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1 Liberty Mutual Research Institute for Safety, 71 Frankland Road, Hopkinton, MA 01748, USA 2 Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, T6G 2G4, Canada
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Abstract |
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(Received 21 February 2006;
accepted after revision 25 May 2006; first published online 1 June 2006)
Corresponding author R. V. Maikala: Liberty Mutual Research Institute for Safety, 71 Frankland Road, Hopkinton, MA 01748, USA. Email: rammohan.maikala{at}libertymutual.com
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Introduction |
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Near-infrared spectroscopy (NIRS) is a non-invasive physiological monitoring technique that measures relative changes in oxygenation and blood volume levels in skeletal muscle continuously in real time during rest and exercise (Chance et al. 1992; Mancini et al. 1994; Ferrari et al. 1997). Muscle oxygenation, defined as the relative saturation of oxyhaemoglobin and oxymyoglobin, depends on the balance between oxygen delivery and oxygen utilization (Mancini et al. 1994). Under conditions with a constant haematocrit, the change in total haemoglobin concentration is proportional to the change in the muscle blood volume within the illuminated field (Piantadosi, 1993).
Validity of the NIRS technique on the skeletal muscle has been previously reported (Mancini et al. 1994). Although measurement of erector spinae oxygenation and blood volume changes using NIRS has been limited to submaximal and maximal isometric contractions during various activities (Jensen et al. 1999; McGill et al. 2000; Albert et al. 2004; Kankaanpää et al. 2005), there are no published data on NIRS responses in the lumbar muscles during WBV. Based on electromyographic studies of the paraspinal muscles in the lower back, it was hypothesized that localized fatigue during WBV could be due to compromised blood flow, resulting in reduced oxygenation and availability of nutrients to these muscles (Pope et al. 1990; Hansson et al. 1991; Zimmerman et al. 1993). However, this hypothesis is yet to be tested objectively.
Thus, the purposes of the present study were to examine in healthy men: (1) the effects of 3, 4.5 and 6 Hz vibration on the relative changes in oxygenation and blood volume in the lumbar erector spinae muscle in a seated posture; (2) the differences in these physiological changes ‘with’ and ‘without’ a backrest support; and (3) alterations in these responses with the addition of rhythmic hand grip contractions during WBV. It was hypothesized that compared to sitting without WBV, erector spinae oxygenation and blood volume responses would decrease during WBV, with the greatest decrease occurring without the backrest. Additionally, compared to WBV alone, NIRS responses would decrease further while performing rhythmic hand grip contraction during WBV.
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Methods |
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Written informed consent that conformed with the Declaration of Helsinki was obtained from 13 right-hand dominant healthy men (mean ± S.D: age, 24.7 ± 3.9 years; mass, 70.8 ± 12.1 kg; height, 1.71 ± 0.07 m; right-hand grip strength, 34.2 ± 9.1 kg). Participants were students recruited from the university population. The experimental protocol was approved by the human ethics review board at the University of Alberta.
Vibration simulator
For this study, a steel vibrating base (Advanced Therapy Products, Inc., Glen Allen, VA, USA) that housed an electromechanical motor connected to a cam mechanism was modified with an external DC speed control device (Fig. 1A). Varying the speed of the motor from 0 to 100% corresponded to vibrating frequencies of 0–6.2 Hz as verified with an oscilloscope (Hitachi digital oscilloscope VC-6050, Japan). The vibrating base was attached with a displacement transducer (Intertechnology, Inc., Don Mills, ON, Canada) to measure the linear displacement of the cam. A Work SeatTM (model WS-40, Advanced Therapy Products, Inc.), with an internal spring-suspension system and a weight-adjustable knob, was bolted to the base assembly. The material for the seat and backrest was of molded vinyl. The seat and backrest configuration resemble the seating system of a small tractor. The adjustable seat could accommodate a maximum sitting weight capacity of 300 lbs. The backrest was adjustable to three seating positions: upright, 3 and 6 deg. The height of the backrest measured from the top of the seat was 44.5 cm. The width of the backrest (measured at the front) was 48.26 cm. The lumbar area measured at the concave region (called ‘dish depth’ by the manufacturer) was 6.67 cm.
