Automotive assemblers engage in physically demanding tasks that involve awkward and upright postures and long periods of repetitive movement. These pose risks to their health, particularly cardiovascular and musculoskeletal strain.

To reduce that risk, engineers have traditionally focused on assessing biomechanical risk factors. Specialists watch workers as they carry out their tasks. They then complete a worksheet that considers various body parts, task intensity, duration and cycle frequency.

While this has been effective in reducing muscular and cardiovascular loads to some extent, subjective measures often overlook the complexity of individual workers’ internal responses—namely, their cardiovascular health.

Despite the common assumption that such physical activity benefits health, emerging evidence suggests a paradoxical relationship between occupational physical activity and cardiovascular health. It has been linked with elevated blood pressure, atherosclerosis and coronary heart disease. This phenomenon is referred to as the physical activity paradox. This discrepancy raises important questions about how the nature, intensity and perception of physical activity at work impact health.

This diagram shows where wireless sensors were placed on workers to collect cardiorespiratory data. Source: Volkswagen

The concept of relative heart rate (RHR) has emerged as a valuable indicator of the intensity of physical work. By comparing the difference between working and resting heart rates with the reserve heart rate (maximum heart rate minus resting heart rate), RHR comprehensively assesses the physiological demands placed on workers during their shifts. 

The balance between a worker’s physical capacity and their job demands can be quantified by maximum acceptable work time (MAWT). This statistic considers individual factors, such as age and maximal oxygen consumption (VO2 max), to help prevent health issues related to overwork. The overwork index—derived from the ratio of each operator’s working time to MAWT—provides a means to evaluate the health risks associated with prolonged working. It highlights the need to balance a worker’s physical abilities and task demands.

Now, thanks to new technologies, such as wearables and the Industrial Internet of Things, it’s possible to measure these parameters on the assembly line in real time.

Past studies on assembly lines have harnessed the capabilities of this technology to measure stress on assemblers and thereby optimize productivity. In one study, for example, the reserve heart rate of workers was monitored through telemetry electrocardiographs (ECG). Management then used that data to adopt a new work strategy that decreased workers’ heart rates significantly.

However, even these studies give a one-sided perspective of the physiological response of workers to the rigors of assembly. We set out to provide a more comprehensive look at the cardiorespiratory effects of work on the assembly line. Specifically, we wanted to determine if there are distinct cardiorespiratory responses to particular workload volumes and if those responses pose any risks to assemblers. Ultimately, our goal is to provide more personalized work interventions, task management and health problem prevention. 

These images show the sequence of movements performed for each task and the specified workstation. Source: Volkswagen

Monitored Assembly Workstations

Our research was conducted at the Volkswagen Autoeuropa assembly plant near Lisbon, Portugal. The factory, which opened in 1995, produces the Volkswagen T-Roc and employs nearly 5,000 people. In 2022, the factory assembled 231,000 vehicles.

Our research focused on three workstations that rely on manual processes:

• H1—tailgate and taillight alignments and attachment of the taillights, involving arm movements in front of or along the upper body.
• H2—alignment of the side doors, rear end and front end, involving upper body bending and applying body weight onto the car.
• H3—mounting of the rearview mirror, cowl, boot panel and trunk symbol, which entails multiple overhead movements.

Sixteen workers were selected through a voluntary participation campaign, all aged 30 to 46.

Each task was given an ergonomic risk score by an ergonomist. We use the European Assembly Worksheet. It includes whole-body and upper-limb assessment of risk factors. To analyze the cardiorespiratory response associated with the ergonomic score, tasks were considered to be low risk with a score equal to or less than 25 (green), moderate risk with a score between 25 and 48 (yellow), and moderate-high risk with a score of 48 or higher (orange). 

Wireless sensors were used to collect ECG; respiratory inductance plethysmography (RIP), and accelerometer (ACC) data from workers during their shifts. The ECG is a noninvasive way to measure the electrical activity of the heart. Cardiovascular load indicators were extracted from these signals, based on reserve heart rate, cardiovascular strain (CVS), and cardiovascular load (CVL). RIP is a noninvasive method of monitoring lung function. The accelerometers provided a measure of effort and workload.

