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NIRS in ergonomics: Its application in industry for promotion of health and human performance at work
International Journal of Industrial Ergonomics 40 (2010) 185–189
Contents lists available at ScienceDirect
International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon
NIRS in ergonomics: Its application in industry for promotion of health and human performance at work Ste´phane Perrey*, Thibaud Thedon, Thomas Rupp EA 2991 Motor Efficiency and Motor Deficiency Laboratory, Faculty of Sport Sciences, University of Montpellier I, 700 Avenue du Pic Saint Loup, 34090 Montpellier, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2008 Received in revised form 4 November 2008 Accepted 25 November 2008 Available online 8 February 2009
Demands of a job may include executing physical actions and/or performing cognitive judgments. The impact of these demands is dependent on the abilities of the individual performing the work. In order to ensure acceptable load levels for promotion of health and human performance at work during repetitive activities such as pushing, pulling, lifting, carrying, reaching and assembling, the physical capabilities of the workers have to be investigated. While the description of pulmonary oxygen consumption kinetics provides a valid and useful indication of cardiorespiratory function during changes in workload, it does not allow the measurement of changes in the oxygen content of the working muscle. Near-infrared Spectroscopy (NIRS) has been successfully employed to measure muscle and brain oxygenation during rest and exercise. Evaluation of central and peripheral responses during working tasks can predict the effectiveness of the workers’ performance. This paper reviews the role of NIRS-derived measures in assessing human performance in the workplace by evaluating demands of executing physical actions and/or performing cognitive tasks in a controlled laboratory environment or in the field.
Keywords: Physical working capacity Skeletal muscle Prefrontal cortex Fatigue
Relevance to industry: Workers often have to exert repeated muscle contractions and make cognitive judgments while operating under a variety of physical constraints. NIRS-derived measurement allows the evaluation of workers with respect to muscular capacities, physical workload situations and the degree of mental effort required to perform the task. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Strenuous exercise or physical work, such as manual materials handling tasks, performed for an 8-h work period can often lead to fatigue (Waters et al., 1993). While the use of new and advanced technology has substantially reduced the amount of heavy physical workload, repetitive manual activities are still observed in various work settings (e.g., Ciriello et al., 1999). An imbalance between job demands and individual capacities may lead to serious health and safety risks (Ramsey et al., 1983; Karlqvist et al., 2003). For example, workers engaged in building, cleaning and in some special safety occupations, such as fire fighters, police officers, and soldiers, are regularly exposed to heavy workloads. In these jobs the cardiorespiratory capacity of workers is the determining factor with regard to endurance performance and work productivity (Ramsey et al., 1983; Schibye et al., 2001; Bilzon et al., 2002). Consequently,
* Corresponding author. Tel.: þ33 (0) 4 67 41 57 61; fax: þ33 (0) 4 67 41 57 08. E-mail address: [email protected] (S. Perrey). 0169-8141/$ – see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2008.11.002
reliable assessment of cardiorespiratory capacity is often relevant for preventing injuries and unhealthy consequences among workers. The assessment of energetic demands of physical jobs and tasks has traditionally been based on measurements of pulmonary oxygen uptake (VO2). Signs of fatigue are usually evident in an individual engaged in prolonged physical work requiring more than 30% of the maximal aerobic capacity (Astrand and Rodahl, 1996). Among the physiological measurements, maximal VO2 is traditionally considered the best parameter to assess cardiorespiratory fitness. Maximal VO2 is dependent upon the overall ability of the body systems to utilize oxygen from the ambient air, to transport it, to deliver it and to utilize it in the working muscles. When assessing cardiorespiratory fitness, graded exercise tests are used to evaluate the ability of the participant to tolerate increasing intensities of exercise while haemodynamic responses are monitored for manifestations of ischemia or other abnormalities. In these conditions, the measurement of gas exchanges and ventilatory responses are essential to determine functional capacity of the respiratory, cardiovascular and metabolic systems. However, direct measurement of VO2 in the workplace is rather cumbersome. Furthermore,
