The effects of handle height, load's CoG height and load on lifting tasks.

The effect of the load's center of gravity (CoG) on manual materials handling tasks received little attention in literature. The motivation of this study was to examine the effects of handle height, load's CoG height and load on lifting tasks. Eighteen participants performed 27 lifting tasks, including 3 handle heights (10, 30, 50 cm) by 3 load's CoG heights (10, 30, 50 cm) by 3 loads (7, 14, 21 kg). The lifting time, maximum box tilt angle, muscular activity (brachioradialis, biceps brachii, deltoid, and erector spinae), maximum CoP velocity, and lifting difficulty were measured. The results showed that lifting time and maximum box tilt angle decreased with increasing handle height. Middle handle height (30 cm) resulted in the lowest muscular activity, maximum CoP velocity, and lifting difficulty. Low load's CoG height decreased lifting time, maximum box tilt angle, muscular activity, and lifting difficulty, however, it also increased maximum CoP velocity. In addition, high load increased lifting time, maximum box tilt angle, muscular activity, maximum CoP velocity, and lifting difficulty.

Effects of hand placement, handles and support on manual holding tasks.

This study recruited 16 participants to examine the effects of hand placement, handles and support on the muscular activities of the musculus biceps brachii and erector spinae, box tilt angle and total center of pressure (CoP) length beneath the feet in manual holding tasks. Each participant was asked to hold a box in two hand placement conditions (symmetrical hand placement and asymmetrical hand placement) × two handle conditions (handles and no handles) × two support conditions (hands-and-body and hands). The results showed that symmetrical hand placement reduced the overall muscular activities of the musculus biceps brachii and erector spinae, box tilt angle and CoP length. The same results were also found in the handles condition and in the hands-and-body support condition. This study recommends that box designers should provide symmetrical handles, and people should keep the box against their front body while holding.

Muscular activity and acceleration of box vibration in manual holding tasks: effects of load and height of the load's center of gravity.

This study recruited 14 participants to examine the effects of load and height of the load's center of gravity (COG) on muscular activities of the brachioradialis, biceps brachii and erector spinae, and on box vibration acceleration (g) in manual holding tasks. Each participant was asked to hold a box in 12 conditions (4 loads × 3 heights of the load's COG). The results showed that muscular activities of the brachioradialis, biceps brachii and erector spinae significantly increased with load; however, they were not affected by the height of the load's COG. In addition, box vibration acceleration increased with load, and decreased with the height of the load's COG. The interaction effect of load and height of the load's COG on box vibration acceleration was also significant. This study recommends that the load magnitude should be decreased for the holding task that requires low vibration.

Foot placement strategy in pushing and pulling.

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.

The effects of arm posture and holding time on holding capability and muscle activity.

The aim of this study was to examine the effects of arm posture and holding time on human holding capability and resulting muscle activity. Fifteen healthy young males were recruited as participants. Maximum holding capacity was examined at 0 (degrees of shoulder forward flexion angle)/90 (degrees of elbow angle), 30/120 and 90/180 arm postures. Maximum acceptable weight of holding was evaluated in three arm postures (0/90, 30/120, 90/180) by three holding times (10 s, 20 s, 30 s). The greatest and lowest maximum holding capacity or maximum acceptable weight of holding occurred at 0/90 and 90/180 arm postures, respectively. Maximum acceptable weight of holding decreased with increasing holding time. While holding maximum acceptable weights, the % of maximum voluntary contraction of brachioradialis, biceps brachii and erector spinae ranged from 14 to 44%, from 14 to 53% and from 25 to 36%, respectively.

Lifting strength in two-person teamwork.

This study examined the effects of lifting range, hand-to-toe distance, and lifting direction on single-person lifting strengths and two-person teamwork lifting strengths. Six healthy males and seven healthy females participated in this study. Two-person teamwork lifting strengths were examined in both strength-matched and strength-unmatched groups. Our results showed that lifting strength significantly decreased with increasing lifting range or hand-to-toe distance. However, lifting strengths were not affected by lifting direction. Teamwork lifting strength did not conform to the law of additivity for both strength-matched and strength-unmatched groups. In general, teamwork lifting strength was dictated by the weaker of the two members, implying that weaker members might be exposed to a higher potential danger in teamwork exertions. To avoid such overexertion in teamwork, members with significantly different strength ability should not be assigned to the same team.

Endurance time, muscular activity and the hand/arm tremor for different exertion forces of holding.

