Abstract
BACKGROUND:
Manual holding task is a potential risk to the development of musculoskeletal injuries since it is prone to induce localized muscle fatigue. Maximum holding endurance time is a significant parameter for the design of manual holding task.
OBJECTIVE:
This study aimed to examine the effects of load and load’s COG height on maximum holding endurance time.
PARTICIPANTS:
Fifteen young and healthy males were recruited as participants.
METHODS:
A factorial design was used to examine the effects of load and load’s COG height on maximum holding endurance time. Four levels of load (15% , 30% , 45% and 60% of the participant’s maximum holding capacity) and two levels of load’s COG height in box (0 cm and 40 cm high from the handle position) were examined.
RESULTS:
Maximum holding endurance time decreased with increasing load and/or increasing load’s COG height. The effect of load’s COG height on maximum holding endurance time decreased with increasing load.
CONCLUSION:
Load, load’s COG height, and the interaction of load and load’s COG height significantly affected maximum holding endurance time. Practitioners should realize the effects of load, load’s COG height, and the interaction of load and load’s COG height on maximum holding endurance time when setting the working conditions of holding tasks.
Introduction
Manual materials handling (MMH) tasks, such as lifting, lowering, holding, carrying, pushing, and pulling, are recognized as the principal source of MMH-related musculoskeletal injuries, especially for low back pain [1, 2]. Although the problem of MMH tasks has been investigated extensively for decades, it is still a significant occupational health issue worldwide today. The compensation cost of MMH tasks is huge. According to the statistics in the United States, the worker’s compensation cost in 2004 due to low back pain was approximate $87.4 billion [3].
Among the MMH tasks, manual holding task is a potential risk to musculoskeletal injuries since it is prone to induce localized muscle fatigue. The muscular endurance time is extensively examined in manual holding task or related studies. For example, Mathiassen and Åhsberg [4] observed that significant individual differences existed in maximum endurance time. Rose et al. [5] showed that the relationship between load and endurance time for passively loaded, fully extended elbow joints resembled that for muscular loading in more normal postures. Rose et al. [6] found that skilled workers have longer endurance and shorter resumption times than non-skilled workers. Iridiastadi and Nussbaum [7] showed that muscle fatigue and endurance during repetitive intermittent static effects were affected by contraction level, duty cycle, and cycle time. Law et al. [8] showed that musculoskeletal discomfort increased with increasing holding time. Yassierli et al. [9] observed that older individuals exhibited longer endurance time and slower development of local muscle fatigue. Mehta and Agnew [10] revealed that concurrent physical and mental workloads decreased muscular endurance time. Farooq and Khan [11] showed that elbow flexion angle had a significant effect on endurance time in a repetitive gripping task. Fallentin et al. [12] found that the reflex latency of back extensors increased after supporting a weight stack while maintaining trunk position. Brouillette [13] showed that maximum endurance time model could not well predict individual endurance time due to inter-individual variability.
Designing an acceptable workload is an effective approach to reduce the severity and frequency of the musculoskeletal injuries of manual holding task. In this respect, the knowledge of holding endurance time is a foundation for designing an acceptable workload of manual holding task. Although previous studies have investigated holding endurance time in some specific holding conditions, they ignored the effect of load’s center of gravity (COG) height on holding endurance time. However, holding or carrying an object whose COG height is higher than people’s hands is occasionally found in our work environments. For example, people often hold or carry a big box at its bottom if the box is absent of handles. Hence, the investigation of the effect of load’s COG height on holding endurance time has practical significance. This study aimed to examine the effects of load and load’s COG height on maximum holding endurance time. The basic hypothesis of this study is that the load and load’s COG height significantly affect maximum holding endurance time.
Methods
Participants
Fifteen male participants, free from any musculoskeletal disorders, were recruited in this experiment. All participants were briefed on the purpose and procedure of the experiment and asked to give their written consent form attesting to the understanding of the risk of the experiment. The mean and standard deviations (SD) for age, weight, and stature were 20.9(0.9) years, 65.1(9.6) kg, and 172.4(4.3) cm, respectively.
Experimental design
A factorial design was used to examine the effects of load and load’s COG height on maximum holding endurance time. Four levels of load (15% , 30% , 45% and 60% of the participant’s maximum holding capacity) were examined. The magnitude of load for each participant was dependent on his own maximum holding capacity. Two levels of load’s COG height (low and high) were examined. They were 0 cm and 40 cm high from the box’s handles position. The dependent variable was maximum holding endurance time. It was defined as the maximum time length that the participant could sustain holding as steady as possible until the occurrence of complete muscle fatigue.
Materials and apparatus
The experimental materials and apparatus included iron shots, box, and hydraulic-driven height-adjustable table. The iron shots were used for determining the weight of load. The box was a light hardboard box (2.5 kg) with dimension of 50 cm×30 cm×60 cm (length×width×height). A pair of rectangle holes (9 cm long and 3 cm width) centered at 10 cm from the bottom of the box served for handles. Two sets of styrofoams were prepared on the box bottom upon which iron shots were loaded, providing the function for locating the load’s COG at 0 cm and 40 cm high from the box’s handles position. One set of styrofoams having a height that elevating the load’s COG that was level to the box’s handles, the other set having a height that elevating the load’s COG that was 40 cm higher than the box’s handles. Selecting a light hardboard box was to lower the effect of box weight on the overall load’s COG height. The hydraulic-driven height-adjustable table was used for adjusting the box’s handles position at the participant’s elbow height. See Fig. 1 for an illustration of the task.
Experimental procedure
The experimental procedure contained two parts. The first was to test the maximum holding capacity of the participants. The second was to examine the effects of load and load’s COG height on maximum holding endurance time.