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to obtain reliable results during laboratory tests, rather sophisticated equipment and testing are required. One solution to this issue might be the use of a simple and sufficiently reliable instrument for assessing the haemodynamic and metabolic responses of specific muscles recruited. Near-infrared Spectroscopy (NIRS) is a non-invasive, nonionizing, real-time monitoring continuous method used to determine oxygenation and haemodynamics in tissue (Van Beekvelt et al., 2001; Boushel et al., 2001). The theory behind NIRS technology has been well explained (e.g., Villringer and Chance, 1997; Boushel et al., 2001). NIRS is sensitive to changes in tissue oxygenation, both at the level of small blood vessels and capillaries and at the intracellular sites of oxygen uptake (Hampson and Piantadosi, 1988). Owing to its ability to provide good temporal and spatial resolution of oxygen availability, this methodology may provide insights into mechanisms regulating regional tissue blood flow and metabolism during the performance of various jobs. This paper aimed to present NIRS as a method to assess performance in workers by evaluating demands of executing physical actions and/or performing cognitive tasks in a controlled laboratory environment or in the field. Knowledge of overall workload may provide more insight into the physiological changes in workers and the potential risks of developing muscle injury. 2. Evaluating physical and mental demands of workers using NIRS Skeletal muscles deoxygenate to varying degrees during physical work in accordance with work intensity and level of fitness (Boushel and Piantadosi, 2000). The NIRS-derived hemoglobin signal obtained during exercise is considered to reflect the balance between oxygen use and delivery, as demonstrated by gradual decrease of oxygen during incremental exercise (Belardinelli et al., 1995; Wilson et al., 1989). During a graded exercise, in addition to providing information on maximal VO2, the pulmonary gas exchange responses to incremental workload can be used to estimate the time at which the lactate threshold (LT) occurs. The LT can be estimated non-invasively by determination of the gas exchange threshold, which is referred as the ventilatory threshold (VT, Wilmore and Costill, 2004; see on Fig. 1 an example). VT is classically used to prescribe exercise intensity, predict endurance performance and exercise tolerance, and diagnose health status in adults that vary in aerobic fitness and training. As exercise intensity progressively increases toward maximum, a point is reached where ventilation increases disproportionately compared with oxygen consumption. Typically, when work rate exceeds 55–70% of maximal VO2, oxygen delivery to the muscles can no longer meet the oxygen requirements (Wilmore and Costill, 2004). One new and interesting method of detecting VT or LT is through NIRS as spectroscopy-derived physiological measures are known to reflect the metabolic changes that occur at VT (Bhambhani et al., 1997, 2007; Rupp and Perrey, 2008) and LT (Grassi et al., 1999) at the muscle site under investigation. Belardinelli et al. (1995) and Bhambhani et al. (1997) utilized NIRS during progressively incremental cycle exercise in healthy men and women with different levels of fitness and compared the observed pattern of deoxygenation with the so-called VT. The pattern of deoxygenation observed in the study of Grassi et al. (1999) appeared similar to that described by Belardinelli et al. (1995) in a group of trained and untrained subjects. This trend suggests that the increased muscle oxygen demand during incremental exercise results in a mismatch between local muscle oxygen delivery and muscle oxygen utilization (Rupp and Perrey, 2008). Also all these authors described a distinct breakpoint of accelerated muscle deoxygenation (see on Fig. 1) in close correlation with the VT. Taken
Fig. 1. The upper graph shows the expired gas analysis and the lower graph shows total hemoglobin (HbTOT), oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb) measurements done by near-infrared spectroscopy on the vastus lateralis muscle during a progressive maximal cycling task in a healthy male. Near the ventilatory threshold (vertical line), the decreasing rate of O2Hb starts to form the inflection point (vertical line) determined by regression of two straight lines. VE, minute ventilation; VE/VO2 and VE/VCO2, ventilatory equivalent for oxygen and carbon dioxide, respectively.
