Ergonomics
ISSN: 0014-0139 (Print) 1366-5847 (Online) Journal homepage: http://www.tandfonline.com/loi/terg20
Peak activation of lower limb musculature during high flexion kneeling and transitional movements David C. Kingston, Liana M. Tennant, Helen C. Chong & Stacey M. Acker To cite this article: David C. Kingston, Liana M. Tennant, Helen C. Chong & Stacey M. Acker (2016): Peak activation of lower limb musculature during high flexion kneeling and transitional movements, Ergonomics, DOI: 10.1080/00140139.2015.1130861 To link to this article: http://dx.doi.org/10.1080/00140139.2015.1130861
Published online: 29 Feb 2016.
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=terg20 Download by: [University of Saskatchewan Library]
Date: 01 March 2016, At: 09:29
Ergonomics, 2016 http://dx.doi.org/10.1080/00140139.2015.1130861
Peak activation of lower limb musculature during high flexion kneeling and transitional movements David C. Kingston , Liana M. Tennant, Helen C. Chong and Stacey M. Acker Department of Kinesiology, University of Waterloo, Waterloo, Canada
ARTICLE HISTORY
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
ABSTRACT
Few studies have measured lower limb muscle activation during high knee flexion or investigated the effects of occupational safety footwear. Therefore, our understanding of injury and disease mechanisms, such as knee osteoarthritis, is limited for these high-risk postures. Peak activation was assessed in eight bilateral lower limb muscles for twelve male participants, while shod or barefoot. Transitions between standing and kneeling had peak quadriceps and tibialis anterior (TA) activations above 50% MVC. Static kneeling and simulated tasks performed when kneeling had peak TA activity above 15% MVC but below 10% MVC for remaining muscles. In three cases, peak muscle activity was significantly higher (mean 8.9% MVC) when shod. However, net compressive knee joint forces may not be significantly increased when shod. EMG should be used as a modelling input when estimating joint contact forces for these postures, considering the activation levels in the hamstrings and quadriceps muscles during transitions.
Received 26 February 2015 Accepted 5 December 2015 KEYWORDS
EMG; knee; occupational ergonomics; footwear; kneeling
Practitioner Summary: Kneeling transitional movements are used in activities of daily living and work but are linked to increased knee osteoarthritis risk. We found peak EMG activity of some lower limb muscles to be over 70% MVC during transitions and minimal influence of wearing safety footwear.
1. Introduction There is a lack of knowledge regarding peak muscular demand in high flexion knee postures. High flexion is defined as postures exceeding 120° at the knee joint (0° being full knee extension), and has been reported to a maximum of 165° (Acker et al. 2011; Hefzy, Kelly, and Cooke 1998). These postures are common when performing activities of daily living (e.g. prayer, childcare) and in industrial occupations (e.g. roofing, flooring). While knee joint kinematics and compressive loads have been studied in high flexion knee postures for arthroplasty research (Kurita et al. 2012; Mündermann et al. 2008; Niki et al. 2013), previous work (Nagura et al. 2006; Yang, Wickwire, and Debski 2010) has identified that information available on the activation of lower limb musculature during kneeling or transitional movements is limited (one study: Gallagher, Pollard, and Porter 2011). This lack of knowledge regarding transitions to or from kneeling postures poses a major challenge in the development of high flexion models and the understanding of acute injury in high flexion. In addition, the applicability of data from total knee arthroplasty patients to healthy CONTACT Stacey M. Acker © 2016 Taylor & Francis
[email protected]
working populations is questionable due to age and physical health considerations. Considerations for transitional movements are vital as workers exposed to getting up from kneeling more than 30 times a day, kneeling with heavy loads and kneeling for greater than one hour in their day have odds ratios of 2.3, 2.9 and 3.0, respectively, for the development of knee osteoarthritis (Coggon et al. 2000; Jensen 2008). With unknown levels of muscle activation during high flexion, it is difficult to incorporate muscle forces into high flexion knee joint modelling without the need for many assumptions in simulations (Hirokawa and Fukunaga 2013; Kim et al. 2009; Komistek et al. 2005; Lin et al. 2010; Nagura et al. 2006; Winby et al. 2009). Muscle activation is crucial for determining the contact forces acting on knee articular surfaces since, in most dynamic activities and in some static scenarios, muscles are the greatest contributors to peak force. (Andriacchi and Favre 2014). Overall, elevated joint contact forces are primarily the cause of knee joint degeneration which leads to osteoarthritis (Andriacchi and Favre 2014; Coggon et al. 2000; Kirkeshov Jensen 2005). A number of mathematical models exist which use inverse dynamics to estimate the joint contact force