The researchers used the European Assembly Worksheet to estimate the stress of various assembly tasks. Source: Volkswagen

Discussion

Our study aimed to investigate the acute response of operators’ cardiorespiratory systems to different workload volumes on a real automobile assembly line. Further analysis was conducted to determine the association of this response with occupational risk. 

The ECG, RIP, and ACC signals of volunteer workers were tracked during their shifts. Metrics based on heart rate (cardiovascular load and variability) and on the resulting breathing patterns (respiratory frequency and respiratory wall coordination) were extracted to quantify the cardiorespiratory effort put into the tasks. These were analyzed at two moments: first and last 10 minutes of 40-minute recordings.

We found a decrease in HRV variables throughout the acquisition time for the H3 station and for the moderate to high ergonomic risk rank. The cardiovascular response of the H1 workstation did not significantly change during the monitored period. As the tasks progressed, the H2 workstation and the moderate ergonomic risk rank had a significant increase in the variation of breathing motion and a decrease in respiratory wall coordination. At the H1 workstation, a decrease in phase synchrony between the thoracic and abdominal walls was verified.

Low-risk assembly activities had lower heart rates than moderate-high risk activities. Source: Volkswagen

Cardiovascular Response

Heart rate is an important indicator of the cardiac system state. This parameter can indicate abnormalities in the electrical activity of the heart, specifically in its rhythm. For instance, when the time between two consecutive beats is too long, this may indicate bradycardia; when it is too short, tachycardia, conditions that may be associated with underlying disease. In addition to being an indicator of disease, heart rate is used in exercise recommendations. By determining a target heart rate, the intensity of exercise can be controlled.

Heart rate variability (HRV) reflects the balance between the parasympathetic and sympathetic branches of the autonomic nervous system, the regulatory function of the respiratory sinus arrhythmia on heart rate, and the regulation of blood pressure. The sympathetic activity increases in response to stressful situations and exercise, whereas parasympathetic activation is dominant in resting conditions, maintaining and conserving energy and regulating basic body functions. Specifically, sympathetic stimulation increases heart rate and contraction force, while parasympathetic stimulation does the opposite. Therefore, the parasympathetic branch dominates at rest and moderate exercise, giving way to increased HRV.

Diminished HRV has been previously linked to fatigue, disease and increased mortality. Moreover, fast-paced tasks lead to lower decision flexibility, a condition previously connected to higher blood pressure. Furthermore, faster repetitions of lower body resistance training provoked higher volume load, cardiovascular and metabolic strain, and perceived exertion. Workers at H3 experienced significant reductions in HRV, pointing to increased cardiovascular stress and potential fatigue, likely due to having the most frequent tasks or shorter cycle times.

To reduce ergonomic risk, engineers have traditionally focused on assessing biomechanical risk factors. Source: Volkswagen

The ergonomic risk score generally agrees with the physiological response. Low-risk tasks have a smaller cardiac load than both moderate and moderate-high risk tasks. The last two are distinguished by HRV. A possible explanation for this reduction only in moderate-high tasks is that they rely on a long time with arm movements above the head, a work posture that has been associated with considerable circulatory load. The fact that moderate and moderate-high ranks can be discriminated by this metric opens the possibility of better quantifying and separating the intermediate zones of the EAWS score.

Furthermore, it is expected that when heart rate increases, HRV decreases and vice-versa. Despite that, in the H3 workstation and moderate-high risk activities, heart rate is maintained, whereas its variability decreases.

Respiratory Response

Quiet breathing yields nearly constant frequency and pattern, or when metabolic demands increase, a rise in these parameters is expected. The movements of both rib cage and abdominal walls are mediated by the respiratory muscles: the diaphragm, the intercostal muscles, abdominal muscles, and accessory muscles of respiration. When the movements of these muscles are not matched in time, it may indicate increased work of breathing, respiratory muscle fatigue, or even neuromuscular disease.