together, the results of the aforementioned studies suggest a close relationship between NIRS-derived indexes of muscle oxygenation and the VT or LT. In industries such as manufacturing, warehousing, and transportation, workers commonly use pallet trucks (pulling task) to move heavy loads several times a day. With progressive fatigue along the day in working muscles, this can increase the risk of a slip or fall by reducing the workers ability to make appropriate gait changes. The physiological cost imposed on workers during industrial pushing tasks for several hours may become high on the lower limbs (e.g., important calf muscle oxygenation decrease) as well as measuring muscle oxygenation from various muscles during simulated progressive maximum ramp pushing or pulling efforts (e.g., push/pull tasks simulating the snow shoveling) is relevant to assess muscle tolerance and exertion level and so in predicting the performance effectiveness of the workers (Maikala and Bhambhani, 2007). Exercise tolerance/intolerance has classically been defined by factors such as oxygen transport and metabolic turnover in the periphery. Little attention has been given to the possibility that the
S. Perrey et al. / International Journal of Industrial Ergonomics 40 (2010) 185–189
central nervous system may be determining these factors by imposing limits to muscular contraction. On one hand, progressive muscle deoxygenation may be directly responsible for impaired peripheral ability to maintain adequate muscle contractions. Muscle afferents, on the other hand can potentially inhibit central motor output and voluntary muscle performance via a reduced activation of the motoneuronal pool (Gandevia, 2001). In a recent review, Kayser (2003) stresses the critical role of the central nervous system in voluntary exercise. While the mechanism(s) of so-called ‘‘central fatigue’’ are largely unknown, researchers have proposed integrative models in which multiple sensory signals may lead to the reduction in central drive and the de-recruitment of muscle fibers as a protective mechanism (e.g., Noakes, 2000). NIRS has provided a unique but faster approach for the assessment of oxygenation patterns in the brain, which may be linked to neuronal drive and exercise tolerance in healthy and diseased populations (Nielsen et al., 2001; Bhambhani et al., 2007; Rupp and Perrey, 2008). During exhaustive or maximally graded exercise, researchers have reported a decrease in cerebral oxygenation and/ or cerebral blood flow (Nielsen et al., 2001; Bhambhani et al., 2007; Rupp and Perrey, 2008). It has been speculated that alterations in oxygenation patterns in the frontal lobe and motor cortex may indicate a reduction in neuronal activation and limit workload capacity (Kayser, 2003; Noakes, 2000; Bhambhani et al., 2007). NIRS monitoring on prefrontal cortex to determine the ability of an individual to make cognitive judgments while operating under physical constraints, is also possible. Time pressure and precision demands are commonplace amongst workers using machines and computers, and these demands are likely contributors to the development of mental stress and musculoskeletal symptoms. Heiden et al. (2005) found that subjects with fatigue showed a significant decrease in forearm oxygenation when time pressure and accuracy were imposed on a painting task, as opposed to mouse use without such demands. During physical work, cerebral haemodynamic changes may be also viewed as psychophysiological measures. To estimate mental stress in the area of industrial settings, it is necessary to investigate the overall function of brain activity in the workplace. Since NIRS is a reliable measure of oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb), NIRS may be used to estimate brain blood volume in the absence of haematocrit change. In this regard, the cognitive tasks (e.g., calculation task, continuous performance task, such as manual materials handling tasks or cyclic manual assembly work in industry) caused functional activation in the frontal region because O2Hb is increased and HHb is decreased (Fig. 2) during such tasks (Fallgatter and Stric, 1997; Villringer et al., 1993). As well, operating an aircraft is considered a stressful task (inducing an increase in the cognitive demand) because momentary misjudgment may result in a serious accident. Furthermore, technological advances and heavy workloads may result in increasing operational demands (takeoff, approach and landing) on both military and civilian pilots. Previous studies have demonstrated that NIRS is a reliable and sensitive method in ‘‘aviation’’ environments and NIRS has been successfully used to measure the in-flight cerebral oxygen status of F-15 fighter pilots during aerial gunnery training (Kobayashi and Miyamoto, 2000) and during air-to-air combat maneuvering (Kobayashi et al., 2002). In a recent study of Kikukawa et al. (2008), the increase in concentration of O2Hb from the basal level during taxi-out/takeoff (cognitive demand task) strongly suggests the O2Hb concentration is associated with cognitive demand (maneuvers) in helicopter pilots, and O2Hb increase in pilots was detectable via NIRS. Additionally, in some specific conditions (e.g., hyperthermia or in high altitude) NIRS can show the reverse behavior with respect to cerebral activation (a drop in O2Hb and a rise in HHb) leading to fatigue (Rupp and Perrey, 2008). Such a deactivation phenomenon
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Fig. 2. Examples of calculation task where subject in a seated position was asked to resolve arithmetical operation (as indicated in the grey box) with time pressure and precision demands. Each time the calculation was done (results are indicated in the right corner of the grey box), it causes cognitive activation in the frontal region as demonstrated by increase in oxyhemoglobin (O2Hb) and decrease in deoxyhemoglobin (HHb).