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
2
D. C. Kingston et al.
occurring at the knee during high flexion (Arnold et al. 2010; Smith et al. 2008; Winby et al. 2009). Many of these models have been validated against instrumented knee implants with the common issue of under-predicting joint loads due, at least in part, to the absence of muscle activation data (Kim et al. 2009; Stylianou, Guess, and Kia 2013; Zhao et al. 2007). Although computer simulations of muscle forces have been completed (Arnold et al. 2013; Lin et al. 2010), no in vivo studies have characterised peak muscle activation during movements to high flexion postures. The peak muscular activation levels presented in this study will contribute to the modelling knowledge base by providing insight into the maximal demand high knee flexion activities pose. Considerable research has focused on industrial demands, but no previous work has measured the effect safety footwear may have on muscle activation during high flexion postures. In many industrial settings, personal protective equipment standards dictate that safety footwear must be worn when working. The mass of safety footwear could increase muscular demand during transitional movements as increased activation would be required to resist the added moment created by the footwear mass at the distal end of the segmental chain. As well, the stiffness of the boot upper, toecap and sole may alter normal ankle range of motion and result in different high flexion postures at the knee, and corresponding muscle activations, than when unshod. These considerations warrant measurement of muscle activation while wearing safety footwear to best represent working conditions. Since peak activation levels of the leg muscles are unknown in high flexion positions, there is limited understanding of the mechanisms through which injury and disease, such as knee osteoarthritis, may occur. In addition, the quantification of peak muscle activation levels during activities of daily living and occupationally relevant movement patterns will identify if particular movements and/or safety footwear pose unique considerations for models of the knee. Therefore, the aim of this study was to evaluate the effect of footwear on lower limb muscular activation during high knee flexion movements. In the pursuit of this aim, peak lower limb muscle activity during high knee flexion postures and movements were quantified, which will add to the limited knowledge of lower limb muscle activity in high flexion. We hypothesised that upward kneeling transitions, from kneeling to standing, will require increased muscle activations when compared to downward transitions due to the upward vertical acceleration of the body’s mass (Isear, Erickson, and Worrell 1997; Tsaopoulos et al. 2007). As well, we hypothesised that safety footwear will increase the peak muscular demand when compared to the barefoot condition due to added mass on the foot.
2. Methods 2.1. Participants Twelve males (age 22.9 ± 1.7 years; mass 78.6 ± 9.3 kg; height 1.75 ± 0.06 m) participated in this study. Exclusion criteria consisted of any low back, shoulder or lower limb injury within the past five years that required medical intervention or time off from work for longer than three days, and/or having a self-reported shoe size outside of 9.5–10.5 M US. Exclusion criteria related to injuries were included to ensure participants were comfortable during transitions and the static kneeling posture, as low-back injuries can inhibit these movements and shoulder injuries could cause pain to participants during the kneeling task due to single arm support. Each participant read and signed an informed consent form approved by the university’s research ethics board.
2.2. Equipment Electromyographic (EMG) activity was monitored bilaterally from the lateral gastrocnemius (LG), medial gastrocnemius (MG), tibialis anterior (TA), semitendinosus (ST), biceps femoris (BF), vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM). Bipolar Ag/AgCl electrodes (BlueSensor N, Ambu Inc., Glen Burnie, MD, USA) were affixed following SENIAM guidelines (Hermens and Freriks 2005). Raw EMG signals were sampled at 2048 Hz (AMT8, Bortec, Calgary, AB, Canada; input impedance = 10 GΩ, built-in bandpass filter (10–1000 Hz), common mode rejection ratio (CMRR) = 120 dB at 60 Hz). Kinetic data were collected at 2048 Hz, synchronous to EMG data, using four embedded force plates (AMTI, Watertown, NY, USA) (Figure 1). For the purposes of this study, the leg which was planted and held the upper body mass during transition periods will be referred to as the ‘lead’ leg (Figure 1(A) – left leg); the opposing leg will be referred to as the ‘trail’ leg (Figure 1(A) – right leg). All participants were required to wear the same pair of size 10 M US safety footwear (Operator Steel Toe, Caterpillar Inc., Model: P709254). The mass of the boots was 1.24 kg each and the upper extended 0.16 m above the lateral malleolus.
