Occupational postural activity and lower extremity discomfort: A review

International Journal of Industrial Ergonomics 40 (2010) 247–256

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Occupational postural activity and lower extremity discomfort: A review Christopher R. Reid*, Pamela McCauley Bush, Waldemar Karwowski, Samiullah K. Durrani Department of Industrial Engineering & Management Systems, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816-2993, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 September 2008 Received in revised form 31 December 2009 Accepted 14 January 2010 Available online 1 February 2010

This review paper provides an appraisal of literature regarding discomfort of the lower extremities of workers in occupational environments. The primary extrinsic occupationally-related causes of job discomfort reviewed in this study are based on individual joint position, whole or partial body posture, and occupational activity. Based on the reviewed literature, postural activity guidelines are proposed summarizing thresholds at which lower extremity discomfort has been reported. Industry relevance: A set of occupational postural activity guidelines is proposed allowing ergonomic and safety practitioners a clearer picture of their individual industry’s work environment’s association with lower extremity worker discomfort. Ó 2010 Elsevier B.V. All rights reserved.

Keywords: Lower extremity Postural activity Body discomfort Joint position Body posture Occupational activity

1. Introduction Several published studies suggest two types of adverse health outcomes for prolonged postural activity at work: (1) body discomfort and (2) work-related musculoskeletal disorders (WMSDs) (Boussenna et al., 1982; Kee and Karwowski, 2003; Kee and Karwowski, 2004; National Research Council, 2001). At the same time, it is evident that postural discomfort can also arise from confounders such as past injury, current health, psychosocial variables, and present diagnosis of WMSDs (Messing et al., 2006; National Research Council, 2001). Better understanding of the plausible cause-and-effect relationships with respect to perceived and reported body discomfort levels and occupational activities can contribute to the prevention and management of work-related musculoskeletal disorders (WMSDs). The above applies to all regions of the body, including the under-studied lower extremities (LE). This review focuses on the lower extremity joints and body segments (pelvis, hip, thigh, knee, lower leg, ankle, and foot) and the occupational variables that affect comfort in these areas. Research on studied lower extremities in occupational settings has taken a retrospective approach, i.e., the occurrence of a disorder results in steps to decipher a cause. At the same time, some studies focus prospectively on disorders that have a pattern in an industry such as the development of stress fractures in track and field

* Corresponding author. Tel./fax: þ1 407 574 2099. E-mail address: [email protected] (C.R. Reid). 0169-8141/$ – see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2010.01.003

athletes (Bennell et al., 1996). Another example is that of osteoarthritis prone workers over 50 years old in floorlaying and construction industries who are exposed to long duration kneeling (Jensen et al., 2000). Although studies such as these are necessary, they only help highlight the causes or risk factors that are involved in the multi-factorial development of a WMSD. Aside from the years required to normally conduct these studies properly, their results leave no form of mitigation strategy to create an awareness of risk for work tasks and activities. On the other hand, discomfort noticed as a result of joint positions and postural activities are immediately noticed and typically do not take long to gestate. Also, it is much easier and faster to quantify discomfort noted from occupational postural activity and develop guidelines as will be shown later in this review. Work procedure guidelines added to epidemiology, physiology, observation, and direct measure results will aid in forming risk models of possible occupational task hazards and their associated cumulative trauma disorders. Derivations of these methodologies have been demonstrated in validated ergonomic risk assessment tools for the lower back (Waters et al., 1993), and upper extremities (McAtamney and Corlett, 1993; Moore and Garg, 1995). Use of them in the design of risk tools that would minimize WMSDs for the LE should prove practical as well. Before any correlation taxonomies can be developed LE body discomfort must first be understood and defined. Body discomfort in terms of postures and activities, can be defined as having sensations of pain, soreness, stiffness, numbness, or tingling, to name a few (Helander and Zhang, 1997; Kee and Karwowski, 2003; Meyer and Radwin, 2007). These same studies reveal that many

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effects of discomfort are due to physiological reasons related to the body’s internal biomechanics and muscle fatigue capacities. These factors actively change as joints articulate, muscles contract, and internal body pressures regulate. Tasks involving prolonged exposure to working postures and or repeated activities are associated to body discomfort (Corlett and Bishop, 1976). Non-neutral joint angles (Van Wely, 1970) and extrinsic contact stress on LE body tissue from work environments (Chung et al., 2005) are also known to be uncomfortable for people if sustained. In addition, holding time of these postures is directly dependent on the perceived levels of discomfort, therefore the torques and fatigues induced through postures indirectly affects the time people can maintain them (postural loading) (Corlett and Bishop, 1976). This review refines the myriad of possible job/task related variables into three general categories: (1) joint positions, (2) body postures, and (3) task performance requirements (occupational activities). Each of these categories, although associated to body discomfort, progressively views the variables of causation at a higher level of association. Joint position looks at each LE joint individually and shows how body discomfort can relate to joint deviation. Body postures looks at the LE from the view of multiple joint positions acting together to attain a primarily static body posture. Body postures commonly studied are standing and chair sitting whereas awkward postures such as tiptoeing while standing, stooping, floor sitting, squatting, kneeling, imbalances, and lying down are less studied. The category of task performance (occupational activities) comprises the same joint positions and body postures from the previous two categories with the addition of dynamic movement. Common movements of this category noted to relate to the LE include walking, pushing, pulling, lifting, lowering, stacking, and stair/ramp climbing. Occupations directly affected by these postural activities may include supermarket cashiers (Ryan, 1989) or post office workers (Sobti et al., 1997) where long duration standing is common. Long duration driving resulting in cumulative exposure to sitting has also been associated with LE discomfort (Chen et al., 2004; McGlothlin, 1996). Finally, prolonged kneeling, squatting, and knee flexion in general have been correlated to LE discomfort for occupations involving farming tasks, manufacturing, and assembly of automobiles, ships, and aircraft (Bruchal, 1995; Lee and Chung, 1999). 2. Joint positions Joint motion is an essential part of human movement in normal daily living activities. Two investigations into body discomfort pertaining to individual joint movement (Genaidy and Karwowski, 1993; Kee and Karwowski, 2003) are reviewed below. Discomfort indicated by these investigations is related to general body discomfort rather than discomfort in any particular body location. Although all the joints of the body were investigated LE joints are the only ones evaluated. Static joint positions are held for 60 s during standing and chair sitting postures. Body discomfort ratings for joint positions are given using numerical values. Indication of increasing discomfort is coupled with higher numerical values. Genaidy and Karwowski’s (1993) research evaluated multiple degrees of freedom (DoF) for each of the LE’s three major joints. Six DoF are reviewed for the hip joint, one for the knee, and two for the ankle. Assessment of discomfort during chair sitting is only completed for the ankle joint’s dorsi and plantar flexion motions. Discomfort ratings are captured during maximum range of motion for each DoF. Table 1 indicates that Genaidy and Karwowski (1993) found hip abduction during standing to be the most uncomfortable hip motion. Hip flexion and hip extension are ranked second and third respectively. Hip adduction and ankle dorsiflexion are shown to be at similar discomfort ratings of 2. Due to chair sitting postures