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0.9gr.m.s. in the vertical direction. A second accelerometer (Crossbow, Inc.) was attached on the subject's neck at the level of the sixth cervical vertebra (C6). Protocols
Each participant completed an arm cranking exercise test on the first day to determine aerobic capacity, followed by random exposure to vibration frequencies 3, 4.5 and 6 Hz on three separate days.
Assessment of the peak oxygen uptake
As evidenced in the literature (Hulshof & Zanten, 1987; Boff & Lincoln, 1988; Griffin, 1990; Bovenzi, 1996), workers exposed to WBV often spend their workday not only in a prolonged sitting posture, but also simultaneously operating manual controls requiring considerable effort from their upper body. Thus, an upper body exercise was chosen for participants' baseline measurements of aerobic fitness. On their first day, the participants completed a fitness test on an arm cranking ergometer (MET 300, Cybex, RonKonKoma, NY, USA). The test was initiated with a 2 min rest period, followed by 2 min of arm cranking at no load and subsequent increments of 25 W every 2 min until voluntary exhaustion, or attainment of two or more of the following termination points: (1) levelling off in the oxygen consumption (increase of 
100 ml min–1) with increasing load; (2) age-predicted maximal heart rate, equated to 220 minus age (in years); (3) respiratory exchange ratio of 
1.10; and (4) rate of perceived exertion 18 (Borg, 1982). Cardiopulmonary variables were monitored using an automated metabolic measurement cart (Sensormedics 2900, Yorba Linda, CA, USA) and a wireless heart rate monitor (Sport Tester, Model 3000, Kempele, Finland). Subjects' peak oxygen uptake values obtained were 24.5 ± 6.6 ml kg–1 min–1.
Vibration tests
Each day subjects were exposed to a different WBV frequency, and before exposure the subject was randomly assigned the backrest condition (‘with’ or ‘without’) and was instructed to adopt a comfortable posture. The subject sat in the vibrating chair with the eyes open, and with the feet placed comfortably on a footrest (Fig. 1B). Before the vibration session, right-hand grip size was adjusted on the dynamometer (JAMAR, Clifton, NJ, USA) to a position that was comfortable for each subject. During sitting ‘with’ and ‘without’ the backrest support, posture (in degrees) at the knee and hip joints was measured by a goniometer (JAMAR).
The test protocol on each day included 6 min of rest without WBV, 8 min of WBV exposure either ‘with’ or ‘without’ backrest condition, 4 min of recovery without WBV, followed by 8 min of WBV with the ‘opposite’ backrest condition, and 4 min of final recovery without WBV. During the fifth minute of each WBV backrest condition, the subject performed rhythmic maximal hand grip contractions on the dynamometer for 1 min at a rate of one every 5 s. Before the start of each test session, the dynamometer handle was adjusted to match the subject's hand size. Subjects were instructed to match their pretest maximal contraction, and each contraction was maintained for at least 2–3 s. The measuring dial is equipped with a needle that retains the reading for the highest level achieved until it is reset. Contractions were performed in the sagittal plane with the elbow fully extended and the arm in a horizontal position (Fig. 1C). Details of the other physiological responses (Fig. 1D) measured during seated WBV have been reported elsewhere (cerebral measurements: Maikala et al. 2005; and cardiovascular measurements: Maikala et al. 2006).
Oxygenation and blood volume measurements
A continuous dual-wave NIRS unit (MicroRunman, NIM Inc., Phila, PA, USA) was used to evaluate relative changes in the lumbar erector spinae oxygenation and blood volume during the vibration tests. This unit consists of: a superficial near-infrared sensor fitted with six light-emitting tungsten lamps placed 2, 3 and 4 cm apart; two photodiode detectors that absorb light in the near-infrared range between 760 and 850 nm; and a display unit that amplifies and displays the absorbency signal generated. The sensor was placed on the erector spinae muscle 3 cm from the mid-line of the spine to the right side in the region the third lumbar vertebra (Maikala et al. 2000). This position of the NIRS sensor was noted to the nearest millimetre and identified with a marker, thus ensuring its correct placement on each subject for all the testing sessions. A piece of clear plastic was wrapped around the sensor to prevent sweat from distorting the absorbency signal. A dark elastic bandage was wrapped around the lumbar region to secure the sensor in place. Erector spinae muscle skinfold thickness at the sensor location was measured twice using a skinfold caliper (Cambridge Scientific Industries, Inc.; Cambridge, MD, USA) before placing the sensor over the muscle belly. Then adipose tissue thickness (the sum of fat and skin layer thicknesses) at the sensor site was calculated as one-half of the skinfold thickness (Van Beekevelt et al. 2001).