2.3. Experimental design This study was a within-subject design in which initial footwear condition (shod or barefoot) was randomly assigned. Participants performed five sets of movements (order partially randomised) in the initial footwear condition, and then repeated five sets of the four movements in the second footwear condition (shod or barefoot). The four movements were split into two general groups, transitions and kneeled movements, for our statistical model. Our
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
Ergonomics
3
Figure 1. Participant performing movement trials. (A) Transitional movement: lead and trail legs are identified; (B) Static kneeling posture; (C) Kneeling task; (D) Dorsiflexed barefoot kneeling position; (E) Plantarflexed barefoot kneeing position. Ten of the twelve participants assumed kneeling position (D), the remaining two assumed kneeling position (E). Surface markers measuring kinematics of the lower limb were used for a separate analysis of centre of pressure changes relative to the knee (Tennant, Kingston, Chong, and Acker 2015).
movements were selected based on unpublished observations of floor layers and tile setters (field- and lab-based). The static kneeling task was a posture assumed by workers to rest the upper body and to perform brief calculations or planning steps, the dynamic kneeling task represented troweling, and the transitions involved similar patterns to those observed in these workers. Independent variables were movement performed and footwear. The dependent variable was peak EMG measured from each muscle.
2.4. Procedures Participant’s skin was prepared for EMG attachment following recommended guidelines (Hermens and Freriks 2005). Two repetitions of maximum voluntary contractions (MVC) were completed for each muscle group in accordance with SENIAM guidelines when possible (Hermens and Freriks 2005; Table 1). The data collection procedure involved two levels of randomisation. First, the initial footwear condition was randomised. Second, within each footwear condition, the order of movements was partially randomised. In order to balance the potential for fatigue and any discomfort that might be present during prolonged kneeling, the movements were grouped into three movement patterns, where the first movement was always a transition from standing to kneeling (stand-to-kneel), the second movement was either a dynamic kneeled task or a static kneeling task and the third movement was always a transition from kneeling to standing (kneel-to-stand). Since both the dynamic and static kneeling tasks require that the participant first
kneels down, before he can stand back up, the order of the two transitional movements (stand-to-kneel and kneel-tostand) could not be randomised, thus this second level of randomisation was not a complete randomisation. The stand-to-kneel transition was performed with participants starting off of the force plates, taking two steps forward on to the force plates, then planting their lead leg (Figure 1(A)) to transition down to the static kneeling posture (Figure 1(B)). The kneel-to-stand transition was performed in reverse order; participants started in static kneeling, lifted their lead leg to approximately a lunge position, then stood up and stepped back off of the force plates. The isolated, static kneeling posture had participants encouraged to rest with their buttocks on their heels when relaxed (Figure 1(B)). The kneeling task began and ended in the static kneeling position but required the participant to lean forward and firmly sweep a brush across the floor using a figure eight pattern while using the other hand as support (Figure 1(C)). This movement was selected to simulate a troweling movement commonly used by tile setters in occupational settings. Movement trials were 10 s long and had the participant kneeling directly onto the force plate without surface padding. Participants were given 30 s of rest, and asked if they required more rest, between each three-movement pattern in an effort to minimise the possibility of fatigue. The only restrictions on movements were that participants were instructed to rest on their heels when kneeling and to perform a single leg kneel or stand during the transitional movements (Figure 1(A)).