Table 1 LE joint motion and accompanied body discomfort rating (Genaidy and Karwowski, 1993) LE joint

Joint motion

Body discomfort rating Chair sitting posture

Standing posture

Hip

Flexion Extension Adduction Abduction Internal rotation External rotation

– – – – – –

4 3 2 5 1 1

Knee

Flexion

–

–

Ankle

Dorsiflexion Plantar flexion

2 1

2 1

Higher discomfort is associated to larger numbers. –, Not collected by the study.

only evaluating two DoF for the ankle, dorsiflexion is considered the most uncomfortable. Kee and Karwowski (2003) performed a contrast and comparison study against that of Genaidy and Karwowski (1993). The methodology employed by Kee and Karwowski (2003) differs from the previous study in that more DoF are evaluated for the ankle joint and for chair sitting postures. Additionally, body discomfort is recorded at different ranges of deviation from neutral position for each DoF in five 25% increments starting from 0 (neutral). Their results, although differing from Genaidy and Karwowski’s (1993) joint motion discomfort rank, does concur that hip motions for standing postures are the most uncomfortable single joint position to hold (Table 2). In light of detail, hip adduction and external rotation are considered to be the highest discomfort ratings for standing. Hip flexion and external rotation are indicated as the two highest discomforts for chair sitting postures. There have also been studies conducted between genders. These studies assess joint body discomfort during standing postures for men (Kee and Karwowski, 2001) and women (Kee and Karwowski, 2004). Additionally, sitting posture is evaluated in Kee and Karwowski’s (2001) research, but only for males. Similar to the preceding studies, Kee and Karwowski (2001, 2004) captured discomfort ratings for joint deviations of 0%, 25%, 50%, 75% and maximum range for each DoF. The DoF observed for the LE joints included all of the aforementioned joint motions (Kee and Karwowski, 2003) except for hip internal/external rotation and knee flexion for chair sitting postures in the male study. These positions were also held static for 60 s.

Table 2 LE joint motion and accompanied body discomfort rating (Kee and Karwowski, 2003). Joint

Joint motion

Body discomfort rating Chair sitting posture

Standing posture

Hip

Flexion Extension Adduction Abduction Internal rotation External rotation

8 – – 4 5 8

4 5 8 5 5 8

Knee

Flexion

–

2

Ankle

Dorsiflexion Plantar flexion Adduction Abduction

3 3 3 3

3 3 3 3

Higher discomfort is associated to larger numbers. –, Not collected by the study.

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Of the body’s major joints, the hip is the most likely to exhibit discomfort in a sustained position (Kee and Karwowski, 2001). This is applicable to both standing and chair sitting postures. Results of the female study (Kee and Karwowski, 2004) show that similar to previous studies, hip joint deviations are the most uncomfortable joint motions to sustain statically. Evidence shows that regardless of gender, the hip seems to be more prone to postural discomfort for each of its DoF when static position duration is a minimum of 60 s (at least for standing postures). Additionally, females were found to be more resilient to discomfort compared to males, but only for joint rotational motions such as hip external rotation. Overall, Kee and Karwowski (2004) found that female joint discomfort levels were verified to be higher than their male counterparts by 13% (p < 0.01). This infers that tasks requiring joints to maintain high angles of deviation are likely to be more uncomfortable for women than for men. Interestingly, another study (Messing et al., 2008), reviewing the Quebec population of Canada (n ¼ 7757) noticed that their female population had a higher prevalence of lower extremity discomfort than did their male counterparts. Unfortunately, causes into these results from this particular study remained ambiguous to the investigators due to possible residual confounding of potential confounder variables such as past injury, type of floor, heat exposure, type of footwear, and pressing of foot pedals. Ironically, even though female discomfort may be high for the LE, evidence shows that women still have a greater LE joint range of motion (ROM) than do men (Chung and Wang, 2009; James and Parker, 1989). Chung and Wang’s (2009) article reveal that ROM for the ankle and hip in Taiwanese females show higher flexibility. This includes ankle dorsiflexion and plantar flexion, as well as hip extension, internal rotation and external rotation. Moreover, Kee and Karwowski (2004) mention that these same jobs, when joined with high strength requirements can quickly lead to worker body discomfort and may be indicative of future WMSDs. Therefore, the authors recommend that task developers take into account the female body’s capacities (joint deviations, strength requirements, and task exposure times) and be more aware of them when creating task procedures and job requirements that use both genders. Understanding joint deviation and its role in body discomfort is one critical step towards better awareness and prevention research in academia and industry. However, there are drawbacks. Kee and Karwowski (2001) note that, in reality, postures and activities are dynamic and incorporate multiple joint positions utilizing multiple DoF sporadically. Most of the time external loads or forces are added to situations at hand and that these situations may also include high repetitions or long static holds (>60 s). 3. Occupational body postures As mentioned previously, a body posture is simply a collection of joint positions for each major joint of the body. As a result, the same variables of static hold time, body endurance, strength requirements, and task exposure time will still apply towards the development of body discomfort (Kee and Karwowski, 2001, 2004). When categorizing work postures, there are three that are mentioned in literature: standing, sitting (assumed to be in a chair), and awkward postures (Gallagher, 2005). 3.1. Standing A multitude of investigations have examined associations between the posture of standing and discomfort (Balasubramanian et al., 2009; Cham and Redfern, 2001; Chung et al., 2003; Messing et al., 2006, 2008; Ngomo et al., 2008; Redfern and Cham, 2000;