This study aimed to examine the effects of exertion force on endurance time, muscular activity and hand/arm tremor during holding. Fifteen healthy young males were recruited as participants. The independent variable was exertion force (20%, 40%, 60% and 80% maximum holding capacity). The dependent variables were endurance time, muscular activity and hand/arm tremor. The results showed that endurance time decreased with exertion force while muscular activity and hand/arm tremor increased with exertion force. Hand/arm tremor increased with holding time. Endurance time of 40%, 60% and 80% maximum holding capacity was approximately 22.7%, 12.0% and 5.6% of that of 20% maximum holding capacity, respectively. The rms (root mean square) acceleration of hand/arm tremor of the final phase of holding was 2.27-, 1.33-, 1.20- and 1.73-fold of that of the initial phase of holding for 20%, 40%, 60% and 80% maximum holding capacity, respectively.

Effects of carrying handles, postures, materials and distances on carrying capability.

This study recruited 14 industrial workers to examine the effects of carrying handles, postures, materials and distances on maximum acceptable weights of carrying (MAWC), and resulting heart rate and body rating of perceived exertion (RPE) for a 20 min intensive carrying task. This study showed that MAWC of carrying with bar handles, carrying with hands-and-body posture, solid materials and 4 m distance were significantly higher than that of carrying with groove handles, carrying with hands posture, liquid materials and 8 m distance, respectively. The resulting heart rates while carrying MAWCs of groove handles, hands-and-body carrying posture, solid materials and 4 m distance were lower than the resulting heart rates while carrying MAWCs of bar handles, hands carrying posture, liquid materials and 8 m distance, respectively. Most pair levels of independent variables resulted in similar body's RPEs except for the pair levels of carrying distance.

The effects of load magnitude and lifting speed on the kinematic data of load and human posture.

This study examined the effects of load magnitude and lifting speed on the kinematic data of load and human posture in a lifting task. Three load magnitudes (10, 20 and 30 kg) and three lifting speeds (fast, normal and slow) were examined in this study. This study showed that participants shortened the load acceleration period on lifting a lighter load than on lifting a heavier load. For normal and slow lifting speeds, participants moved and lifted the load closer to their body when lifting a heavy load. Participants tended to maintain their postures by using an ankle strategy when in heavier load or faster lifting conditions. The profiles of angle velocity of knee and ankle joints demonstrated the important role of the lower extremities in the acceleration of the load in the initial stage of fast lifting. In addition, participants could not easily control the momentum transmitted to the ankle joint for lifting the heavy load.

Maximum holding endurance time: Effects of load and load's center of gravity height.

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.

Grip force and heart rate responses to manual carrying tasks: effects of material, weight, and base area of the container.

This study recruited 16 industrial workers to examine the effects of material, weight, and base area of container on reduction of grip force (ΔGF) and heart rate for a 100-m manual carrying task. This study examined 2 carrying materials (iron and water), 4 carrying weights (4.4, 8.9, 13.3, 17.8 kg), and 2 base areas of container (24 × 24 cm, 35 × 24 cm). This study showed that carrying water significantly increased ΔGF and heart rate as compared with carrying iron. Also, ΔGF and heart rate significantly increased with carrying weight and base area of container. The effects of base area of container on ΔGF and heart rate were greater in carrying water condition than in carrying iron condition. The maximum dynamic effect of water on ΔGF and heart rate occurred when water occupied ~60%-80% of full volume of the container.

Subjective perception of load heaviness.

This study examined human subjective perception of load heaviness. Forty-two (3 boxes × 14 weights) and 27 (3 boxes × 9 weights) experimental conditions were randomly presented to male and female participants, respectively. The results showed that the participants were not able to discriminate the effect of the box on perceived weight. The participants underestimated the weight for low weights and overestimated it for high weights. The females perceived a greater increase in weight than the males for the same increase in weight. The participants' linguistic term for perceived weight was positively correlated to the magnitude of weight. Approximately 50% of the males perceived a weight of 20 kg or over as risky, while ~60% of the females perceived a weight of 14 kg or over as risky. This study supposes that the gender difference in muscular capability is responsible for the effect of gender on the risk perception of weight.

Analysis of working postures at a construction site using the OWAS method.

This study used OWAS to analyze the working postures of construction workers on building the foundations of a log cabin. Three construction workers, with an average work experience of 40 years, participated in this study. Eight elementary jobs of building the foundations of a log cabin were videotaped at a construction site and analyzed later in the laboratory. For an overall distribution of trunk postures, OWAS identified that a bent and twisted trunk posture (34%), which fell into action category 3, was the major poor posture for construction workers. This study also identified that tying beams with steel bars, assembling column templates, and cement grouting of the ground were the 3 principal jobs in which workers building the foundations exhibited poor working posture. This article suggests ways to reduce and evaluate poor posture in a dynamic construction site.