The assembly line processes depend on various arm movements. Considering that solely through arm elevation, metabolic and ventilation demands have been demonstrated to rise, it was expected that these tasks would increase workers’ respiratory strain. Moreover, these movements rely on muscles of the rib cage that stabilize arm position and posture, diminishing their contribution to respiration, leading the diaphragm and abdominal muscles to compensate to keep up with physiological requirements. The diaphragm contracts not just during respiration, but also when fast repetitive changes in posture and limb position occur, to raise ventilation pressures.

At H1, decreased phase synchrony between thoracic and abdominal movements suggest respiratory distress, likely associated with the movements performed near to the upper body demanded by its tasks. H2 shows similar distress, with a marked increase in abdominal motion variation and reduced respiratory wall coordination, indicating a mismatch in muscle effort during respiration, likely due to the bending and weight application required by these tasks.

Unlike the other workstations, the subjects at the H3 maintained the coordination between their rib cage and abdominal respiratory movements. Thus, it indicates that these tasks do not cause as much breathing distress as the ones in the other workstations.

The respiratory response by ergonomic risk rank showed that the moderate risk results point to a rise in the abdominal exertion with a decline in rib cage contribution. Our data indicate a growth in breathing effort from the first to the last 10 minutes of our recordings.

The results also point to the low-risk activities having a lower breathing effort compared with other ranks. It appears that the tasks of the moderate-high risk workstations have more controlled respiration, even though they take a larger toll on the cardiac system. Here, it is seen that the breathing load is not compatible with the attributed risk rank.

New technologies, such as wearables and the Industrial Internet of Things, make it possible to monitor workers’ cardiorespiratory health on the assembly line in real time. Source: Volkswagen

Practical Implications for Manufacturing and Assembly

The findings of this study support the incorporation of health monitoring systems, such as wearables, on assembly lines. These systems could provide critical data to be used in the adjustment of workloads, task rotation, or scheduling breaks to manage fatigue and reduce long-term health risks. For instance, reports of the workers’ general status could be sent to the occupational physician and alert workers and supervisors of any alarming situation. 

In the construction industry, some studies have already included this type of monitoring, determining task demand by monitoring respiratory variables. Additionally, alert systems based on HRV were implemented to activate when signs of fatigue were detected, thereby informing supervisors that workers required a break. Regarding ergonomic intervention, these measurements could be used to create risk quantification tools.

Limitations and Future Work

Future studies should consider factors such as a worker’s experience and time of day, since these may influence physiological adaptations. Furthermore, sample size should be increased, giving us a broader vision of the factory’s population while increasing the statistical power. Another suggestion is the creation of a risk score that includes cardiorespiratory responses and biomechanical risk, giving way to more individual-specific workplace management.

Conclusions

Our analysis of cardiorespiratory measurements showed that the workstations with characteristic workload volumes have distinct demand and adaptation mechanisms to the tasks. The H3 workstation with the shortest cycle time is the only one that presents a decrease in HRV measures. Conversely, it is the workstation that puts less stress on the respiratory system. H2, with the longest cycle tasks, and H1, with the medium length cycle, present higher breathing efforts.

When analyzing tasks with different ergonomic risks, the low-risk activities had lower heart rates than moderate-high risk activities. They also had a lower maximum heart rate and RHR. Respiratory distress did not coincide with ergonomic risk rank. The moderate-risk tasks conveyed the most asynchronous breathing patterns.

The results of this study revealed important information supporting that cardiorespiratory adaptations should be accounted for in occupational settings.

Editor’s note: This article is a summary of a research paper co-authored by Hui Liu of the University of Bremen in Germany, as well as Dania Furk, Luís Silva, Mariana Dias, Phillip Probst and Hugo Gamboa of the NOVA University of Lisbon in Portugal. To read the entire paper, click here.

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