in the prefrontal cortex during performance of mental tasks has also been described in a simultaneous positron emission tomography–NIRS study (Hoshi et al., 1994). What emerges is that physical and mental demands of jobs are not only a challenge to the oxygenation of the muscle, but also of the brain. 3. NIRS as a functional tool in the follow up of muscle function during physical work The relative importance of oxygen supply to the muscles and oxygen utilization in muscle tissue is still the subject of debate in graded maximal exercise (Richardson et al., 1999). Also, oxygen consumption of the working muscles increases in close relation to an increase in blood flow during submaximal exercise at constant load (Vøllestad et al., 1990) suggesting that the oxygen supplyconsumption balance is not only intensity dependent but also time dependent. In other words, it emerges excess in VO2 uptake (i.e., a VO2 slow component) leading to early muscle fatigue (Bringard and Perrey, 2004). Workloads where there is no sustained metabolic acidosis can usually be sustained for long periods with a modest effort. Most activities in industry settings are full of challenges in which the energy demands of the working muscles might quickly go from rest to moderate to heavy loading. In an unsteady-state, the rate at which skeletal muscle oxidative metabolism adjusts to a new metabolic requirement is one of the factors that determines physical work tolerance due to oxygen deficit (Whipp et al., 2005). Knowledge of this rate will then provide a framework to explore the consequences of impaired muscle function. A physiological steady state is usually attained within about 3 min in young healthy adults, but sooner in those of high aerobic fitness and considerably longer with cardio-pulmonary dysfunction. The on-transient VO2
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response is multifactorial, and relies upon the capacity within the working muscle to rapidly change the delivery and utilization of oxygen. While the determination of VO2 kinetics is valuable for the functional evaluation of muscle oxidative metabolism (Whipp et al., 2005), the monitoring with high-time resolution of oxygen utilization through the use of NIRS within the working muscle may help to identify peripheral limitations to this metabolic adaptation (Fig. 3). NIRS-derived signals determine muscular VO2 kinetics during exercise where metabolic condition changes diversely (DeLorey et al., 2003; Grassi et al., 2003; Volianitis et al., 2003). By assessing the pulmonary gas exchange and the deoxygenation state of the working muscles by NIRS during task (Fig. 3), the factors (constant time of on-transient VO2 and NIRS response and amplitude of the slow component) that determine exercise tolerance can be assessed in workers. In industry, there are a variety of manual materials handling activities, such as pushing, pulling, lifting, carrying, reaching and assembling. This type of physical work with a monotonous or a repetitive muscle activity pattern over time is associated with an increased risk of upper extremity and low-back disorders (Hagberg and Wegman, 1987). Studying the physical work in a controlled laboratory environment (Kell and Bhambhani, 2003) helps to identify the high-risk activities and conditions that can lead to injury. For example, performance during an incremental test to failure with a controlled form of exercise (e.g., lifting technique in Kell and Bhambhani, 2003) or a submaximal exercise test (Maronitis et al., 2000) with NIRS monitoring can be utilized to evaluate muscle oxidative function in preventing muscle fatigue and thus assisting injury prevention. This information can be used to set limits and redesign jobs to increase safety (Waters et al., 1993). Evidence based on the use of NIRS has also shown that the endurance of lower-back muscles was closely linked to the availability of oxygen (Yoshitake et al., 2001), although decrease in oxygen due to elevated intramuscular pressure was demonstrated only at moderate to higher levels of muscle contraction. Interestingly, Yang et al. (2007) examined the status of muscle oxygenation in the low back over an entire workday. These authors demonstrated that the requirement of oxygen for the low-back muscles in a typical job associated with lifting increased over time. Yang et al. (2007) also reported that participants who had no experience in lifting demonstrated an increase in muscle tension and needed more muscle oxygenation than the experienced lifters.
Fig. 3. Graph shows an example of simultaneous measurements of whole-body oxygen uptake (VO2) and back muscle oxygenation with near-infrared spectroscopy during constant workload arm exercise in supine position (with a swimming simulator device) performed at a high relative load intensities This graph demonstrates that cardiorespiratory and localized muscle oxygenation do not attain a steady state after about 3 min of exercise: VO2 continues to increase (the so-called VO2 slow component) while tissue oxygenation index (TOI) over the back muscle decreases. Back muscles are likely to play some limitation to the overall metabolic adaptation characterized with VO2 measurement. Thus, muscle tolerance of the individual appears low and muscle fatigue will appear quickly.
4. Conclusion The ability to obtain reliable quantitative measurements of muscle oxygen consumption is necessary to understand the mechanisms of local muscle metabolism at rest, during and after exercise. The concurrent measurement of both pulmonary oxygen uptake and muscle oxygenation helps to provide measures of both systemic and peripheral responses to exercise and work. NIRS is proving to be an effective method for the monitoring of muscle metabolism and brain haemodynamics during performance of job activities, which will help in understanding tissue physiology and associated disorders. In an ongoing effort to help safety practitioners match physical work demands to worker capabilities, NIRSderived measurements are encouraged in the workplace.
