Foot placement strategy in pushing and pulling

Abstract

BACKGROUND:

Pushing and pulling tasks are very common in daily and industrial workplaces. They are one major source of musculoskeletal complaints.

OBJECTIVE:

This study aimed to examine the foot placement strategy while pushing and pulling.

PARTICIPANTS:

Thirteen young males and ten young females were recruited as participants.

METHODS:

A two (pushing and pulling) by four (48 cm, 84 cm, 120 cm, and 156 cm) factorial design was used.

RESULTS:

Exertion direction and exertion height significantly affected foot placement strategy. Pushing task needed more anteroposterior space than pulling task. The percentages of female/male for trailing foot position ranged from 77% to 90% (pushing) and from 80% to 93% (pulling) across the exertion heights.

CONCLUSION:

Practitioners should provide an anteroposterior space approximately to 70% body stature for workers to exert their maximum pulling and pushing strengths.

Keywords

Force exertion capability workplace design

1 Introduction

Pushing and pulling tasks are very common in daily and industrial workplaces. Pushing and pulling tasks account for approximately 20% of low back injuries claims in US [1]. They are also connected to musculoskeletal complaints of the upper extremities [2].

The empirical data for human pushing/pulling strength is the basis of ergonomics redesign on pushing/pulling tasks. Many empirical data for pushing/pulling strengths have been reported in literature [3 –16]. The investigation on pushing/pulling tasks continues recently. For example, Hoffman et al. [17] found that pushing/pulling task resulted in significant off-axis forces. Bennett et al. [18] found that the muscle activities of the shoulders and upper extremity in pushing and pulling tasks were affected to a greater degree by pushing and pulling technique than those of the lower limbs. Chateauroux and Wang [19] asked participants to pull a handbrake on an adjustable car mock-up as they would do when parking their own car. The results showed that maximum static handbrake pulling force and normal handbrake pulling force were dependent on handbrake position, age and gender. Todd [20] measured the physiological responses associated with pushing and pulling. They indicated that pushing elicited significantly lower heart rate and oxygen uptake than pulling. Lin et al. [21] showed that pushing strength varied among handle configurations, comparing to the reference handle configuration (horizontal, straight), the 45- degree rotated and 15-degree tilted handles allowed 6.7% more pushing output, while the horizontal and 15-degree tilted handles resulted in 2.8% less. Lin et al. [22] also observed that pulling direction and handle height significantly affected one-handed pulling strength. Pulling from the side of the body resulted in the greatest strength, followed by front and across pulls. Pulling strength decreased as the handle height increased from 61 cm above the floor to above the shoulder.

Despite the pushing/pulling strength being extensively examined, very few studies have examined foot placement strategy while pushing/pulling. The foot placement strategy is important in the design of pushing/pulling space. Hence, the objective of this study was to examine the effects of exertion direction and exertion height on foot placement strategy. Another objective was to examine the effects of gender (male vs. female) on foot placement strategy.

2 Method

2.1 Participants

A total of thirteen males and ten females participated in this study. The mean (SD) age, body height and mass were 21.4 (1.3) years, 173.6 (2.8) cm and 65.9 (6.1) kg for male participants, respectively; and were 20.7 (0.9) years, 162.6 (3.4) cm and 52.4 (3.4) kg for female participants, respectively.

2.2 Experimental design

The independent variables were exertion directions (pushing and pulling) and exertion heights (48 cm, 84 cm, 120 cm, and 156 cm, measuring from the floor). This permitted a factorial design of a total of eight conditions. The dependent variables were participants’ leading foot position and trailing foot position. The leading foot position and trailing foot position were the horizontal distances from the toe of leading foot and heel of trailing foot to the vertical plane of exertion, respectively. Figure 1 depicts the schematic representation for the measurements of participants’ leading foot position and trailing foot position.

Fig.1

Terminologies for the measurements of foot placement strategy.

2.3 Task procedure

Participants wore flat-soled sport shoes during experiment. They were directed to exert their maximum two-handed isometric strength in eight conditions (two exertion directions x four exertion heights) in a random order. Participants were allowed to use their own optimal posture and foot placement strategy for achieving maximum strength. Participants built up gradually their strength with hands parallel at approximately 60 cm apart to the maximum on the handle bar without jerking. The participants were required to keep their feet on the floor during all tests to assure participants’ body stability while exerting. Participants’ leading foot position and trailing foot position were measured for each test. Three repetitions of all measurements were analyzed in this study. Prior to the formal experiments, participants were asked to perform practice exercises to familiarize the procedure.

3 Results

Table 1 summarizes the significant results (p values) of analysis of variance (ANOVA). Table 1 shows that exertion direction, exertion height and the interaction of exertion direction and exertion height significantly affected trailing foot position. The effect of exertion direction on leading foot position was also significant. However, only exertion height significantly affected female participants’ leading foot position.