The sensor was calibrated using the NIRCOM software (MicroRunman, NIM Inc.). The vertical penetration depth of the light was set to 4 cm, the maximum depth allowed by the manufacturer. Both oxygenation and blood volume were measured in optical density (OD) units, which was defined as the logarithmic ratio of intensity of light (I) calibrated at each wavelength (760 and 850 nm) to the intensity of measured light at the same wavelength. Real-time measurements were recorded on-line at 1 Hz.
The following equations were used (MicroRunman User Manual, NIM Inc., Phila, PA, USA, 1999) to calculate localized oxygenation and blood volume changes:
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Data analysis of oxygenation and blood volume
The real-time NIRS values were averaged every 20 s using a customized Microsoft ExcelTM software program especially designed for this study. Oxygenation and blood volume responses before the start of the WBV session and after the first WBV exposure (Rest and Recovery 1; Fig. 2) were identified as ‘baseline’ values. ‘Minimum’ values were identified during each WBV session of ‘with’ and ‘without’ backrest support for the two conditions of WBV only and WBV combined with hand grip contractions. Then, for each participant, the ‘physiological change’ in oxygenation and blood volume for each of these backrest and workload conditions was calculated as the difference between the ‘baseline’ and the ‘minimum’. A typical example of calculating the ‘physiological change’ is shown in Fig. 2. Statistical analyses were performed on the ‘physiological change’ values calculated for each WBV condition.
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A three-way analysis of covariance, using the peak oxygen uptake as the covariate, and three repeated measures factors (frequency [3, 4.5 and 6 Hz], backrest [‘with’ and ‘without’] and workload [WBV only and WBV combined with maximal hand grip contractions]) with a fully crossed design was used to evaluate the differences in muscle oxygenation and blood volume responses. Peak oxygen uptake obtained from the arm cranking was used to examine whether the aerobic fitness level of the subjects had any influence on the localized oxygenation and blood volume responses in the lumbar muscles. Three within-subject factors (frequency, backrest support and posture [hip versus knee]) as repeated measures were used to evaluate the postural differences at the knee and hip joints during sitting. Also, two within-subject factors (frequency and backrest support) as repeated measures were used to evaluate the differences in acceleration at the cervical region. To minimize the violation of the assumption of homogeneity of variance, the ‘Greenhouse-Geisser’ adjustment was selected (Stevens, 1996). An 
level of P
0.05 was considered statistically significant. Subsequent post hoc tests were performed using the Bonferroni analysis.
The relationships between selected variables (posture measured at the hip joint and head acceleration measured from the sixth cervical region) and ‘physiological change’ in oxygenation and blood volume responses were evaluated using Pearson product moment correlations. A Spearman correlation test was used to examine the relationship between logarithmic value of adipose tissue thickness and NIRS responses for the three vibration tests. Since the adipose tissue thickness of the subjects in the present study was not normally distributed, a non-parametric correlation test was used to examine its effect on the NIRS measurements. The Statistical Package for Social Sciences SPSS (version 10) was used for all statistical analyses (SPSS Inc., Chicago, IL, USA).
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Results |
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Effect of whole-body vibration frequency
No significant differences were observed in oxygenation and blood volume changes for all frequencies (3 Hz, 0.029 ± 0.20 OD; 4.5 Hz, 0.026 ± 0.02 OD; and 6 Hz, 0.030 ± 0.03 OD, all n.s.). Corresponding comparisons for the blood volume were not significantly different as well (3 Hz, 0.033 ± 0.03 OD; 4.5 Hz, 0.049 ± 0.04 OD; and 6 Hz, 0.045 ± 0.02 OD; all n.s.).
Effect of backrest support
Sitting ‘without’ backrest resulted in further reduction in oxygenation and blood volume changes versus sitting ‘with’ backrest support (oxygenation, 0.024 ± 0.01 versus 0.032 ± 0.02 OD, P = 0.02; and blood volume, 0.039 ± 0.02 versus 0.045 ± 0.03 OD, P = 0.05).
Effect of hand grip exercise
Rhythmic hand grip contractions during WBV resulted in a greater reduction in NIRS responses compared to the WBV-only condition (oxygenation, 0.025 ± 0.02 versus 0.032 ± 0.02 OD, P = 0.003; and blood volume, 0.039 ± 0.03 versus 0.045 ± 0.03 OD, P = 0.04).