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Contents lists available at ScienceDirect
International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon
NIRS in ergonomics: Its application in industry for promotion of health and human performance at work Ste´phane Perrey*, Thibaud Thedon, Thomas Rupp EA 2991 Motor Efficiency and Motor Deficiency Laboratory, Faculty of Sport Sciences, University of Montpellier I, 700 Avenue du Pic Saint Loup, 34090 Montpellier, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2008 Received in revised form 4 November 2008 Accepted 25 November 2008 Available online 8 February 2009
Demands of a job may include executing physical actions and/or performing cognitive judgments. The impact of these demands is dependent on the abilities of the individual performing the work. In order to ensure acceptable load levels for promotion of health and human performance at work during repetitive activities such as pushing, pulling, lifting, carrying, reaching and assembling, the physical capabilities of the workers have to be investigated. While the description of pulmonary oxygen consumption kinetics provides a valid and useful indication of cardiorespiratory function during changes in workload, it does not allow the measurement of changes in the oxygen content of the working muscle. Near-infrared Spectroscopy (NIRS) has been successfully employed to measure muscle and brain oxygenation during rest and exercise. Evaluation of central and peripheral responses during working tasks can predict the effectiveness of the workers’ performance. This paper reviews the role of NIRS-derived measures in assessing human performance in the workplace by evaluating demands of executing physical actions and/or performing cognitive tasks in a controlled laboratory environment or in the field.
Keywords: Physical working capacity Skeletal muscle Prefrontal cortex Fatigue
Relevance to industry: Workers often have to exert repeated muscle contractions and make cognitive judgments while operating under a variety of physical constraints. NIRS-derived measurement allows the evaluation of workers with respect to muscular capacities, physical workload situations and the degree of mental effort required to perform the task. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Strenuous exercise or physical work, such as manual materials handling tasks, performed for an 8-h work period can often lead to fatigue (Waters et al., 1993). While the use of new and advanced technology has substantially reduced the amount of heavy physical workload, repetitive manual activities are still observed in various work settings (e.g., Ciriello et al., 1999). An imbalance between job demands and individual capacities may lead to serious health and safety risks (Ramsey et al., 1983; Karlqvist et al., 2003). For example, workers engaged in building, cleaning and in some special safety occupations, such as fire fighters, police officers, and soldiers, are regularly exposed to heavy workloads. In these jobs the cardiorespiratory capacity of workers is the determining factor with regard to endurance performance and work productivity (Ramsey et al., 1983; Schibye et al., 2001; Bilzon et al., 2002). Consequently,
* Corresponding author. Tel.: þ33 (0) 4 67 41 57 61; fax: þ33 (0) 4 67 41 57 08. E-mail address: [email protected] (S. Perrey). 0169-8141/$ – see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2008.11.002
reliable assessment of cardiorespiratory capacity is often relevant for preventing injuries and unhealthy consequences among workers. The assessment of energetic demands of physical jobs and tasks has traditionally been based on measurements of pulmonary oxygen uptake (VO2). Signs of fatigue are usually evident in an individual engaged in prolonged physical work requiring more than 30% of the maximal aerobic capacity (Astrand and Rodahl, 1996). Among the physiological measurements, maximal VO2 is traditionally considered the best parameter to assess cardiorespiratory fitness. Maximal VO2 is dependent upon the overall ability of the body systems to utilize oxygen from the ambient air, to transport it, to deliver it and to utilize it in the working muscles. When assessing cardiorespiratory fitness, graded exercise tests are used to evaluate the ability of the participant to tolerate increasing intensities of exercise while haemodynamic responses are monitored for manifestations of ischemia or other abnormalities. In these conditions, the measurement of gas exchanges and ventilatory responses are essential to determine functional capacity of the respiratory, cardiovascular and metabolic systems. However, direct measurement of VO2 in the workplace is rather cumbersome. Furthermore,