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Ryan, 1989; Van Wely, 1970). Standing differs from other postures in that total body weight is supported and distributed bilaterally along each leg of the LE and then into each foot. Chung et al. (2003) notes that the standing posture requires knee flexion angles to be less than 30 (along the vertical axis). Research to be reviewed on standing discomfort can be dispersed between types of foot stances, prolonged standing durations, and the impact of floor surface material for the foot. Foot stance has been shown to be one cause of standing related discomfort (Chung et al., 2003; Van Wely, 1970). Van Wely (1970) points out that pigeon toed standing (when toes are pointed in lateral directions) seems to lead to foot pain, especially if performed for long durations. Chung et al. (2003), expanded on the notion even further by attempting to quantify distances between feet during wide foot stance positions. The foot stance positions were divided between the feet being (1) parallel to shoulder breadth and spaced medio-laterally apart and (2) separated in anterior-posterior directions. Each of these two groups was mentioned to have distances between the feet of approximately 40 cm (15.75 in.), where an anterior-posterior stance is about 1.5 foot lengths apart. Of the two stance groups, their results found the latter to be more uncomfortable for people to maintain. Results also mention that as distance between heels increased discomfort also increased slightly. The investigators continued by suggesting that if one is using a heel anterior-posterior stance, that distances whilst standing be reserved to a mean measurement of 37.1 cm (14.6 in.) or less. This measurement is based on the mean foot length of 24.7 cm (9.72 in.) in the study that they conducted. So it can be inferred that the mean of this distance would vary depending on the mean of the foot length of the population being observed. Although an association between discomfort and standing balance had not been reviewed by the investigators of this study, Chung et al. (2003) did mention that Kirby et al.’s (1987) study noted the positioning of the feet and their distance apart from each other is directly related to one’s balance while standing. Studies involving prolonged standing were the next group evaluated in this standing-discomfort review. An assessment of supermarket employees was conducted by Australia’s Victorian Occupational Health and Safety Commission (Ryan, 1989). The examiner concluded that the checkout stand areas contained the highest number of discomforts accounted for throughout the stores. They focused particularly on cashiers. Cashiers spent 90% of their work day standing in their work areas. This factor led to a significant association with pain in the lower leg, ankle, and foot portions of the LE (lower leg: R2 ¼ 0.837, t ¼ 4.53, p ¼ 0.011; ankle/ foot: R2 ¼ 0.951, t ¼ 8.79, p ¼ 0.001). In agreement with Ryan’s (1989) results, other studies recognized the relationship between prolonged standing durations and leg discomfort (Balasubramanian et al., 2009; Messing et al., 2006; Ngomo et al., 2008). Using surface electromyography (EMG), Balasubramanian et al. (2009) noticed a significant difference (p < 0.05) existed between the fatigue rates of prolonged static standing and dynamic standing (consistent movement within a standing area) for the gastrocnemius muscles of the lower legs. Messing et al. (2006) too observed that periods of prolonged standing with little to no movement can cause low levels of comfort for both of these same body regions [lower leg (Odds Ratio [OR]: 3.69; 99% CI: 2.19, 6.23) and ankle/foot (OR: 3.89; 99% CI: 2.53, 5.99)]. They mention that discomfort in the LE regions of the lower leg, ankle, and foot are not solely associated with prolonged standing but also standing with areas of limited freedom of movement (Messing et al., 2006). Their review of the literature particularly noted that as walking frequency decreases, discomfort changes locations from the acute plantar regions of the foot to a broader LE distribution that includes the lower legs as well. Their

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results also mention that the option of sitting at will while working seemed to be more connected to older aged individuals and those with a higher annual income. Therefore, they recommend the option for jobs to allow employees to be able to sit or stand at their own discretion. In a following study, Messing et al. (2008) compared prolonged fixed standing tasks to that of standing tasks with a sitting option. This associated to discomfort for the lower leg/calf and the ankle/foot for both men (lower leg/calf: OR ¼ 1.75; 95%CI: 0.94, 3.29; ankle/foot: OR ¼ 3.04; 95%CI: 2.08, 4.44) and women (lower leg/calf: OR ¼ 2.87; 95%CI: 1.86, 4.43; ankle/foot: OR ¼ 3.08; 95%CI: 2.15, 4.43). Ngomo et al. (2008) suggests that the significant association that resulted between the static standing and lower leg/foot discomfort (p ¼ 0.046) of their research may be caused by the combination of lowered blood pressure in the legs, mechanical compression on internal tissue, and possibly muscle tissue damage. Pope et al. (2003) were looking to find correlations between occupational postural activity and hip pains by reviewing subjects that had at least 24 h of pain from over the past month. Their epidemiological study (n ¼ 352) found that over time, people who stand for more than 2 h (cumulatively at a time per incident) without a change of posture for occupations where they have been working for more than one year, showed an association to hip discomfort (OR: 1.19; 95%CI: 0.8, 1.78) (Pope et al., 2003). Furthermore, the investigators mention that workers exposed to long term standing throughout occupations over a period of 16 years had an increase in associated hip pain (OR: 1.46; 95%CI: 1.00, 2.14). Prolonged standing itself is not the only contributor of LE pain. Floor surface conditions can also influence the result of perceived discomfort for the LE (Cham and Redfern, 2001). Floor surfaces that are perceived as ‘‘hard’’ are noted to have higher levels of LE discomfort and fatigue (Redfern and Chaffin, 1995; Redfern and Cham, 2000). Softer floor surfaces or floor mats are perceived as more comfortable (Redfern and Chaffin, 1995). However, the authors found that the results of their study showed extremely soft floor surfaces can be just as uncomfortable as hard surfaces. Cham and Redfern (2001) advise that when choosing anti-fatigue flooring, high elasticity, low energy absorption, and high stiffness are noted to show higher levels of comfort and anti-fatigue. They also mention that the resulting LE discomfort associated to prolonged standing may not become noticeable until at least 3 h of constant exposure (p < 0.05).