4
D. C. Kingston et al.
Table 1. MVC SENIAM guidelines (Hermens and Freriks 2005) for techniques used in this study. All contractions were performed twice separately for each leg. Muscle(s) Tibialis anterior
Gastrocnemii
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
Semitendinosus Biceps femoris
Vastus lateralis Rectus femoris Vastus medialis
SENIAM guidelines
Study protocol
• Support above the ankle joint • Ankle joint in dorsiflexion and the foot in inversion • Apply pressure against the medial dorsal surface of the foot in the direction of plantar flexion and eversion
• Participant seated on edge of a massage table with legs hanging over the side • Followed guideline
• Plantar flexion of the foot with emphasis on pulling the heel upward more than pushing the forefoot • Apply pressure against the forefoot as well as against the calcaneus
• Participant seated in leg press machine with a leg straight, but not locked at the knee • Attempt plantar flexion of the foot against weighted resistance
• Press against the leg proximal to the ankle in the direction of knee extension
• Participant prone on a massage table • Flex knee joint to an angle of ~65º with 0º being full extension • Attempt knee flexion while manually resisting in the direction of knee extension
• Extend the knee without rotating the thigh while applying pressure against the leg above the ankle in the direction of flexion
• Participant seated in leg extension exercise machine • Flex knee joint to an angle of ~45º with 0º being full extension • Attempt extension of the knee against weighted resistance
2.5. Data processing Custom Matlab software was used for data processing (The Mathworks Inc., Natick, MA, USA). Raw EMG data were full-wave rectified and low-pass filtered using a unidirectional 2nd-order Butterworth digital filter with a cut-off frequency of 2 Hz (Shultz, Perrin, and Adams 2001; Winter 2009) to produce a linear envelope. Respective maxima from linear enveloped MVC trials were determined and then used to normalize the linear envelopes of each muscle to a percentage of MVC (% MVC). Peak normalized EMG amplitude was identified from each movement, averaged across intra-subject trials, then ensemble averaged across subjects. All values are reported in mean % MVC ± 1 SD format. Vertical ground reaction force data, used to identify the start and end of some movements, were low-pass filtered using a bidirectional 2nd-order Butterworth digital filter with a 6-Hz cut-off frequency (Longpré, Potvin, and Maly 2013; Winter 2009). The start of stand-to-kneel was defined when the vertical ground reaction force on one of the anterior plates (Figure 1, anterior plates are the right pair in each window) exceeded 10 N, indicating initial contact between the lead foot and the force plate (preceding Figure 1(A)). The end of stand-to-kneel was defined when the lead leg VL activation remained below 5% MVC for one second (De Luca 1997), indicating the participant had come to rest in the static kneeling posture (Figure 1(B)). The start of kneel-tostand was defined when the vertical ground reaction force on the force plate under the lead knee dropped from the
resting level by 10 N, indicating the participant had initiated movement from the kneeling posture by beginning to lift the lead knee off the force plate. The end of standto-kneel was defined as the last instance in the trial when the vertical ground reaction force on the force plate under the lead leg fell below 10 N, indicating that both knees were now off the plates and the participant was about to step back to upright standing (after Figure 1(A)). In the simulated task (Figure 1(C)), the start of movement was defined when bilateral VL activation exceeded 5% MVC (De Luca 1997). The end of the movement was defined as the last instance when bilateral VL activation fell below 5% MVC. The entire collection trial was assessed for static kneeling (Figure 1(B)).
2.6. Statistical analysis Statistical analysis of peak EMG data was completed in SPSS 20 (IBM, Armonk, NY) using separate two-way repeated measures analysis of variance (RM ANOVA) for each movement group, transitional movements (standto-kneel and kneel-to-stand) and kneeled movements (static kneeling and kneeled task), with a series of eight RM ANOVA (one for each muscle) for each leg performed. Independent variables were movement performed and footwear. The dependent variable was peak EMG measured from each muscle. The alpha level for these comparisons was preset at 0.05 with Bonferroni adjustments to account for multiple comparisons performed in post hoc
Ergonomics
5
Table 2. Statistical results (p-values) from transitional movements two-way RM ANOVA.