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to obtain reliable results during laboratory tests, rather sophisticated equipment and testing are required. One solution to this issue might be the use of a simple and sufficiently reliable instrument for assessing the haemodynamic and metabolic responses of specific muscles recruited. Near-infrared Spectroscopy (NIRS) is a non-invasive, nonionizing, real-time monitoring continuous method used to determine oxygenation and haemodynamics in tissue (Van Beekvelt et al., 2001; Boushel et al., 2001). The theory behind NIRS technology has been well explained (e.g., Villringer and Chance, 1997; Boushel et al., 2001). NIRS is sensitive to changes in tissue oxygenation, both at the level of small blood vessels and capillaries and at the intracellular sites of oxygen uptake (Hampson and Piantadosi, 1988). Owing to its ability to provide good temporal and spatial resolution of oxygen availability, this methodology may provide insights into mechanisms regulating regional tissue blood flow and metabolism during the performance of various jobs. This paper aimed to present NIRS as a method to assess performance in workers by evaluating demands of executing physical actions and/or performing cognitive tasks in a controlled laboratory environment or in the field. Knowledge of overall workload may provide more insight into the physiological changes in workers and the potential risks of developing muscle injury. 2. Evaluating physical and mental demands of workers using NIRS Skeletal muscles deoxygenate to varying degrees during physical work in accordance with work intensity and level of fitness (Boushel and Piantadosi, 2000). The NIRS-derived hemoglobin signal obtained during exercise is considered to reflect the balance between oxygen use and delivery, as demonstrated by gradual decrease of oxygen during incremental exercise (Belardinelli et al., 1995; Wilson et al., 1989). During a graded exercise, in addition to providing information on maximal VO2, the pulmonary gas exchange responses to incremental workload can be used to estimate the time at which the lactate threshold (LT) occurs. The LT can be estimated non-invasively by determination of the gas exchange threshold, which is referred as the ventilatory threshold (VT, Wilmore and Costill, 2004; see on Fig. 1 an example). VT is classically used to prescribe exercise intensity, predict endurance performance and exercise tolerance, and diagnose health status in adults that vary in aerobic fitness and training. As exercise intensity progressively increases toward maximum, a point is reached where ventilation increases disproportionately compared with oxygen consumption. Typically, when work rate exceeds 55–70% of maximal VO2, oxygen delivery to the muscles can no longer meet the oxygen requirements (Wilmore and Costill, 2004). One new and interesting method of detecting VT or LT is through NIRS as spectroscopy-derived physiological measures are known to reflect the metabolic changes that occur at VT (Bhambhani et al., 1997, 2007; Rupp and Perrey, 2008) and LT (Grassi et al., 1999) at the muscle site under investigation. Belardinelli et al. (1995) and Bhambhani et al. (1997) utilized NIRS during progressively incremental cycle exercise in healthy men and women with different levels of fitness and compared the observed pattern of deoxygenation with the so-called VT. The pattern of deoxygenation observed in the study of Grassi et al. (1999) appeared similar to that described by Belardinelli et al. (1995) in a group of trained and untrained subjects. This trend suggests that the increased muscle oxygen demand during incremental exercise results in a mismatch between local muscle oxygen delivery and muscle oxygen utilization (Rupp and Perrey, 2008). Also all these authors described a distinct breakpoint of accelerated muscle deoxygenation (see on Fig. 1) in close correlation with the VT. Taken
Fig. 1. The upper graph shows the expired gas analysis and the lower graph shows total hemoglobin (HbTOT), oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb) measurements done by near-infrared spectroscopy on the vastus lateralis muscle during a progressive maximal cycling task in a healthy male. Near the ventilatory threshold (vertical line), the decreasing rate of O2Hb starts to form the inflection point (vertical line) determined by regression of two straight lines. VE, minute ventilation; VE/VO2 and VE/VCO2, ventilatory equivalent for oxygen and carbon dioxide, respectively.