what the authors call ‘‘inactive sitting’’ which is noted as prolonged sitting with little or no leg movement. Contrast is noted to occur with increased leg activity during these chair sitting periods and is offered by the authors as an initial solution. Pope et al. (2003) notice a correlation between hip discomfort and cumulative chair sitting greater than 2 h per incident (OR: 1.82; 95%CI: 1.19, 2.77). Further detail on this correlation reveals that it is particularly linked to people whose repeated exposures throughout occupations have been accumulating for more than 18 years. In contrast, no significant association is mentioned for cumulative sitting greater than 30 minutes per hour in the study done by Andersen et al. (2007) (hazard ratio: 1.0; 95%CI: 0.6, 1.5). It should be noted though that Andersen et al.’s (2007) study differed from Pope et al.’s (2003) by looking at the cumulative amount of time in minutes per hour instead of hours per day. Occupations tied to LE discomfort and prolonged periods of chair sitting have been evaluated by little research (Anderson and Raanaas, 2000; Chen et al., 2004; Taiwan Institute of Occupational Safety and Health (IOSH), 1999). Participants of a population based study (n ¼ 1115) of professional and taxi cab drivers are mentioned by Chen et al. (2004) to have developed knee pains after driving for more than 6 hours per day (OR: 2.52; 95%CI: 1.36, 4.65). In addition, the authors state that further research be conducted to find a correlation between these noted knee pains and the possibility of developing knee osteoarthritis from prolonged driving exposure. 3.3. Awkward postures Awkward postures are typically unusual and sometimes uncomfortable postures that are assumed by individuals in order to accomplish their work goals. Gallagher (2005) mentions that when these awkward postures are forced on workers due to their occupational environment, they are then deemed as restricted postures (p. 51). An example of ‘‘restricted postures’’ is working inside a commercial aircraft’s wing (a confined space) where technicians are reduced to crawling on hands and knees and in some cases moving themselves while lying prone or supine. While Gallagher (2005) considers lying down, kneeling, squatting, and stooping as restricted postures, it can also be assumed that floor sitting may be so labeled as well, depending on environmental circumstances. Otherwise, floor sitting would remain as an awkward posture. Additional awkward postures are tiptoeing while standing and imbalanced stances while managing another posture.

3.2. Chair sitting Research has used direct measure as a means for understanding chair sitting and its impact on discomfort (De Looze et al., 2003). De Looze et al. (2003) reviewed the efficacy of the tools of EMG, pressure distribution mapping, and postural analysis. Of the three, pressure distribution was noted to be closely related to reports of chair sitting discomfort by participants. Variables mentioned that influence discomfort ratings include distribution of body weight across the seat pan of the chair as well as back support offered to the lumbar region of the spinal column. Foot rests are also noted to provide relief while sitting in chairs, associating distress to the leg and knee regions when not being employed (Van Wely, 1970). LE discomfort for chair sitting has also been associated with physical symptoms in several studies (Winkel and Jorgensen, 1986a, b). Venous pooling (swelling) is noted to occur in the lower legs and feet during prolonged periods of sitting (at least 4 h) which is reported to be associated with discomfort as well (Winkel and Jorgensen, 1986b). Winkel and Jorgensen (1986a) noticed that additional symptoms tending to occur along with venous pooling included increased average heart rate and lower flexor hallucis longus muscle temperature. All of these symptoms correlate to

3.3.1. Tiptoeing while standing As mentioned above, awkward postures are those that are used by people in order to attain a desired goal. While common awkward postures studied may have working zones at or below the waistline, reaching for extended distances in superior–anterior directions above the head may require the extra couple of inches gained from a tiptoeing posture. While tiptoeing is conducted during the standing posture, it is not ‘‘normally’’ assumed and is therefore a derivation of standing (Chung et al., 2003, 2005). The discomfort rating observed by Chung et al. (2003) for bilateral standing versus tiptoed standing was twice as low revealing that maintaining this posture (greater than 1 min) is significantly more likely to cause discomfort to a male’s LE regions (a ¼ 0.05). Females were not observed in this study. 3.3.2. Stooping Stooping postures are those that require a forward torso bend while keeping the legs and knees as close to neutral as possible. Depending on the height of the work area, stooping may involve spacing the legs apart along medio-lateral directions in the transverse plane to increase or decrease working reach. Research

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accomplished by Meyer and Radwin (2007) compared stooping versus prone postures for working in agricultural work environments. The simulated task represented picking fruits or vegetables from plants low to the ground. Stooping postures were maintained for two 15-min work sessions with each session allowing 2 min of continuous work followed by 30 s of rest. Perceived discomfort, EMG ratings, and heart rate were the three variables that were collected. All three measures portrayed the stooping posture entailing a higher level of discomfort and fatigue on the body. In particular, LE body regions affected included the anterior and posterior portions of the knee, the thigh, the lower leg’s calf muscle, and particularly the hamstring. The hamstring’s mean perceived discomfort rating was 6.17 out of 10, making it the most affected region during the employment of this posture. EMG root means square (RMS) measures displayed an18% higher muscle utilization for the stooping versus prone posture. In addition, mean heart rate was 35% greater than resting heart rate whereas prone was only 17% greater. Interest in biomechanical influence on perceived discomfort was observed in another study for stooping postures (Boussenna et al., 1982). Unlike Meyer and Radwin’s (2007) investigation of discomfort and muscle fatigue, Boussenna et al. (1982) looked for association between discomfort and joint torque while stooping. In their review of the literature, the authors point out that while an EMG shows muscle utilization during a posture, it does not allow one to perceive the relationship of discomfort and soft tissue tension over articulating surfaces. They defined joint torque as the resisting (net) torque created by the body segments superior to the joint being assessed in order to maintain the posture or position. The method utilized during this study involved four stooping postures that were relative to the height of the shoulders above the ground. These four positions started with first 100% shoulder height (back is in neutral posture), followed by three 25% decrements of forward torso flexion until only 25% of the shoulder’s maximum height was observed relative to the ground. All positions required legs to be straight with little to no knee flexion. Participants sustained each of these positions as long as their perceived discomfort levels would allow. Results of this study illustrate that the more the torso flexed forward, the more LE joint torques increased (p < 0.001). Increasing joint torque along the LE also was associated with increasing discomfort levels. So as the stoop’s torso flexion brought the shoulder height closer to the ground, participants were less likely to show long static posture hold times. Demonstration of these joint tensions’ effect on discomfort along the hip, knee, and ankle was clearly noted. But also realized was how the joint tension affected the body segments superior or inferior to them. Boussenna et al. (1982) mention that torque at the hip joint influences discomfort along the lower back, gluteus maximus, and hamstring muscle regions. However, there was no significant statistical correlation for this association. Significant positive correlation is noticed though for knee joint tension and the affected discomfort levels in the hamstring and lower leg’s gastrocnemius regions (0.969; p < 0.025). Positive significant correlation is also noted for the ankle torque’s affect on the posterior lower leg region superior to itself (0.999; p < 0.01). 3.3.3. Squatting and knee flexion Determining whether a person is either squatting or just performing a knee flexion posture depends on the degree of knee flexion that a person uses while standing. The review performed by Chung et al. (2003) points out that a squat posture is that in which a degree of knee flexion is 90 or greater along the vertical axis (standing with legs straight and knees unflexed would be considered as 0 ). Squatting also requires that additional joints be flexed