Correlations between selected variables and oxygenation and blood volume changes
Since angular measurements at the knee joint were not significantly different ‘with’ and ‘without’ backrest support (Table 2), correlations were calculated for hip joint and oxygenation and blood volume responses only. Interestingly, only at 4.5 Hz was there a significant relationship observed between hip angle and oxygenation responses (Table 3). The relationships between acceleration measured at the sixth cervical vertebra and oxygenation and blood volume responses were not significant (Table 4). Furthermore, Pearson correlations obtained between acceleration measured from the cervical region and hip-joint angles during sitting ‘with’ backrest were also not significant, suggesting that greater accelerations reported in the ‘with’ backrest condition were not influenced by the greater hip angles (Table 5). Adipose tissue thickness calculated from erector spinae skinfolds was 5.7 ± 2.5 mm (mean ± S.D.). Spearman correlations between adipose tissue thickness and mean changes in oxygenation and blood volume data are shown in Table 6. The only correlations obtained were for the blood volume changes at 3 Hz during the WBV-only condition.
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Discussion |
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Using the photon diffusion principle, NIRS can detect changes in oxygenation and blood volume of skeletal muscle during any state (Chance et al. 1992; Ferrari et al. 1997). It is important to note that NIRS signals are treated primarily as a representative of capillary and venous haemoglobin saturation (Mancini et al. 1994), and that NIRS does not measure blood flow. Since NIRS monitors oxygen saturation at the levels of smaller blood vessels, a decrease in oxygenation and blood volume trends during WBV (with a concomitant and simultaneous increase in whole-body oxygen uptake [Maikala et al. 2006] and cerebral responses [Maikala et al. 2005]) confirmed deficiency in the vascular supply to the lower back muscles during WBV. An imbalance between oxygen supply and uptake may lead to a greater deoxygenation in the muscle, reducing muscle oxygen saturation. Such a decrease in oxygenation during WBV may also result from an increase in intramuscular pressure above perfusion pressure (Rundell et al. 1997), thus limiting blood flow to the working muscle. Yoshitake et al. (2001) hypothesized that a decrease in both oxygenation and blood volume responses observed during maximal back extension in a prone position might be due to ischaemic muscle activity in the lower back region. Few mechanisms for such a decrease in oxygenation were hypothesized (McGill et al. 2000): compromised blood flow in these smaller blood vessels (owing to intramuscluar pressure exceeding intravascular pressure) thus reducing blood volume; reduced perfusion and an increase in oxygen extraction from the muscle; or decrease in skin blood flow. However, the NIRS signal primarily monitors muscle rather than skin oxygenation (Mancini et al. 1994). During recovery (Fig. 2), blood supply to these paraspinal muscles returned towards the baseline, demonstrating an increase in muscle blood volume and reoxygenation.
Effect of whole-body vibration frequency
Although subjects demonstrated frequency-dependent NIRS responses, with the greatest decrease of oxygenation and blood volume observed at the spinal resonance frequency of 4.5 Hz, the results were not influenced by the change in WBV frequency (Table 1). In the same subjects, when comparing localized lumbar muscle responses with whole-body physiological responses (Maikala et al. 2006), neither pulmonary oxygen uptake (in l min–1) nor heart rate (in beats min–1) were influenced by the vibration frequency: oxygen uptake, 0.323 ± 0.15 (3 Hz) versus 0.350 ± 0.14 (4.5 Hz) versus 0.345 ± 0.140 (6 Hz); heart rate, 86 ± 14 (3 Hz) versus 90 ± 14 (4.5 Hz) versus 88 ± 12 (6 Hz). However, these changes in whole-body physiological responses observed during WBV resemble light physical work.
Using NIRS, Yamada et al. (2005) found decreased oxygenation levels in the vastus lateralis muscle during squatting exercise on a vibration platform. Hansson et al. (1991) hypothesized that exposure to WBV would result in a reduced arterial supply to the back muscles. At the spinal resonance frequency of 4.5 Hz and during sitting for a period of 5 min with and without WBV, these authors demonstrated that vibration increases both the speed and the magnitude of the development of muscle fatigue. However, their results were based on electromyographic findings. Moderate correlations between the decrease in oxygenation (determined by NIRS) and median frequency (determined by electromyography) for the lumbar muscle were previously reported during few exercise modes (Maronitis et al. 2000; Yoshitake et al. 2001; Albert et al. 2004). Since a similar reduction in both blood volume and oxygenation in the erector spinae muscles was evident in the present study, one can speculate that NIRS can be a useful monitoring technique for understanding lumbar muscle physiology when combined with other physiological modalities (e.g. Maikala et al. 2005, 2006).