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
Muscle LEAD TRAIL
TA LG MG BF ST VL RF VM TA LG MG BF ST VL RF VM
Main effect – Footwear 0.022* 0.281 0.56 0.431 0.276 0.863 0.669 0.761 0.124 0.013* 0.045* 0.094 0.07 0.331 0.585 0.261
Main effect – Movement 0.2 0.188 0.352 0.547 0.002* 0.001* 0.025* 0.002* 0.359 0.013* 0.826 0.67 0.003*
ISSN: 0014-0139 (Print) 1366-5847 (Online) Journal homepage: http://www.tandfonline.com/loi/terg20
Peak activation of lower limb musculature during high flexion kneeling and transitional movements David C. Kingston, Liana M. Tennant, Helen C. Chong & Stacey M. Acker To cite this article: David C. Kingston, Liana M. Tennant, Helen C. Chong & Stacey M. Acker (2016): Peak activation of lower limb musculature during high flexion kneeling and transitional movements, Ergonomics, DOI: 10.1080/00140139.2015.1130861 To link to this article: http://dx.doi.org/10.1080/00140139.2015.1130861
Published online: 29 Feb 2016.
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=terg20 Download by: [University of Saskatchewan Library]
Date: 01 March 2016, At: 09:29
Ergonomics, 2016 http://dx.doi.org/10.1080/00140139.2015.1130861
Peak activation of lower limb musculature during high flexion kneeling and transitional movements David C. Kingston , Liana M. Tennant, Helen C. Chong and Stacey M. Acker Department of Kinesiology, University of Waterloo, Waterloo, Canada
ARTICLE HISTORY
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
ABSTRACT
Few studies have measured lower limb muscle activation during high knee flexion or investigated the effects of occupational safety footwear. Therefore, our understanding of injury and disease mechanisms, such as knee osteoarthritis, is limited for these high-risk postures. Peak activation was assessed in eight bilateral lower limb muscles for twelve male participants, while shod or barefoot. Transitions between standing and kneeling had peak quadriceps and tibialis anterior (TA) activations above 50% MVC. Static kneeling and simulated tasks performed when kneeling had peak TA activity above 15% MVC but below 10% MVC for remaining muscles. In three cases, peak muscle activity was significantly higher (mean 8.9% MVC) when shod. However, net compressive knee joint forces may not be significantly increased when shod. EMG should be used as a modelling input when estimating joint contact forces for these postures, considering the activation levels in the hamstrings and quadriceps muscles during transitions.
Received 26 February 2015 Accepted 5 December 2015 KEYWORDS
EMG; knee; occupational ergonomics; footwear; kneeling
Practitioner Summary: Kneeling transitional movements are used in activities of daily living and work but are linked to increased knee osteoarthritis risk. We found peak EMG activity of some lower limb muscles to be over 70% MVC during transitions and minimal influence of wearing safety footwear.
1. Introduction There is a lack of knowledge regarding peak muscular demand in high flexion knee postures. High flexion is defined as postures exceeding 120° at the knee joint (0° being full knee extension), and has been reported to a maximum of 165° (Acker et al. 2011; Hefzy, Kelly, and Cooke 1998). These postures are common when performing activities of daily living (e.g. prayer, childcare) and in industrial occupations (e.g. roofing, flooring). While knee joint kinematics and compressive loads have been studied in high flexion knee postures for arthroplasty research (Kurita et al. 2012; Mündermann et al. 2008; Niki et al. 2013), previous work (Nagura et al. 2006; Yang, Wickwire, and Debski 2010) has identified that information available on the activation of lower limb musculature during kneeling or transitional movements is limited (one study: Gallagher, Pollard, and Porter 2011). This lack of knowledge regarding transitions to or from kneeling postures poses a major challenge in the development of high flexion models and the understanding of acute injury in high flexion. In addition, the applicability of data from total knee arthroplasty patients to healthy CONTACT Stacey M. Acker © 2016 Taylor & Francis
[email protected]
working populations is questionable due to age and physical health considerations. Considerations for transitional movements are vital as workers exposed to getting up from kneeling more than 30 times a day, kneeling with heavy loads and kneeling for greater than one hour in their day have odds ratios of 2.3, 2.9 and 3.0, respectively, for the development of knee osteoarthritis (Coggon et al. 2000; Jensen 2008). With unknown levels of muscle activation during high flexion, it is difficult to incorporate muscle forces into high flexion knee joint modelling without the need for many assumptions in simulations (Hirokawa and Fukunaga 2013; Kim et al. 2009; Komistek et al. 2005; Lin et al. 2010; Nagura et al. 2006; Winby et al. 2009). Muscle activation is crucial for determining the contact forces acting on knee articular surfaces since, in most dynamic activities and in some static scenarios, muscles are the greatest contributors to peak force. (Andriacchi and Favre 2014). Overall, elevated joint contact forces are primarily the cause of knee joint degeneration which leads to osteoarthritis (Andriacchi and Favre 2014; Coggon et al. 2000; Kirkeshov Jensen 2005). A number of mathematical models exist which use inverse dynamics to estimate the joint contact force