Postural and muscular responses while viewing different heights of screen.

This study aimed to examine the effects of visual display terminal (VDT) viewing angle on human postural angle and muscular activity. The participants' neck, thoracic bending, and trunk inclination angles; and the activity of sternocleidomastoid, trapezius, splenius capitis, and erector spinae at 5 viewing angles (+40°, +20°, 0°, -20°, and -40°) of a VDT screen were collected for 1 min. This study showed that neck and thoracic bending angles increased with viewing angle, while viewing angle did not significantly affect trunk inclination angle. In addition, the activity of trapezius and erector spinae increased when viewing a higher or lower VDT screen height compared with viewing a horizontal VDT screen height; however, the activity of splenius capitis decreased with viewing angle.

Effects of range and mode on lifting capability and lifting time.

This study examined the effects of 3 lifting ranges and 3 lifting modes on maximum lifting capability and total lifting time. The results demonstrated that the maximum lifting capability for FK (from floor to knuckle height) was greater than that for KS (from knuckle height to shoulder height) or FS (from floor to shoulder height). Additionally, asymmetric lifting with initial trunk rotation decreased maximum lifting capability compared with symmetric lifting or asymmetric lifting with final trunk rotation. The difference in total lifting time between KS and FS was not significant, while FK increased total lifting time by ~20% compared with FS even though the travel distance was 50% shorter.

Effects of hand posture, breathing type, arm posture and body posture on hand errors.

This study consisted of 2 experiments. Experiment 1 examined the effects of hand posture, arm posture and body posture on hand error, while experiment 2 examined the effects of hand posture, breathing type and body posture on hand error. This study showed that more hand errors occurred in the nondominant hand, extended arm, normal breathing and standing compared with errors in the dominant hand, flexed arm, inspire-hold and sitting, respectively. This study advised people to use their dominant hand, flex their arm, inspire and hold the breath and support their body while performing fine manipulation tasks. Finally, hand error varied dramatically across the participants, indicating the need to screen individuals for fine manual manipulation tasks.

Asymmetric lifting capabilities for different container dimensions.

This study recruited 14 young male participants to examine human 4-h maximum acceptable weight of lifting (MAWL) and maximum weight of lifting (MWL) for different modes of asymmetric lifting and containers. The results showed that asymmetric lifting with trunk rotation decreased MAWL and MWL by 9.1 and 17.3%, respectively, and asymmetric lifting with body turn decreased MAWL and MWL by 6.1%, when compared with the symmetric lifting. The decreasing effects of container width and MAWL and MWL were greater than those of container length. Participants selected MAWL of ~33-37% of their MWL capability.

Preferred monitor height in computer workstations.

The height of the monitor directly affects human posture and visual demands in using a computer. Current recommendations on the height of monitors are conflicting.

Maximum symmetric and asymmetric isoinertial lifting capabilities from floor to knuckle height.

Ten young male participants were recruited to test their maximum symmetric and asymmetric isoinertial lifting capabilities from floor to the knuckle height in different container dimensions varied in both width and length. The results showed that the order from the highest to lowest lifting capability for the lifting modes was symmetric lifting, asymmetric lifting with leg rotation and asymmetric lifting with trunk rotation, and for the container dimensions was 50 x 35 x 15 cm, 70 x 35 x 15 cm and 50 x 50 x 15 cm. Participants' lifting capabilities differed significantly (p<0.05) among lifting modes and container dimensions. This study recommends that asymmetric lifting with leg rotation should be encouraged as compared with asymmetric lifting with trunk rotation when performing heavy asymmetric lifting tasks.

Two-handed isometric pulling strengths at different floor-slope angles.

This study recruited 10 Taiwanese men (M age=20.4 yr., SD=1.9) to examine the effects of exertion height (30%, 50%, 70%, and 90% stature) and ascending floor-slope angle (0 degrees, 5 degrees, 10 degrees, and 15 degrees) on maximum horizontal isometric pulling strengths. Analysis showed pulling strength decreased linearly with exertion height and increased linearly with floor-slope angle. The pulling strength at the exertion height of 90% stature was only 35% to 40% of that of 30% stature. The pulling strength of the 15 degrees floor-slope angle was approximately 127.6% to 150.9%, dependent on exertion height, of that of the 0 degrees floor-slope angle. The linear correlations between pulling strength and exertion height and between pulling strength and floor-slope angle were linear.