Effect of backrest support
With backrest support, subjects tend to transfer most of their body weight to the backrest, thus reducing load on the lower back caused by the upper body weight (Andersson et al. 1991). However, such a posture might also transmit greater vibration to the head. Griffin (1990) and Paddan & Griffin (1988a,b) reported that transmission of vertical vibration to seated subjects depends on the vibration of the backrest. In contrast, Magnusson et al. (1992) found that use of a backrest and its inclination had minor effects on the attenuations of WBV, and concluded that a backrest does not necessarily reduce the effect of vibration on spinal loading. These authors did not report the acceleration values of the backrest studied. Although acceleration of the backrest attached to the vibration shaker was not measured in the present investigation, presence of a backrest played a small but significant role in acceleration measured from the cervical region (‘with’ backrest values being 2% higher than ‘without’ backrest). Based on the inconsistent correlation values reported in Table 4, postural adjustment might not have resulted in greater transmission of vibration to the head. This also holds true for ‘physiological change’ in oxygenation and blood volume responses.
In the present investigation, acceleration measured from the cervical region was significantly highest at 4.5 Hz. Skin-mounted placement of the accelerometer and the anatomical region selected in the present study might have contributed to this discrepancy. Pope et al. (1986) reported a significant artifact resulting from surface- or skin-mounted accelerometers, whereas Magnusson et al. (1992) recommended a pin-mounted method for the best results. Furthermore, it was demonstrated that optimal measurement of acceleration and transmissibility at the head can be obtained by inserting a bite-bar accelerometer between the teeth of participants (Griffin, 1990; Pope et al. 1989). Since we collected whole-body physiological responses using a mouthpiece (Maikala et al. 2006) and cerebral NIRS responses from the forehead (Maikala et al. 2005) simultaneously, we approximated subjects' head acceleration by placing an accelerometer at the cervical region.
On the same subjects of the present study, Maikala et al. (2006) demonstrated greater pulmonary oxygen uptake responses (by 11%) and heart rate (by 5.5%) during sitting ‘without’ backrest compared to the ‘with’ backrest condition. Similarly, during sitting ‘without’ backrest, ‘physiological change’ in oxygenation and blood volume amounted to a 27 and 11% decrease, respectively, compared to values obtained sitting ‘with’ backrest (Table 1). This suggests that subjects sitting ‘without’ a backrest experience greater energy expenditure but a simultaneous decrease in localized NIRS responses in the lumbar muscles. During a simulated truck driving, upright sitting caused more fatigue than any other posture (Wilder et al. 1994). Thus, such a great decrease in NIRS responses during sitting ‘without’ backrest suggests that oxygen availability and utilization were not equally balanced in this posture. During sitting ‘without’ a backrest, it has been speculated that sustained isometric contraction of the paravertebral muscles in maintaining an erect posture for a prolonged period will slow down the pumping mechanism, therefore disturbing the nutritional diffusion (Junghanns, 1990). This would decrease the blood flow, resulting in deprivation of oxygenation availability to these postural muscles (Fig. 2).
Masuda et al. (2005) demonstrated the influence of posture on erector spinae muscle thickness, tissue oxygenation and blood volume. However, studies pertaining to changes in lumbar muscle oxygenation and blood volume in postural variations during WBV have not been reported to date. Thus, it might be reasonable to compare the present posture-related findings with other NIRS studies on posture. Maikala et al. (2000) showed a decrease in both oxygenation and blood volume in the right side of lumbar erector spinae muscles during maximal extension of the back in sitting and standing positions. These authors reported significant intraclass correlation coefficients of 0.83 and 0.84 for minimum oxygenation during maximal back extension in sitting and standing, respectively, and 0.99 for the minimum blood volume during both postures. Szmedra et al. (2001) demonstrated a greater decrease in oxygenation and blood volume in the vastus lateralis muscles when skiers adopted a lower posture during the giant slalom technique compared to the slalom skiing technique. These authors attributed such a phenomenon in the muscles to a greater static load imposed on the posture during the giant slalom, but only secondary to a higher percentage of maximal voluntary muscle contraction experienced.