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
2
D. C. Kingston et al.
occurring at the knee during high flexion (Arnold et al. 2010; Smith et al. 2008; Winby et al. 2009). Many of these models have been validated against instrumented knee implants with the common issue of under-predicting joint loads due, at least in part, to the absence of muscle activation data (Kim et al. 2009; Stylianou, Guess, and Kia 2013; Zhao et al. 2007). Although computer simulations of muscle forces have been completed (Arnold et al. 2013; Lin et al. 2010), no in vivo studies have characterised peak muscle activation during movements to high flexion postures. The peak muscular activation levels presented in this study will contribute to the modelling knowledge base by providing insight into the maximal demand high knee flexion activities pose. Considerable research has focused on industrial demands, but no previous work has measured the effect safety footwear may have on muscle activation during high flexion postures. In many industrial settings, personal protective equipment standards dictate that safety footwear must be worn when working. The mass of safety footwear could increase muscular demand during transitional movements as increased activation would be required to resist the added moment created by the footwear mass at the distal end of the segmental chain. As well, the stiffness of the boot upper, toecap and sole may alter normal ankle range of motion and result in different high flexion postures at the knee, and corresponding muscle activations, than when unshod. These considerations warrant measurement of muscle activation while wearing safety footwear to best represent working conditions. Since peak activation levels of the leg muscles are unknown in high flexion positions, there is limited understanding of the mechanisms through which injury and disease, such as knee osteoarthritis, may occur. In addition, the quantification of peak muscle activation levels during activities of daily living and occupationally relevant movement patterns will identify if particular movements and/or safety footwear pose unique considerations for models of the knee. Therefore, the aim of this study was to evaluate the effect of footwear on lower limb muscular activation during high knee flexion movements. In the pursuit of this aim, peak lower limb muscle activity during high knee flexion postures and movements were quantified, which will add to the limited knowledge of lower limb muscle activity in high flexion. We hypothesised that upward kneeling transitions, from kneeling to standing, will require increased muscle activations when compared to downward transitions due to the upward vertical acceleration of the body’s mass (Isear, Erickson, and Worrell 1997; Tsaopoulos et al. 2007). As well, we hypothesised that safety footwear will increase the peak muscular demand when compared to the barefoot condition due to added mass on the foot.
2. Methods 2.1. Participants Twelve males (age 22.9 ± 1.7 years; mass 78.6 ± 9.3 kg; height 1.75 ± 0.06 m) participated in this study. Exclusion criteria consisted of any low back, shoulder or lower limb injury within the past five years that required medical intervention or time off from work for longer than three days, and/or having a self-reported shoe size outside of 9.5–10.5 M US. Exclusion criteria related to injuries were included to ensure participants were comfortable during transitions and the static kneeling posture, as low-back injuries can inhibit these movements and shoulder injuries could cause pain to participants during the kneeling task due to single arm support. Each participant read and signed an informed consent form approved by the university’s research ethics board.
2.2. Equipment Electromyographic (EMG) activity was monitored bilaterally from the lateral gastrocnemius (LG), medial gastrocnemius (MG), tibialis anterior (TA), semitendinosus (ST), biceps femoris (BF), vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM). Bipolar Ag/AgCl electrodes (BlueSensor N, Ambu Inc., Glen Burnie, MD, USA) were affixed following SENIAM guidelines (Hermens and Freriks 2005). Raw EMG signals were sampled at 2048 Hz (AMT8, Bortec, Calgary, AB, Canada; input impedance = 10 GΩ, built-in bandpass filter (10–1000 Hz), common mode rejection ratio (CMRR) = 120 dB at 60 Hz). Kinetic data were collected at 2048 Hz, synchronous to EMG data, using four embedded force plates (AMTI, Watertown, NY, USA) (Figure 1). For the purposes of this study, the leg which was planted and held the upper body mass during transition periods will be referred to as the ‘lead’ leg (Figure 1(A) – left leg); the opposing leg will be referred to as the ‘trail’ leg (Figure 1(A) – right leg). All participants were required to wear the same pair of size 10 M US safety footwear (Operator Steel Toe, Caterpillar Inc., Model: P709254). The mass of the boots was 1.24 kg each and the upper extended 0.16 m above the lateral malleolus.