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as well such as the ankle, torso, and, depending on working height location, possibly the shoulder (Lee and Chung, 1999). Continuing on, Chung et al. (2003) mentions that for angles less than 90 but not less than 60 , the term ‘‘severe knee flexion’’ is given. ‘‘Mild knee flexion’’ is that in which the angle threshold from the vertical axis is 30 or greater, but still less than 60 . In addition, neither squatting nor knee flexion requires contact with the ground nor any surface (such as for kneeling postures). In the squatting studies performed by Chung et al. (2003, 2005) participants were instructed to maintain squatting postures for 60 s with a 2 min break in between observations. The resulting consensus from both studies found that as participants increased the angle of deviation their reported mean discomfort ratings would increase as well (a ¼ 0.05). In fact, the level of discomfort plateaus after surpassing the 90 marker (Chung et al., 2005). According to Chung et al.’s (2003) results, severe knee flexion is judged as the highest level of discomfort even when compared to other postures such as standing, sitting, kneeling, or imbalanced. Interestingly, their reports also mention that squatting discomfort levels are comparable to mild knee flexion levels and both show no significant differences (a ¼ 0.05). A US study into school teacher related occupational disorders was conducted for preschool employees (Grant et al., 1995). Investigators found that 6 of 18 (33%) of their participants complained of LE discomfort. Further inspection revealed that squatting made up 6.8% of the work day. In the agricultural industry, pepper picking is a required labor that has not fully transitioned to automation. When compared to kneeling and stooping, squatting was found to be the most expedient method of collecting peppers although it was noted as causing LE fatigue and discomfort (Jin et al., 2009). In another study, this time carried out by Lee and Chung (1999), squatting postures were analyzed in Korean steel manufacturing ship yards. Typical worker tasks include cutting, welding, and grinding. They also mention that other occupations may be affected by squatting postures aside from ship manufacturing, such as automobile manufacturing, farming, and machine repair shops. The difference between this research and that of the previous two (Chung et al., 2003, 2005) is that Lee and Chung (1999) studied appropriate stool seat height compared to just a squatting posture. Subjects (n ¼ 8 males) were instructed to maintain these postures for 16 min continuously with subjective ratings being taken every 2 min. Results indicate that prolonged squatting (without a stool) for more than 4 min showed a significant increase in reported discomfort levels for the whole body, thigh, and especially the lower leg regions (p < 0.05). Use of low stools were considered equivalent to squatting postures due to their use allowing workers access to low lying work areas near the floor surface. The suitable stool height indicated to alleviate discomfort was found to be 10 cm from the floor surface (p < 0.05). Even so, discomfort was shown to be delayed by only 2 min before onset, indicating that, again, dynamic change of posture is necessary in order to help lessen discomfort. In a prospective study conducted in western Denmark, investigators looked for risk factors that led to discomfort in portions of the body including the hip, knee, lower leg, and foot (Andersen et al., 2007). These investigators noticed that there is an association with pain for workers that squat cumulatively for more than 5 min per hour (HR: 1.2; 95%CI: 0.8, 1.8). Lee and Chung (1999) propose that the reason the lower leg region of the LE may be affected as much as their study shows, is possibly due to the lower leg supporting the entire body’s weight while squatting. Another study, which reviewed postural activities and knee discomfort complaints for southern Hampshire in Britain, noted that a cumulative squatting posture greater than 1 h per 8 h work day was associated with147 of the 762 working men studied

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for knee discomfort symptoms (Baker et al., 2003). Symptoms of discomfort have also been reported for the thigh region’s quadriceps muscle group (Olendorf and Drury, 2001). Multiple studies pinpoint the reason for this to this muscle group’s static posture sustaining contractions and the tightening of the tendons across the knee joint into the superior–anterior portions of the lower leg (Boussenna et al., 1982; Chung et al., 2005; Olendorf and Drury, 2001). Again, as mentioned previously in this review, this is an example of how joint biomechanics can be one set of variables that lead to LE discomfort. 3.3.4. Kneeling Occupations requiring workers to kneel rather than stand employ the knee as a weight bearing surface in place of the foot (Bruchal, 1995). In their investigation of LE postures and perceived discomfort, Chung et al. (2003, 2005) noticed that along with squatting and knee flexion, kneeling postures are also commonly used in Asian and African culture working environments. Their studies of kneeling focused on four types of kneeling postures. The first is kneeling with both the knees in full flexion while the ankles are in full plantar flexion. The second kneeling posture observed, is kneeling with the knees at 90 flexion so that the lower legs are horizontally perpendicular to the vertical thigh and torso regions of the body. The third posture is single knee kneeling where only one knee is in contact with the floor. The fourth and last kneeling posture involves a crawling posture with both hand and knee surface contact. Reported LE mean discomfort ratings from Chung et al. (2003) reveal that kneeling with both knees in full flexion and one-legged kneeling postures were significantly more uncomfortable than the 90 flexed kneeling and kneeling in a crawling posture (a ¼ 0.05). Additionally, the authors also note that kneeling in the crawling posture is significantly more comfortable than that of 90 knee flexion (a ¼ 0.05). Chung et al. (2005) commented on the fact that kneeling with knees fully flexed causes symptoms of discomfort such as tingling and numbness in the lower legs. They attribute this to vascular compression caused by the upper body’s weight applying pressure to the lower leg segments. Population studies of British working males aged 20–59 revealed that kneeling adds to discomfort within the knee itself as well (Baker et al., 2003). Particular attention was given to those that reported cumulative kneeling, greater than 1 h throughout their work day (159 of 762 reported men). 3.3.5. Floor sitting A second seated posture that is commonly seen in many Asian cultures and countries such as Korea, India, and Thailand, involves derivations of floor sitting positions (Chung et al., 2005; Laohacharoensombat et al., 2005; Nag and Nag, 2007). Three types of floor sitting styles are discussed by Chung et al. (2003, 2005). The first involves sitting with the legs stretched straight and anterior to the body with no knee flexion. The second posture is similar, but the hips as well as the knees are allowed to be flexed to the point where the thighs are brought closer to the chest of the person. The third posture is that of cross-legged floor sitting while the knees are flexed. Both studies found the crossed-legged floor posture to be the most comfortable of the three to maintain. Additionally, they reveal that except for cross-legged sitting, these postures display higher mean discomfort ratings than chair sitting ones. Both Chung et al. (2003, 2005) studies propose that one reason behind the discomfort noticed from the first two postures is that due to the lack of a chair’s back support, an individual must maintain proper lumbar lordosis through their own effort. Hence, a more uncomfortable body posture results. Chung et al. (2005) note that the subjective discomfort level of these postures may be even higher for Western cultures more accustomed to the use of chairs. This

