force Acceptable to 75% of Male Workers:

Correlation coefficients and standard errors for the above regression equations are provided in

A few studies have assessed oxygen uptake (VO_{2}) and/or heart rate (HR) for psychophysically determined maximum acceptable pushing and pulling forces (_{2} might be too high for certain combinations of pushing distances and frequencies.

Most of the scientific studies on pushing and pulling have utilized psychophysics, either maximum isometric or isokinetic pushing and pulling strengths for a single exertion or maximum acceptable pushing and pulling forces for repetitive pushing and pulling. Between these two types of data, maximum acceptable forces provide the most comprehensive data for recommending acceptable levels of pushing and pulling forces for designing and analyzing pushing/pulling tasks in industry as these data reflect the effects of handle height, frequency of exertion and pushing/pulling distance. It is believed that the

Regarding maximum acceptable pushing and pulling forces, the assumption in psychophysics is that an individual can determine his or her maximum pushing and pulling initial and sustained forces that would not lead to an adverse health outcome. In this regard, two different studies (

From a biomechanical perspective large pushing and pulling forces may produce large stresses to both low back as well as shoulder joints. Only a few investigators have quantified stresses to both low back and shoulder joints from pushing and pulling of loads (

Another concern regarding psychophysically determined maximum acceptable pushing and pulling forces is that the sustained pushing and/or pulling forces for certain combinations of frequency, distance and height may cause excessive physical fatigue.

A second concern with psychophysically determined maximum acceptable pushing and pulling forces is that initial maximum forces for low frequency pushing and pulling tasks may be difficult to determine using the adjustment methodology employed for psychophysical studies. Since the methodology relies on the subjects’ ability to increase or decrease the forces between various pushes and pulls, it is unclear how the subject can accurately adjust the forces for very infrequent activities, such as those performed only a few times per day. Therefore, it is suggested that biomechanical limits should also be considered when designing or evaluating very infrequent pushes and pulls.

Psychophysically determined forces are a little higher for pushing than for pulling, implying that pushing is preferable over pulling. However, biomechanical evidence of an advantage between pushing and pulling is inconclusive (

A comprehensive review of maximum acceptable pushing and pulling forces shows that the 1991 guidelines from Snook and Ciriello for pushing and pulling forces are still valid for pushing and pulling carts. For very low frequency pushing and pulling tasks (e.g. less often than one effort per hour), biomechanical criteria should be considered to confirm that compressive and shear forces produced from maximum acceptable forces do not exceed recommended biomechanical limits. Similarly, these low frequency maximum acceptable pushing and pulling forces should be evaluated to make sure that they do not produce unacceptably high moments and stresses to shoulder joints. For high frequency tasks, physiological criteria may be helpful to determine that maximum acceptable forces are within the workers’ physiological limits.

Regression equations fitted to the psychophysical data to estimate initial and sustained forces acceptable to 75% of female and 75% of male workers should be useful to employers and practitioners who design and analyze pushing and pulling tasks in industry. At present it is unclear whether it is preferable to push or pull. Similarly, it is difficult to make recommendations for optimum handle height, as pushing and pulling tasks could be stressful to both the low back and the shoulders. There is a critical need for comprehensive epidemiological studies linking exposure to pushing and pulling tasks and risk of low back pain and/or shoulder disorders. These studies must be well designed and focused on assessing risk associated with pushing and pulling tasks.

The views expressed in this article are those of the authors and do not necessarily represent the views of NIOSH.

Correlation coefficients and standard errors for regression equations fitted to

Task | Type of force | Gender | Eq. # | ^{2} | S.E. |
---|---|---|---|---|---|

Push | Initial | Female | 1 | 0.93 | 0.90 |

Push | Sustained | Female | 2 | 0.92 | 0.87 |

Pull | Initial | Female | 3 | 0.93 | 0.95 |

Pull | Sustained | Female | 4 | 0.92 | 0.90 |

Push | Initial | Male | 5 | 0.91 | 1.95 |

Push | Sustained | Male | 6 | 0.94 | 1.30 |

Pull | Initial | Male | 7 | 0.92 | 1.93 |

Pull | Sustained | Male | 8 | 0.93 | 1.45 |

Combinations of distance and frequency for maximum acceptable sustained push/pull forces (

Gender | Distance (m) | Frequency (1 exertion every)
| |
---|---|---|---|

Push | Pull | ||

Females | 2.1 | 6 s, 12 s | 6 s, 12 s |

Females | 7.6 | 15 s, 22 s | 15 s, 22 s |

Females | 15.2 | 25 s, 35 s, 1 m | 25 s, 35 s, 1 m |

Females | 30.5 | 1 m, 2 m | 1 m, 2 m |

Females | 45.7 | 1 m, 2 m | 1 m, 2 m |

Females | 61.0 | 2 m | 2 m |

Males | 2.1 | 6 s | 6 s |

Males | 7.6 | 15 s, 22 s | 15 s, 22 s |

Males | 15.2 | 25 s, 35 s | 25 s, 35 s, 1 m |

Males | 30.5 | 1 m | 1 m |

Males | 45.7 | 1 m, 2 m | 1 m, 2 m |

Males | 61.0 | 2 m | 2 m |

Exceeds 8-h physiological criteria (1.0 l/min for males) for 64 and 95 cm handle heights.

Exceeds 8-h physiological criteria (1.0 l/min for males) for 64 cm handle height only.

This article provides a concise discussion of important factors relevant to designing and analyzing pushing/pulling tasks. Regression equations to estimate initial and sustained pushing and pulling forces acceptable to 75% male and female workers are provided and can be used to design and analyze pushing and pulling tasks common in industry.