together, the results of the aforementioned studies suggest a close relationship between NIRS-derived indexes of muscle oxygenation and the VT or LT. In industries such as manufacturing, warehousing, and transportation, workers commonly use pallet trucks (pulling task) to move heavy loads several times a day. With progressive fatigue along the day in working muscles, this can increase the risk of a slip or fall by reducing the workers ability to make appropriate gait changes. The physiological cost imposed on workers during industrial pushing tasks for several hours may become high on the lower limbs (e.g., important calf muscle oxygenation decrease) as well as measuring muscle oxygenation from various muscles during simulated progressive maximum ramp pushing or pulling efforts (e.g., push/pull tasks simulating the snow shoveling) is relevant to assess muscle tolerance and exertion level and so in predicting the performance effectiveness of the workers (Maikala and Bhambhani, 2007). Exercise tolerance/intolerance has classically been defined by factors such as oxygen transport and metabolic turnover in the periphery. Little attention has been given to the possibility that the
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central nervous system may be determining these factors by imposing limits to muscular contraction. On one hand, progressive muscle deoxygenation may be directly responsible for impaired peripheral ability to maintain adequate muscle contractions. Muscle afferents, on the other hand can potentially inhibit central motor output and voluntary muscle performance via a reduced activation of the motoneuronal pool (Gandevia, 2001). In a recent review, Kayser (2003) stresses the critical role of the central nervous system in voluntary exercise. While the mechanism(s) of so-called ‘‘central fatigue’’ are largely unknown, researchers have proposed integrative models in which multiple sensory signals may lead to the reduction in central drive and the de-recruitment of muscle fibers as a protective mechanism (e.g., Noakes, 2000). NIRS has provided a unique but faster approach for the assessment of oxygenation patterns in the brain, which may be linked to neuronal drive and exercise tolerance in healthy and diseased populations (Nielsen et al., 2001; Bhambhani et al., 2007; Rupp and Perrey, 2008). During exhaustive or maximally graded exercise, researchers have reported a decrease in cerebral oxygenation and/ or cerebral blood flow (Nielsen et al., 2001; Bhambhani et al., 2007; Rupp and Perrey, 2008). It has been speculated that alterations in oxygenation patterns in the frontal lobe and motor cortex may indicate a reduction in neuronal activation and limit workload capacity (Kayser, 2003; Noakes, 2000; Bhambhani et al., 2007). NIRS monitoring on prefrontal cortex to determine the ability of an individual to make cognitive judgments while operating under physical constraints, is also possible. Time pressure and precision demands are commonplace amongst workers using machines and computers, and these demands are likely contributors to the development of mental stress and musculoskeletal symptoms. Heiden et al. (2005) found that subjects with fatigue showed a significant decrease in forearm oxygenation when time pressure and accuracy were imposed on a painting task, as opposed to mouse use without such demands. During physical work, cerebral haemodynamic changes may be also viewed as psychophysiological measures. To estimate mental stress in the area of industrial settings, it is necessary to investigate the overall function of brain activity in the workplace. Since NIRS is a reliable measure of oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb), NIRS may be used to estimate brain blood volume in the absence of haematocrit change. In this regard, the cognitive tasks (e.g., calculation task, continuous performance task, such as manual materials handling tasks or cyclic manual assembly work in industry) caused functional activation in the frontal region because O2Hb is increased and HHb is decreased (Fig. 2) during such tasks (Fallgatter and Stric, 1997; Villringer et al., 1993). As well, operating an aircraft is considered a stressful task (inducing an increase in the cognitive demand) because momentary misjudgment may result in a serious accident. Furthermore, technological advances and heavy workloads may result in increasing operational demands (takeoff, approach and landing) on both military and civilian pilots. Previous studies have demonstrated that NIRS is a reliable and sensitive method in ‘‘aviation’’ environments and NIRS has been successfully used to measure the in-flight cerebral oxygen status of F-15 fighter pilots during aerial gunnery training (Kobayashi and Miyamoto, 2000) and during air-to-air combat maneuvering (Kobayashi et al., 2002). In a recent study of Kikukawa et al. (2008), the increase in concentration of O2Hb from the basal level during taxi-out/takeoff (cognitive demand task) strongly suggests the O2Hb concentration is associated with cognitive demand (maneuvers) in helicopter pilots, and O2Hb increase in pilots was detectable via NIRS. Additionally, in some specific conditions (e.g., hyperthermia or in high altitude) NIRS can show the reverse behavior with respect to cerebral activation (a drop in O2Hb and a rise in HHb) leading to fatigue (Rupp and Perrey, 2008). Such a deactivation phenomenon
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Fig. 2. Examples of calculation task where subject in a seated position was asked to resolve arithmetical operation (as indicated in the grey box) with time pressure and precision demands. Each time the calculation was done (results are indicated in the right corner of the grey box), it causes cognitive activation in the frontal region as demonstrated by increase in oxyhemoglobin (O2Hb) and decrease in deoxyhemoglobin (HHb).