Table 1
Summary of the p values from ANOVA on leading foot position and trailing foot position

Source Males Females

leading foot trailing foot leading foot trailing foot

Participants <0.0001 <0.0001 <0.0001 <0.0001

Repetition 0.2360 0.8773 0.8576 0.6700

Direction <0.0001 <0.0001 <0.0001 <0.0001

Height 0.0732 <0.0001 0.0103 <0.0001

Direction x Height 0.6041 <0.0001 0.3426 <0.0001

Source	Males	Females
Participants	<0.0001	<0.0001	<0.0001	<0.0001
Repetition	0.2360	0.8773	0.8576	0.6700
Direction	<0.0001	<0.0001	<0.0001	<0.0001
Height	0.0732	<0.0001	0.0103	<0.0001
Direction x Height	0.6041	<0.0001	0.3426	<0.0001

Figures 2 and 3 shows the participants’ leading foot position and trailing foot position across exertion heights, respectively. Several results were observed. First, pushing tasks were associated with further leading foot position and trailing foot position than pulling tasks. Second, the effect of exertion height on trailing foot position was greater than that on leading foot position. Third, the effect of exertion height on trailing foot position was greater in pushing task than that in pulling task. Fourth, male participants placed their feet further from the handle than female participants did.

Fig.2

The effects of exertion direction and height on male participants’ foot placement.

Fig.3

The effects of exertion direction and height on female participants’ foot placement.

Figure 4 shows the result of the participants’ postural angle (θ) between the line of the trailing foot position to the handle and the line of floor. Figure 4 shows that the θ angle differed in exertion direction, exertion height and gender. Participants selected a lower θ angle when pushing as compared with pulling, and the θ angle seemed to increase linearly with the exertion height. In addition, male participants selected lower θ angle than female participants when pushing/pulling.

Fig.4

The effects of exertion height on participants’ optimal postural angle (θ) between the line of the trailing foot to the handle and the line of floor in the sagittal plane.

4 Discussion

This study showed that exertion direction affected participants’ foot placement strategies. Both participants’ leading and trailing feet were further from the handles in pushing than in pulling. For example, male participants placed their leading foot and trailing foot approximately 15 to 18 cm and 15 to 31 cm, respectively, further from the handle in pushing than in pulling across the four exertion heights, and female participants placed their leading foot and trailing foot approximately 6 to 11 cm and 2 to 26 cm, respectively, further from the handle in pushing than in pulling across the four exertion heights. This study found that participants tried to lean forward, pivoting about the trailing foot, in pushing to maximize their pushing strength. On the contrary, they tried to lean backward, pivoting about the leading foot, in pulling to maximize their pulling strength. This difference in exertion mechanism was responsible for the differences of leading foot positions and trailing foot positions between pushing and pulling tasks.

Exertion height also affected participants’ foot placement strategies. This study found that participants placed their feet further from the handle at medium exertion height than at high or low exertion height. This study attributed this result to the difference of exertion posture while pushing/pulling. It should be noted that high or low exertion height limited participants’ exertion postures and thus foot placement strategies. For example, participants could not lean forward for pushing or lean backward for pulling to maximize their pushing strength. On the contrary, medium exertion height provided more degree of freedom for participants to select their exertion posture and maximize the foot positions.

Male participants needed more anteroposterior space for the trailing foot as compared with female participants while pushing/pulling. The percentages of female/male trailing foot position ranged from 77% to 90% (pushing) and 80% to 93% (pulling) across the exertion heights. This study attributed this to that males are higher than females in stature since people with higher stature had more degree of freedom to place their trailing foot at the same exertion height. Since pushing task needed more anteroposterior space than pulling task, the anteroposterior space for pushing task was also enough for pulling task. Based on the trailing foot position observed in this study, we suggested that the anteroposterior space should provide at least 120 cm and 110 cm long for male participants and female participants, respectively, to achieve their maximum pushing strength. This space was approximately to 70% body stature of the participants.

Exertion direction, exertion height and gender resulted in different θ angles. The further position for the trailing foot from the handle while pushing than pulling was responsible for the lower θ angle in pushing than in pulling. This study found that θ angle seemed to increase linearly with the exertion height. This linearity provided a guideline for practitioners to design the anteroposterior exertion space for a specific exertion height. Finally, the θ angle of male participants was lower than that of female participants. This might be attributed the stature difference between male and female participants since males were higher in stature that allowed them to place their trailing foot further from the handle when pushing/pulling.

5 Conclusion

Exertion direction and exertion height significantly affected foot placement strategy in pushing and pulling tasks. The anteroposterior space for maximum pushing strength was greater than that for maximum pulling strength. Practitioners should provide an anteroposterior space approximately to 70% body stature for workers to exert their maximum pulling and pushing strengths.

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