2.3. Experimental design This study was a within-subject design in which initial footwear condition (shod or barefoot) was randomly assigned. Participants performed five sets of movements (order partially randomised) in the initial footwear condition, and then repeated five sets of the four movements in the second footwear condition (shod or barefoot). The four movements were split into two general groups, transitions and kneeled movements, for our statistical model. Our
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
Ergonomics
3
Figure 1. Participant performing movement trials. (A) Transitional movement: lead and trail legs are identified; (B) Static kneeling posture; (C) Kneeling task; (D) Dorsiflexed barefoot kneeling position; (E) Plantarflexed barefoot kneeing position. Ten of the twelve participants assumed kneeling position (D), the remaining two assumed kneeling position (E). Surface markers measuring kinematics of the lower limb were used for a separate analysis of centre of pressure changes relative to the knee (Tennant, Kingston, Chong, and Acker 2015).
movements were selected based on unpublished observations of floor layers and tile setters (field- and lab-based). The static kneeling task was a posture assumed by workers to rest the upper body and to perform brief calculations or planning steps, the dynamic kneeling task represented troweling, and the transitions involved similar patterns to those observed in these workers. Independent variables were movement performed and footwear. The dependent variable was peak EMG measured from each muscle.
2.4. Procedures Participant’s skin was prepared for EMG attachment following recommended guidelines (Hermens and Freriks 2005). Two repetitions of maximum voluntary contractions (MVC) were completed for each muscle group in accordance with SENIAM guidelines when possible (Hermens and Freriks 2005; Table 1). The data collection procedure involved two levels of randomisation. First, the initial footwear condition was randomised. Second, within each footwear condition, the order of movements was partially randomised. In order to balance the potential for fatigue and any discomfort that might be present during prolonged kneeling, the movements were grouped into three movement patterns, where the first movement was always a transition from standing to kneeling (stand-to-kneel), the second movement was either a dynamic kneeled task or a static kneeling task and the third movement was always a transition from kneeling to standing (kneel-to-stand). Since both the dynamic and static kneeling tasks require that the participant first
kneels down, before he can stand back up, the order of the two transitional movements (stand-to-kneel and kneel-tostand) could not be randomised, thus this second level of randomisation was not a complete randomisation. The stand-to-kneel transition was performed with participants starting off of the force plates, taking two steps forward on to the force plates, then planting their lead leg (Figure 1(A)) to transition down to the static kneeling posture (Figure 1(B)). The kneel-to-stand transition was performed in reverse order; participants started in static kneeling, lifted their lead leg to approximately a lunge position, then stood up and stepped back off of the force plates. The isolated, static kneeling posture had participants encouraged to rest with their buttocks on their heels when relaxed (Figure 1(B)). The kneeling task began and ended in the static kneeling position but required the participant to lean forward and firmly sweep a brush across the floor using a figure eight pattern while using the other hand as support (Figure 1(C)). This movement was selected to simulate a troweling movement commonly used by tile setters in occupational settings. Movement trials were 10 s long and had the participant kneeling directly onto the force plate without surface padding. Participants were given 30 s of rest, and asked if they required more rest, between each three-movement pattern in an effort to minimise the possibility of fatigue. The only restrictions on movements were that participants were instructed to rest on their heels when kneeling and to perform a single leg kneel or stand during the transitional movements (Figure 1(A)).