in the prefrontal cortex during performance of mental tasks has also been described in a simultaneous positron emission tomography–NIRS study (Hoshi et al., 1994). What emerges is that physical and mental demands of jobs are not only a challenge to the oxygenation of the muscle, but also of the brain. 3. NIRS as a functional tool in the follow up of muscle function during physical work The relative importance of oxygen supply to the muscles and oxygen utilization in muscle tissue is still the subject of debate in graded maximal exercise (Richardson et al., 1999). Also, oxygen consumption of the working muscles increases in close relation to an increase in blood flow during submaximal exercise at constant load (Vøllestad et al., 1990) suggesting that the oxygen supplyconsumption balance is not only intensity dependent but also time dependent. In other words, it emerges excess in VO2 uptake (i.e., a VO2 slow component) leading to early muscle fatigue (Bringard and Perrey, 2004). Workloads where there is no sustained metabolic acidosis can usually be sustained for long periods with a modest effort. Most activities in industry settings are full of challenges in which the energy demands of the working muscles might quickly go from rest to moderate to heavy loading. In an unsteady-state, the rate at which skeletal muscle oxidative metabolism adjusts to a new metabolic requirement is one of the factors that determines physical work tolerance due to oxygen deficit (Whipp et al., 2005). Knowledge of this rate will then provide a framework to explore the consequences of impaired muscle function. A physiological steady state is usually attained within about 3 min in young healthy adults, but sooner in those of high aerobic fitness and considerably longer with cardio-pulmonary dysfunction. The on-transient VO2
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response is multifactorial, and relies upon the capacity within the working muscle to rapidly change the delivery and utilization of oxygen. While the determination of VO2 kinetics is valuable for the functional evaluation of muscle oxidative metabolism (Whipp et al., 2005), the monitoring with high-time resolution of oxygen utilization through the use of NIRS within the working muscle may help to identify peripheral limitations to this metabolic adaptation (Fig. 3). NIRS-derived signals determine muscular VO2 kinetics during exercise where metabolic condition changes diversely (DeLorey et al., 2003; Grassi et al., 2003; Volianitis et al., 2003). By assessing the pulmonary gas exchange and the deoxygenation state of the working muscles by NIRS during task (Fig. 3), the factors (constant time of on-transient VO2 and NIRS response and amplitude of the slow component) that determine exercise tolerance can be assessed in workers. In industry, there are a variety of manual materials handling activities, such as pushing, pulling, lifting, carrying, reaching and assembling. This type of physical work with a monotonous or a repetitive muscle activity pattern over time is associated with an increased risk of upper extremity and low-back disorders (Hagberg and Wegman, 1987). Studying the physical work in a controlled laboratory environment (Kell and Bhambhani, 2003) helps to identify the high-risk activities and conditions that can lead to injury. For example, performance during an incremental test to failure with a controlled form of exercise (e.g., lifting technique in Kell and Bhambhani, 2003) or a submaximal exercise test (Maronitis et al., 2000) with NIRS monitoring can be utilized to evaluate muscle oxidative function in preventing muscle fatigue and thus assisting injury prevention. This information can be used to set limits and redesign jobs to increase safety (Waters et al., 1993). Evidence based on the use of NIRS has also shown that the endurance of lower-back muscles was closely linked to the availability of oxygen (Yoshitake et al., 2001), although decrease in oxygen due to elevated intramuscular pressure was demonstrated only at moderate to higher levels of muscle contraction. Interestingly, Yang et al. (2007) examined the status of muscle oxygenation in the low back over an entire workday. These authors demonstrated that the requirement of oxygen for the low-back muscles in a typical job associated with lifting increased over time. Yang et al. (2007) also reported that participants who had no experience in lifting demonstrated an increase in muscle tension and needed more muscle oxygenation than the experienced lifters.
Fig. 3. Graph shows an example of simultaneous measurements of whole-body oxygen uptake (VO2) and back muscle oxygenation with near-infrared spectroscopy during constant workload arm exercise in supine position (with a swimming simulator device) performed at a high relative load intensities This graph demonstrates that cardiorespiratory and localized muscle oxygenation do not attain a steady state after about 3 min of exercise: VO2 continues to increase (the so-called VO2 slow component) while tissue oxygenation index (TOI) over the back muscle decreases. Back muscles are likely to play some limitation to the overall metabolic adaptation characterized with VO2 measurement. Thus, muscle tolerance of the individual appears low and muscle fatigue will appear quickly.
4. Conclusion The ability to obtain reliable quantitative measurements of muscle oxygen consumption is necessary to understand the mechanisms of local muscle metabolism at rest, during and after exercise. The concurrent measurement of both pulmonary oxygen uptake and muscle oxygenation helps to provide measures of both systemic and peripheral responses to exercise and work. NIRS is proving to be an effective method for the monitoring of muscle metabolism and brain haemodynamics during performance of job activities, which will help in understanding tissue physiology and associated disorders. In an ongoing effort to help safety practitioners match physical work demands to worker capabilities, NIRSderived measurements are encouraged in the workplace.
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