may explain why research for this posture has not been more prevalent. In addition, it would be interesting to see if future studies could identify additional reasons for the discomfort disparity noted between floor sitting and chair sitting postures, such as possibly an individual’s lower extremity flexibility. 3.3.6. Lying down In his review of awkward postures, Gallagher (2005) mentions that lying down greatly reduces the lifting capability of the body due to the removal of the LE’s muscle groups. His review mentions only one exception which is a two handed lift while in the supine posture (noted to exceed that of even lifting while standing). Of the research conducted for LE discomfort, very few if any, have looked at the comfort of working in postures that involve lying down. These postures can include lying on the stomach (prone), lying on the back (supine), or lying on the side of the body. A prone postural study (n ¼ 15 males) was performed in order to test its feasibility for working in agricultural fields picking low lying fruits and vegetables (Meyer and Radwin, 2007). The postural setup of the prone position involved the use of a mobile padded workstation that the subject would lay on. This workstation, similar to a massage table included face and neck support as well as an ‘‘S’’ curvature to account for a comfortable hip and knee flexion posture. Height of the workstation allowed subjects to be within arms reach of the work area lying low to the ground. Prone discomfort results were compared to stooping posture results in order to show commonalities and differences between this proposed working posture and a current stooping working posture used in the field. Lower levels of discomfort were reported for not just the LE but also for the whole body. The discomfort scale was from 0 to 10 with 10 being the most uncomfortable. The negligible but highest mean discomfort levels reported from the results were for the hamstring, quadriceps, and calf muscles with mean scores of 0.67, 0.40, and 0.33 (p < 0.05). In stark contrast, those same body areas for stooping reported mean discomfort levels of 6.17, 3.33, and 2.97 respectively (p < 0.05). 3.3.7. Imbalance The in-depth LE discomfort studies conducted by Chung et al. (2003, 2005) also observed imbalanced postures. Participants were instructed to lean their body’s center of mass onto their right foot. Four of these imbalanced postures were evaluated. The first two were standing postures that entailed spreading the feet apart medio-laterally for the first posture and then in an anteriorposterior method for the second. The latter two imbalanced postures required knee flexion postures. The first of these had a 30 knee flexion angle while the last had an actual squat position (>90 ). Again, as in their previous observations with other postures, subjects were requested to hold these postures for 60 s. Their results reveal that the latter two postures were more uncomfortable to maintain. Of these two knee flexion postures, they note that imbalanced squatting has the highest level of discomfort and is followed by 30 knee flexion (a ¼ 0.05) (Chung et al., 2003). Interestingly, three one-legged standing postures were also evaluated in the study of Chung et al. (2003), two of which resulted in higher discomfort levels than even imbalanced squatting. They were not included under imbalanced postures but given their own category of one-foot standing postures. The first was just a plain right-footed stance with no knee flexion while the left leg was held above the floor. The second posture was the same as the first except, that the right standing knee was given a flexion angle of 30 (mild knee flexion). The last posture was a derivation of the second posture with 60 of flexion instead (severe knee flexion). As noticed by the trend of previous postures requiring maintenance of body

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posture while the knees are flexed, one-leg standing on bended knee is no less uncomfortable. 60 of flexion was rated with a higher mean level of discomfort than any other one-legged and imbalanced posture. This posture is followed in the results by 30 of knee flexion while standing on the right foot. Olendorf and Drury (2001) noticed in their validation study of the OWAS method (Karhu et al., 1977), that this same result occurred for their onelegged standing postures as well. The only posture exceeding the 60 knee flexion single foot standing-discomfort level is the severe knee flexion angle posture of 90 (Chung et al., 2003). 4. Occupational activities Discomfort is not limited to limb segment positions and body postures alone. Discomfort also may arise due to activities that are performed for the sake of finishing a task. Activity differs from posture in this study because activity is dynamic while posture is stationary. Therefore the impact of activities such as walking, lifting, lowering, etc. on LE body discomfort levels will be reviewed. An example of such a study was conducted for a soft drink beverage delivery company (n ¼ 9 males) (McGlothlin, 1996). The investigator reviewed several postures and activities that were utilized by the beverage delivery employees which included sitting (while driving), kneeling, squatting, pushing, pulling, lifting, lowering, stacking or unstacking items, walking, and stair or ramp climbing. The investigation showed that knee discomfort symptoms were the most reported by employees for the LE.

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investigators calculated an association between the lower back and hip discomforts (OR: 8.6; 95%CI: 7.1, 10.3) and noted that both were caused by heavy lifting activities. In agreement, Pope et al. (2003) mentioned that 46% of their research participants reported lower back discomfort along with hip discomfort. They propose that there is possibly some form of referred pain between the two regions and that they are likely to be caused by mechanical risk factors. Cumulative lifting and pushing or pulling was also reviewed in a study (n ¼ 1513) by Andersen et al. (2007). These MMH factors are shown to be associated to both lower back (cumulative lifting) and lower extremity (cumulative pushing or pulling) discomfort as was inferred by Pope et al. (2003). For lower back pain, lifting 100 kg (220.5 lbs) or more per hour at work was considered as a risk (HR: 1.5; 95%CI: 1.0, 2.3). People who push or pull a cumulative load of more than 355 kg (782.6 lbs) per hour had a relationship with hip, knee, and foot pain (HR: 1.6; 95%CI: 1.0, 2.5). Additionally, there may be tasks that indirectly affect the comfort balance of the lower extremities. Examples of this are mentioned by Messing et al. (2006) for both males and females. An affinity with LE discomfort is noticed for women performing highly repetitive hand and arm tasks. The areas of the LE that affect them include the lower leg (OR: 1.50; 99% CI: 0.90, 2.49) and the ankle/ foot (OR: 1.73; 99% CI: 1.11, 2.70) regions. Messing et al. (2008) also noted the affiliation between lower leg/calf discomfort and repetitive hand and arm movements (AOR: 1.70; 99% CI: 1.08, 2.66). They mention that these activities are conducted during prolonged periods of standing which may explain their relationship to the discomforts noticed in the LE.