Based on the posture-related electromyography studies, several researchers have hypothesized that lower back muscle fatigue might result from the prolonged cyclic firing of paraspinal muscles during WBV (Wilder et al. 1982; Seroussi et al. 1989; Zimmerman et al. 1993; Pope et al. 1998). According to Seroussi et al. (1989), fatigue observed in the lumbar erector spinae muscles after exposure to WBV may affect the load-bearing capacity of the trunk muscles. Greater cyclic compression of the spine was observed during sitting in an anterior lean posture (Zimmerman et al. 1993). Prolonged exposure to this position, in addition to cyclic compression, may result in decreased nutrient diffusion, leading to earlier onset of erector spinae muscle fatigue (Zimmerman et al. 1993). It is important to note that for registering erector spinae activity, most of these electromyography studies adopted a sitting posture in a slightly flexed position or loaded subjects with a small weight. However, since NIRS can successfully identify oxygenation and blood volume changes in any postural variation (Ferrari et al. 1997; Rundell et al. 1997; Maikala et al. 2000; Szmedra et al. 2001), we believed it was not necessary to load the subjects with any weights in the present study.
Effect of hand grip exercise
According to Asmussen (1981), rhythmic hand grip contractions (similar to the protocol employed in the present study) can be characterized as ‘dynamic work’ because short-lasting muscle contractions are interspersed with relaxation phases. During rhythmic activity, the possibility of an increase in venous return is much greater because muscular activity during relaxation phase returns blood to the heart and such phenomena might further delay the absence of the active muscle pump (Asmussen, 1981). Thus, physiological responses during rhythmic hand grip contractions may be less than those obtained during typical static work. However, during seated WBV, the muscle pump in the legs may be inactive, resulting in pooling of blood, thereby decreasing venous return. When subjects performed maximal rhythmic hand grip contractions during WBV, Maikala et al. (2006) demonstrated that pulmonary oxygen uptake and heart rate increased significantly, by 36 and 17%, respectively, from the WBV-only condition. At the same time, during hand grip contractions, the same subjects exhibited a greater reduction in oxygenation (by 22%) and blood volume (by 13%) compared to the WBV-only condition, suggesting a significant decrease in lumbar muscle haemodynamics when subjects performed physical activity during exposure to WBV.
Andersson et al. (1974) observed an increase in myoelectric activity in the lumbar muscles when subjects changed vehicle gears during driving. In race-car driving, shifting gears with excessive force is common, and tight grip in combination with slamming from one gear to another observed in this profession will not only destroy the mechanisms of vehicle dynamics but also influences human performance. To replicate the operation of hand controls, we chose hand grip contractions as a simple work simulation for operators handling various controls during driving. In the present study, contractions were performed at the rate of one every 5 s. During pilot studies, the maximal effort with rest in between contractions did not result in muscle fatigue. The rationale for continuing the intermittent hand grip exercise for a period of 1 min was to prevent the problem of breath holding.
Figure 2 shows evidence that oxygenation and blood volume responses decreased from baseline values during the WBV-only condition, and continued to decline during vibration combined with work. This reduction in blood volume and oxygenation during maximal rhythmic hand grip contractions (Fig. 2) also suggests that muscle coactivation is possible, even at the erector spinae level, although the primary muscles involved during this hand grip activity were the forearm muscles only (Kahn et al. 2000). Interestingly, in the present study, some of the subjects did not show any steep decrease and a few demonstrated an increase in blood volume changes during WBV combined with rhythmic hand grip contractions. This may result from diffusion limitation between the capillaries and the monitored erector spinae muscle. Boushel et al. (1998) suggested a similar hypothesis in upper extremity muscles during rhythmic hand grip exercise. These authors speculated that this unchanged value might result from a contribution of blood from less active tissues and impeded flow owing to the mechanical forces generated during muscle contraction. Overall, based on the NIRS findings, the present study demonstrated that concomitant exposure to activities such as hand grip work during WBV in addition to the ‘postural load’ experienced owing to sitting for a prolonged period of time decreases oxygenation and blood volume responses, further burdening the activity of lower back muscles.