4
D. C. Kingston et al.
Table 1. MVC SENIAM guidelines (Hermens and Freriks 2005) for techniques used in this study. All contractions were performed twice separately for each leg. Muscle(s) Tibialis anterior
Gastrocnemii
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
Semitendinosus Biceps femoris
Vastus lateralis Rectus femoris Vastus medialis
SENIAM guidelines
Study protocol
• Support above the ankle joint • Ankle joint in dorsiflexion and the foot in inversion • Apply pressure against the medial dorsal surface of the foot in the direction of plantar flexion and eversion
• Participant seated on edge of a massage table with legs hanging over the side • Followed guideline
• Plantar flexion of the foot with emphasis on pulling the heel upward more than pushing the forefoot • Apply pressure against the forefoot as well as against the calcaneus
• Participant seated in leg press machine with a leg straight, but not locked at the knee • Attempt plantar flexion of the foot against weighted resistance
• Press against the leg proximal to the ankle in the direction of knee extension
• Participant prone on a massage table • Flex knee joint to an angle of ~65º with 0º being full extension • Attempt knee flexion while manually resisting in the direction of knee extension
• Extend the knee without rotating the thigh while applying pressure against the leg above the ankle in the direction of flexion
• Participant seated in leg extension exercise machine • Flex knee joint to an angle of ~45º with 0º being full extension • Attempt extension of the knee against weighted resistance
2.5. Data processing Custom Matlab software was used for data processing (The Mathworks Inc., Natick, MA, USA). Raw EMG data were full-wave rectified and low-pass filtered using a unidirectional 2nd-order Butterworth digital filter with a cut-off frequency of 2 Hz (Shultz, Perrin, and Adams 2001; Winter 2009) to produce a linear envelope. Respective maxima from linear enveloped MVC trials were determined and then used to normalize the linear envelopes of each muscle to a percentage of MVC (% MVC). Peak normalized EMG amplitude was identified from each movement, averaged across intra-subject trials, then ensemble averaged across subjects. All values are reported in mean % MVC ± 1 SD format. Vertical ground reaction force data, used to identify the start and end of some movements, were low-pass filtered using a bidirectional 2nd-order Butterworth digital filter with a 6-Hz cut-off frequency (Longpré, Potvin, and Maly 2013; Winter 2009). The start of stand-to-kneel was defined when the vertical ground reaction force on one of the anterior plates (Figure 1, anterior plates are the right pair in each window) exceeded 10 N, indicating initial contact between the lead foot and the force plate (preceding Figure 1(A)). The end of stand-to-kneel was defined when the lead leg VL activation remained below 5% MVC for one second (De Luca 1997), indicating the participant had come to rest in the static kneeling posture (Figure 1(B)). The start of kneel-tostand was defined when the vertical ground reaction force on the force plate under the lead knee dropped from the
resting level by 10 N, indicating the participant had initiated movement from the kneeling posture by beginning to lift the lead knee off the force plate. The end of standto-kneel was defined as the last instance in the trial when the vertical ground reaction force on the force plate under the lead leg fell below 10 N, indicating that both knees were now off the plates and the participant was about to step back to upright standing (after Figure 1(A)). In the simulated task (Figure 1(C)), the start of movement was defined when bilateral VL activation exceeded 5% MVC (De Luca 1997). The end of the movement was defined as the last instance when bilateral VL activation fell below 5% MVC. The entire collection trial was assessed for static kneeling (Figure 1(B)).
2.6. Statistical analysis Statistical analysis of peak EMG data was completed in SPSS 20 (IBM, Armonk, NY) using separate two-way repeated measures analysis of variance (RM ANOVA) for each movement group, transitional movements (standto-kneel and kneel-to-stand) and kneeled movements (static kneeling and kneeled task), with a series of eight RM ANOVA (one for each muscle) for each leg performed. Independent variables were movement performed and footwear. The dependent variable was peak EMG measured from each muscle. The alpha level for these comparisons was preset at 0.05 with Bonferroni adjustments to account for multiple comparisons performed in post hoc
Ergonomics
5
Table 2. Statistical results (p-values) from transitional movements two-way RM ANOVA.
Downloaded by [University of Saskatchewan Library] at 09:29 01 March 2016
Muscle LEAD TRAIL
TA LG MG BF ST VL RF VM TA LG MG BF ST VL RF VM
Main effect – Footwear 0.022* 0.281 0.56 0.431 0.276 0.863 0.669 0.761 0.124 0.013* 0.045* 0.094 0.07 0.331 0.585 0.261
Main effect – Movement 0.2 0.188 0.352 0.547 0.002* 0.001* 0.025* 0.002* 0.359 0.013* 0.826 0.67 0.003*