4.1. Manual material handling 4.2. Walking The cause-and-effect relationship between postures and discomfort has been discussed profusely prior to this section. It should not then be a surprise that sitting (while driving), kneeling, and squatting postures affect the knee and other areas of the LE. Activities on the other hand, such as pushing, pulling, lifting, lowering, stacking and unstacking can be considered as manual material handling (MMH) which normally requires the use of the LE. A Canadian study (n ¼ 7770) conducted in the Quebec province, reviewed correlations between the working population and reported LE discomfort (Messing et al., 2006). A significant association was detected amongst women and heavy material handling. Messing et al. (2006) mentions that in particular this relationship affected both the lower leg (OR: 2.56; 99% CI: 1.30, 5.04) and ankle/ foot (OR: 1.76; 99% CI: 0.93, 3.31) regions of the LE. Regrettably, an exact figure for what is considered as ‘‘heavy’’ is not declared by the investigators. A later (Messing et al., 2008) study confirmed that women in work environments requiring handling heavy loads consistently throughout their work shift did indeed have a significant tie to lower leg and calf discomforts (adjusted odds ratio [AOR]: 2.78; 99.9% CI: 1.44, 5.39). A British study performed by Pope et al. (2003) noticed a correlation between people whose jobs required lifting and moving objects greater than 23 kg (50.7 lbs) and their reports of hip discomfort. More intriguing, is that these workers were working with these exposures in these work environments for more than 13 years (OR: 1.90; 95%CI: 1.30, 2.78). Another study (n ¼ 5042) focused on male and female British post office workers between 70 and 75 years old (Sobti et al., 1997). Similar to Pope et al. (2003), Sobti et al. (1997) noted that there is a significant correlation with hip discomfort and regularly lifting more than 25 kg (55 lb) per item per average work day. This correlation was especially noticeable for those people in the study exposed to this risk for more than 20 years (Risk Ratio: 1.50; 95%CI: 1.24, 1.82). Interestingly, Sobti et al. (1997) also report that heavy lifting tasks also cause noted discomfort in the lower back region (p < 0.001). The study’s

The activity of walking for task related purposes at work is also appraised in this review. A study conducted in the United Kingdom (n ¼ 127) reviewed load carrying (24 kg/52.9 lbs) while on a 1 h field marching exercise (Birrell and Haslam, 2009). This exercise resulted in a noted discomfort to the feet amongst both genders (p ¼ 0.001). In addition, women reported a higher level of hip discomfort than their male counterparts (p ¼ 0.030). Based on their review of literature, Birrell and Haslam (2009) suggest that this hip pain noted by the females in the march could be associated to the stride parameters unintentionally set by the pace of the males in their marching group. Males typically have longer legs and longer stride lengths. This leaves females with the options of increasing their stride lengths or increasing their stride frequency (both of which add increased stress to the hip area of their pelvis). Baker et al. (2003) noticed an association between walking more than 2 miles in a single work day and knee discomfort. Although there is no statistical correlation available for their associated reference, they did state that approximately 35% (266 of 762) of the men studied mentioned the complaint. More empirical evidence is available from the results of Pope et al.’s (2003) study, although the discomfort location is with the hip joint instead. They mention that walking more than 2 miles per work day associated with workers that had more than 15 years of cumulative occupational exposure (OR: 1.65; 95%CI: 1.13, 2.41). This exposure time is reduced to half (>7 years) when the environmental variables include walking over rugged terrain (OR: 2.65; 95%CI: 1.43, 4.90). 4.3. Stair climbing Another LE activity noted to be associated with discomfort is stair or ramp climbing. Stair climbing in particular has had a stronger draw for investigation due to its association with osteoarthritis of both the knee (Cooper et al., 1994; Coggon et al., 2000) and hip (Lau et al., 2000). In Pope et al.’s (2003) retrospective study,

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Table 3 Occupational postural relationships with the LE body region they affect. Posture

Joint or segment discomfort location Lower back

Standing Stooping Chair sitting Floor sitting Knee flexion Squatting Kneeling Tiptoeing Imbalance Lying down

Hip

Thigh

Buttock

Knee

Lower Leg

Ankle

Foot

Overall LE

X

X X X

X

X X

X X X

X X

X X

X X

X

X

X X X

X

a slight association was noticed for hip pain and stair climbing. Details of the results reveal that stair climbing of more than 20 flights of stairs per work day associated to those people exposed to it for more than 14 years (OR: 1.40; 95%CI: 0.87, 2.25). Though considered a weak significant correlation, Sobti et al. (1997) did notice that an association did exist between knee pain and people climbing more than 30 stair flights for 1 to 15 years of occupational exposure (RR: 1.17; 95%CI: 0.99, 1.38). Insight into the biomechanical effect that climbing a set of stairs has on knee discomfort tells that the anterior–posterior shear force of the knee is close to the body weight of an individual (Costigan et al., 2002). Additionally, the author’s results also mention that even higher levels of stress are incurred by the knee from compressive forces (3–6 times body weight). 4.4. Vibration exposure Men have been found to have an association with discomfort and whole body vibration in two studies (Messing et al., 2006, 2008). Both the lower leg and ankle/foot are the affected areas in both studies. Messing et al. (2006) had lower leg and ankle/foot discomfort correlations being OR: 3.48 (99% CI: 1.92, 6.32) and OR: 2.40 (99% CI: 1.40, 4.10), respectively. In their later study of 2008, Messing et al. found the association to be AOR: 3.70 (99.9% CI: 2.02, 6.76) for the lower leg/calf and AOR: 2.67 (99.9% CI: 1.58, 4.52) for the ankle/foot. Neither of these studies gave an associated quantity exposure measure such as duration, vibration frequency, or count. 5. Discussion Many LE regions are affected by a variety of work-related postures and activities. The results of this review indicate that the knee, lower leg, and foot are the three joints or segments most affected by work postures (Table 3). Work activities appear to generally affect the hip, lower leg, ankle, and foot (Table 4). Based on reviewed studies (Table 3), stooping postures affect the most LE body regions, causing the highest overall level of LE discomfort.