Tests of maximum holding capacity
Two weeks prior to the formal experiments, each participant was tested his maximum holding capacity. The participant was asked to wear leisure clothes and sneakers in tests. Before tests, the participant did warm-up stretches for a few minutes, then, stood erect in front of the box with feet placed parallel at approximately 45 cm apart plus one-foot length. The participant grasped the handles of the box. The height of the handles was adjusted to fit a 90-degree arms posture. Figure 1 shows the schematic picture of the participant’s arms posture in this study. Then, the box was randomly loaded with iron shots centered at box bottom. The participant was asked to lift-and-hold the box clearly off the table for at least 3 seconds. If the participant succeeded, the weight of the box was increased, in increments of 1 to 5 kg, and the participant was asked to try the lift-and-hold task again. The procedure repeated until the participant failed the lift-and-hold task. The participant was provided at least 2 minutes or longer of rest between two consecutive progressive trials, eliminating the effect of fatigue in determining the maximum holding capability. The 2 minutes rest time followed the standardized procedure of static strength testing [14]. No motivational factors were provided to the participant to avoid competition.
Formal experimental procedure
In formal experiments, all participants were asked to perform 8 (4 loads×2 load’s COG heights) experimental conditions in a random order. The participant wore leisure clothes and sneakers. Before experiments, the participant did warm-up stretches for a few minutes. The holding posture was identical to that in the test of maximum holding capacity. The participant was asked to lift and hold the box clearly off the table and keep it as steady as possible until he could not sustain the holding task due to the occurrence of complete muscle fatigue. The participant visually controlled the box position during holding to assure his holding posture unchanged. In this study, a barrier mark was set over the box which left only 2 cm allowance for the participant to control the height of the box during holding. The maximum holding endurance time was recorded at the end of the experiment. Only one experimental condition was performed by each participant in a day.
Each participant was given a brief and standardized demonstration of the task and allowed to perform practice exercises to familiarize himself with the procedure prior to the formal experiments.
Results
Table 1 lists the maximum holding endurance times for the 8 (4 loads×2 load’s COG heights) experimental conditions. Table 1 shows that maximum holding endurance time decreased with increasing load and/or increasing load’s COG height. The maximum holding endurance time decreased from 605.9 s to 39.7 s as load increased from 15% maximum holding capacity to 60% maximum holding capacity in the condition of low load’s COG, and decreased from 531.5 s to 37.3 s as load increased from 15% maximum holding capacity to 60% maximum holding capacity in the condition of high load’s COG. The difference of maximum holding endurance time between low and high load’s COG also decreased with increasing load. It decreased from 74.4 s to 2.4 s as load increased from 15% maximum holding capacity to 60% maximum holding capacity. The results of analysis of variance in Table 2 indicate that load, load’s COG height, and the interaction of load and load’s COG height significantly affected maximum holding endurance times. Duncan’s multiple range test further indicated that the maximum holding endurance times of 15% , 30% , 45% and 60% maximum holding capacity were significantly different from one another (p < 0.05). In addition, the maximum holding endurance times of low load’s COG were significantly different from those of high load’s COG (p < 0.05). Figure 2 shows the interaction effect of load and load’s COG height on maximum holding endurance time. ’Post hoc analysis with simple ANOVA blocking on participants was further performed to examine the effect of load’s COG height on maximum holding endurance time in each load condition. The results showed that the effect of load’s COG height on maximum holding endurance time was only significant at 15% maximum holding capacity (P < 0.05).
Discussion
In agreement with our hypothesis, this study showed that maximum holding endurance time decreased with increasing load and/or increasing load’s COG height. Maximum holding endurance time decreased with increasing load was in consistent with the result of Nag’s study [15]. This study found that maximum holding endurance time decreased from 605.9 s to 39.7 s as load increased from 15% maximum holding capacity to 60% maximum holding capacity for low load’s COG condition, and decreased from 531.5 s to 37.3 s as load increased from 15% maximum holding capacity to 60% maximum holding capacity for high load’s COG condition.
In this study, maximum holding endurance time was determined by the time to the occurrence of complete muscle fatigue, shorter maximum holding endurance time stood for more rapid occurrence of complete muscle fatigue. Hence, this study showed that heavier load or higher load’s COG height quickened the occurrence of complete muscle fatigue. Heavier load quickened the occurrence of complete muscle fatigue was due to a lesser fresh blood and sugar or oxygen was received by the muscle when performing heavier static muscular contraction [16]. Higher load’s COG position quickened the occurrence of complete muscle fatigue might be attributed to greater load’s instability as load’s COG height increased. From the view point of biomechanics, the load was supported by the exertion forces of two hands. However, the exertion forces of the two hands could not balance each other perfectly due to inconsistent physiological tremor, neuromuscular control, muscular capability, and time to muscle fatigue. This resulted in load instability and thus extra moment to the hands. The load instability and extra moment was greater in high load’s COG condition than that in low load’s COG condition. This result quickened the occurrence of complete muscle fatigue. However, it should be noted that the effect of load’s COG height on maximum holding endurance time decreased with increasing load. For example, maximum holding endurance time of low load’s COG was 74.4 s longer than that of high load’s COG in the load of 15% maximum holding capacity, while only 2.4 s longer than that of high load’s COG in the load of 60% maximum holding capacity. In addition, the difference of maximum holding endurance time between low and high load’s COG was only significant at 15% maximum holding capacity. The effect of load’s COG height on maximum holding endurance time decreased with increasing load might be attributed to that muscle fatigue occurred very soon as the load increased. The result of this study implies that practitioners should be encouraged to lower the load and load’s COG height for a prolonged manual holdingtask.
Footnotes
Acknowledgments
This study was supported by a grant from the Ministry of Science and Technology, ROC. This funding is gratefully acknowledged.