However, an important question arises. How are these decreases in oxygenation and blood volume in the lumbar muscles related to natural situations where workers are often exposed to WBV and other concomitant variables such as hand control work? To our knowledge, there is no normative data of ‘physiological change’ in NIRS responses of WBV populations, which makes it difficult to relate our findings in an objective way. For example, we used a maximal hand grip exercise to derive NIRS responses during WBV. Based on Fig. 2, during sitting ‘with’ backrest support, the subject demonstrated 0.062 OD while performing hand grip exercise. However, during sitting ‘without’ backrest support, the same subject demonstrated 0.095 OD while performing hand grip work. This simple result suggests that the use of a backrest should be studied with the aim of decreasing the load on the lumbar muscles, and demonstrates the importance of understanding the characteristics of the backrest prior to designing the seat–backrest interface for any vehicle. These NIRS results, in combination with acceleration values, can also be extrapolated to provide an understanding of the transmissibility of vibration, at the source (seat–base interface) or through a backrest, thus reducing exposure to vibration. More importantly, by comparing exercise-related localized NIRS responses with the WBV-only condition, it should be possible to design and develop guidelines for hand controls that might require less (submaximal) effort than the one used in the present study. Furthermore, we used an exercise protocol performed at the shoulder level. However, by simulating hand grip work within the workspace envelope of a typical operator, it is possible to study functional responses in the back muscles and develop ergonomic workstations, thus enhancing performance and safety of the exposed population.
Influence of adipose tissue thickness on NIRS measurements
Van Beekvelt et al. (2001) reported that adipose tissue metabolism is lower than muscle metabolism; therefore, the muscle oxygenation determined using NIRS might be underestimated. During cuff ischaemia, McCully & Hamaoka (2000) demonstrated a greater change in the physiological range in subjects with less subcutaneous fat compared to those with more subcutaneous fat. In the present study, adipose tissue thickness was not significantly correlated with oxygenation measurements (at a source–detector separation of 4 cm) among the three vibration tests (Table 6). This suggests that at the same source–detector separation of 4 cm, oxygenation values obtained during the different experimental conditions were not influenced by adipose tissue thickness. This observation is in contrast to the findings of van Beekvelt et al. (2001), who demonstrated a significant decrease in oxygenation with increases in adipose tissue thickness. Interestingly, in the present study, negative correlations were obtained for blood volume responses only (Table 6) during sitting ‘with’ backrest support (at 3 Hz). It is not clear, however, why adipose tissue thickness influenced only blood volume responses at 3 Hz and not at higher frequencies, and also during sitting ‘with’ and not ‘without’ backrest support.
Based on the NIRS measurements from the vastus lateralis muscle, Feng et al. (2001) identified an optimal source–detector distance of 35–40 mm for an adipose tissue thickness range of 2.5–5 mm; 45 mm for a thickness of 8 mm; and 50 mm for a thickness of 17 mm. In the present study, however, a fixed source–detector distance of 4 cm, the maximum separation allowable by the MicroRunman Model, was used (irrespective of subjects' skinfold thickness, which ranged from 7 to 24 mm). It is possible that this fixed distance could have influenced the present NIRS findings. Using an ultrasound imaging technique, Jensen et al. (1999) measured the distance from the skin surface to the lumbar erector spinae muscle at the NIRS sensor site, and reported a distance of 9.0 ± 0.3 mm (at the cranial aspect) and 9.3 ± 0.5 mm (at distal aspect), respectively. Thus, measurements obtained with the existing NIRS set-up (e.g. source–detector separation, photon penetration depth) in the present investigation are well within the range for the lumbar muscle. However, other discrepancies, such as NIRS technique (e.g. continuous wave versus spatially resolved), type of muscle and its location tested, exercise protocol investigated (static versus dynamic), and sample size might influence the NIRS measurements.
Influence of aerobic fitness
Boff & Lincoln (1988) and Griffin (1990) suggested that physiological responses to WBV may be influenced by the level of fitness. However, these authors did not mention typical fitness values of occupational groups exposed to WBV. In terms of peak oxygen uptake responses, subjects in the present study ranged between a minimum of 855 ml min–1 to a maximum of 2929 ml min–1, with a mean of 1736 ml min–1. This mean value falls within the oxygen uptake values summarized for arm cranking by Sawka (1986). Karlqvist et al. (2003) defined ‘excess of metabolic level’ on a typical working day as metabolic demands exceeding 33% of the individual's aerobic capacity. Also, the National Institute (USA) for Occupational Safety and Health proposed that, in order to avoid undue fatigue, the time-weighted average oxygen uptake during an 8 h work day should not exceed 33% of the individual's maximal aerobic capacity obtained on a standardized protocol (Waters et al. 1993). Based on the low pulmonary oxygen uptake and heart rate values obtained for three different WBV exposures, we demonstrated that subjects in the present study could easily sustain energy expenditures during WBV-related tasks throughout a typical work shift.