X

X X X X X X X

Opposite of this are lying down postures, which as of yet has not had enough empirical research to establish its rank amongst the other postures. Table 4 reveals that walking, pushing, and pulling activities affect LE areas the most with stair climbing the least. This taxonomy of postural activity and LE regions should not be taken as final. Postural activities may affect other areas of the LE but were not included in this review due to a lack of statistical significance in the reviewed studies or the relevant studies were not yet published at the time of this review. Postures involving knee flexion relate to LE discomfort in several of the studies reviewed. Much of the attention that is given to this LE joint clarifies the association between discomfort, fatigue, and biomechanical forces such as shear, tension, compression, and pressure (Messing et al., 2008; Ngomo et al., 2008). The degree of deviation from a neutral standing posture is shown to influence the level of subjective discomfort especially when knee flexion angles approach 90 . Therefore, it should be no surprise why the squatting posture and stair climbing activity are given their place in the guidelines of Table 5. This guideline is a cumulative representation of postural limitations. Surpassing these limits can plausibly lead to noticeable LE discomfort according to the statistical associations of their originating studies. These limits refer to postural activity by exposure quantities such as physical position (foot stance or joint flexion angle), single incident, time allocation (work day or per hour), or weight. ‘‘Subjective’’ is a word reiterated continuously throughout this review. In context, it represents the individual levels of pain and aggravation that participants noticed for postures and activities that they used while working. De Looze et al. (2003) explain the term by commenting on an individual’s acceptable discomfort capacities. These capacities are influenced not only by the postural activities being used but also by occupational environment and even the task itself. By adapting this information and the information made available by the National Research Council’s (2001) results on the origins of WMSDs, two distinct groups of risk variables are noticed: extrinsic and intrinsic. Extrinsic variables have been discussed all through this document in the form of joint

Table 4 Occupational activity relationships with the LE body region they affect. Activity

Walking Pushing Pulling Lifting Lowering Stacking Stair/ramp climbing Vibration activity

Joint or segment discomfort location Lower back

Hip

X

X X X X X X X

Thigh

Buttock

Knee

Lower leg

Ankle

Foot

Overall LE

X X X

X X X X X X

X X X X X X

X X X X X X

X X X X X X X

X

X

X

X

C.R. Reid et al. / International Journal of Industrial Ergonomics 40 (2010) 247–256

255

Table 5 LE discomfort guideline with maximum exposure quantities and the LE body area that each posture or activity affects. Posture or activity

Continuous exposure quantity

LE body area affected

Statistical association

Source

Standing (anterior–posterior foot stance) (men) Standing Standing Tiptoeing (men) Chair sitting Chair sitting Chair sitting (while driving) Squatting (men) Squatting Squatting (men) Lifting and lowering Lifting Pushing and pulling Load Carrying Load carrying (women) Walking Stair climbing Stair climbing

37.1 cm (14.6 in.) between heels

LE

–

Chung et al. (2003)

>2 h/incident 3 h/incident >1 min/incident >2 h/incident >4 h/incident >6 h/incident 90 of knee flexion >5 min/h >4 min/incident >23 kg (50.7 lbs)/item >25 kg (55 lbs)/item >355 kg (782.6 lbs)/h 24 kg (52.9 lbs)/incident 24 kg (52.9 lbs)/incident >2 miles/work day >20 flights/work day >30 flights/work day

Hip LE LE Hip Lower leg; foot Knee Thigh Hip; knee; foot Lower leg; thigh Hip Hip Hip; knee; foot Foot Hip Hip Hip Knee

OR:1.19, 95%CI: 0.8,1.78 p < 0.05 a ¼ 0.05 OR:1.82, 95%CI: 1.19,2.77 – OR:2.52, 95%CI: 1.36,4.65 a ¼ 0.05 HR:1.2, 95%CI: 0.8,1.8 p < 0.05 OR:1.90, 95%CI: 1.30,2.78 RR:1.50, 95%CI: 1.24,1.82 HR:1.6, 95%CI: 1.0,2.5 p ¼ 0.001 p ¼ 0.030 OR:1.65, 95%CI: 1.13,2.41 OR:1.40, 95%CI: 0.87,2.25 RR:1.17, 95%CI: 0.99,1.38

Pope et al. (2003) Cham and Redfern (2001) Chung et al. (2003) Pope et al. (2003) Winkel and Jorgensen (1986b) Chen et al. (2004) Chung et al. (2005) Andersen et al. (2007) Lee and Chung, 1999 Pope et al. (2003) Sobti et al. (1997) Andersen et al. (2007) Birrell and Haslam (2009) Birrell and Haslam (2009) Pope et al. (2003) Pope et al. (2003) Sobti et al. (1997)

–, Not mentioned by the study; OR, odds ratio; RR, relative risk; HR, hazard ratio.

positions, work postures, and work activities. Not included in this review are the extrinsic risks of environmental and psychosocial factors such as temperatures, fear, or intimidation. Intrinsic risk factors also pose an influence as noted by Messing et al. (2006). Examples of intrinsic variables that may influence discomfort levels may be past injuries, current health status, and even a presently active WMSD. So although postural activity is a major influence to occupational discomfort, a holistic analysis approach should be incorporated that includes assessment of other extrinsic and intrinsic variables. With such an approach practitioners will not only be treating the symptoms of discomfort but perhaps also be solving the problems that could lead to WMSDs.

This could plausibly begin building a relationship between discomfort and disorder. Hypothetically speaking, a tool designed with a principle such as this would be of the degree of predictive value and allow practitioners a truer sense of postural activity risk to individual employees in an occupation or job. For now though, tools such as guidelines and models can be used to steer work environment design and ergonomic mitigation.

Acknowledgments The authors would like to thank Karl J. Schaefer for his assistance in the proofing and editing of this review article.

6. Conclusion The Discomfort Guideline given in Table 5 above helps to formalize the postural activities that can lead to fatigue and discomfort. Exposures are captured either through observation of the participants (e.g., duration or frequency) or through direct measure of them and their environment (e.g., distance, goniometry, or weight). Although this information can be perceived as being conclusive, it should only be taken as approximate due to the high variability that each person experiences for levels of discomfort. The association between postural activity discomfort and WMSDs is not fully understood. One fact that is known to researchers of LE discomfort and disorders is the similarity between the use of postures and activities. Andersen et al. (2007) state that the posture of squatting is linked to discomfort of the knee. In that same respect, squatting has also been linked to knee osteoarthritis (Cooper et al., 1994) and knee meniscal disorders (Baker et al., 2003). Therefore, practitioners may automatically assume that continuous cumulative exposure to these postural activities will eventually lead to a disorder in the location where discomfort was previously noticed. One should not be so quick to draw these conclusions though until it is empirically proven. Further research needs to be conducted to narrow the gap between these two levels of morbidity. Continuing with the squatting example for the knee, a future study might attempt to investigate the possible matching of psychophysical pain thresholds of individuals in this posture with biomechanical data (force and pressure measurements) and then compare that data to known physiological tissue failure limits of knee components (whether acute or cumulative fatigue induced).

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