Using the peak oxygen uptake obtained from an incremental arm cranking test until exhaustion as a covariate, the results suggested that erector spinae oxygenation and blood volume responses in men were not dependent upon the level of their aerobic fitness (P > 0.05). This is in contradiction to the hypotheses of previous literature (Boff & Lincoln, 1988; Griffin, 1990). If participants had been tested for their static or dynamic lower back muscle endurance (Albert et al. 2004; Kankaanpää et al. 2005), rather than aerobic fitness only, and if this muscle-specific endurance variable had been related to localized physiological responses in the lumbar muscles during WBV, an influence of lower back muscular fitness would most probably have been evident in the participants.
Study limitations
In the present study, owing to the type of vibrating apparatus used, displacements corresponding to different frequencies were observed. This set-up resulted in different acceleration magnitudes as well. However, these accelerations were averaged to obtain a mean value of 0.9gr.m.s.. Hence, the results of this study are not intended to be compared directly with the International Organization for Standardization (1997) of evaluation of human exposure to WBV. Although the influence of backrest vibration and characteristics of the seat were not investigated in the present study, these variables also would have played a significant role in the physiological responses observed. The appropriateness of a maximal hand grip exercise during WBV can also be questioned. Since there is no literature on either maximal or submaximal hand effort on the back muscles during WBV, we chose a maximal exercise for the present study. With the results from the present study, one can hypothesize that if work is performed at submaximal levels during WBV, lumbar muscles, depending on the activity, might demonstrate less desaturation compared to the values obtained during maximal performance values.
Regarding the NIRS observations, the physiological measurements obtained from the NIRS instrumentation have very large variances (Table 1). It is not clear whether the variances are inherent in the nature of NIRS measurements or due to new (and maybe relevant) haemodynamic behaviour not identifiable by the existing tissue oxygenation and blood volume measurements. Using a larger sample size might have reduced the large variances in the NIRS measurements. Since an optimal sample size for NIRS-related studies is not yet documented, calculation of the sample size for the present study was based on the cardiorespiratory measurements (Maikala et al. 2006) that were derived from Stevens (1996). Therefore, it is suggested that sample sizes for future studies which incorporate both cardiorespiratory and tissue haemodynamic measurements should be calculated on the basis of statistical power that can be obtained specifically with the NIRS variables of interest. Moreover, reference measurements of oxygen saturation for the lumbar muscles were not calculated in the present study. For any tissue under investigation, physiological calibration for reference measurements is typically achieved through ischaemic conditions (Chance et al. 1992; Boushel et al. 1998). Any subsequent changes in tissue oxygenation observed during a physical activity can be scaled to the overall difference that is observed during and following the ischaemic conditions. Since occlusion of blood flow to the lumbar muscles for physiological calibration is difficult, for each frequency of exposure, we calculated the ‘physiological change’ as the difference between baseline and WBV conditions (Fig. 2). Thus, we suggest that one should explore a proper calibration approach to the lumbar muscles so that normalization of NIRS measurements can be fmade.
Conclusions
The effects of sitting ‘with’ and ‘without’ backrest and work performance in healthy men during exposure to three different WBV frequencies on lumbar erector spinae oxygenation and blood volume responses were investigated. A decrease in oxygenation and blood volume in the paraspinal muscles was observed during WBV compared to sitting without WBV. ‘Physiological change’ in oxygenation and blood volume values were influenced by the backrest, demonstrating the importance of backrest support. WBV combined with hand grip contractions resulted in a further reduction in NIRS responses compared to the WBV-only condition, suggesting that greater deprivation of oxygen occurs in the lumbar muscles during work. The influence of skinfold thickness on NIRS measurements was inconclusive. Furthermore, the level of aerobic fitness was not evident on the ‘physiological change’ in oxygenation and blood volume measurements.
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