Correlates of the shift in heart rate variability with postures and walking by time-frequency analysis

Heart rate (HR) variability derived from electrocardiogram (ECG) can be used to assess the function of the autonomic nervous system. HR exhibits various characteristics during different physical activities attributed to the altered autonomic mediation, where it is also beneficial to reveal the autonomic shift in response to physical-activity change. In this paper, the physical-activity-related HR behaviors were delineated using a portable ECG and body acceleration recorder based on a personal digital assistant and the smoothed pseudo Wigner-Ville distribution. The results based upon eighteen subjects performing four sequential 5-min physical activities (supine, sitting, standing and spontaneous walking) showed that the high-frequency heartbeat fluctuations during supine and sitting were significantly larger than during standing, and that the ratio of low- to high-frequency fluctuation during standing was significantly higher than during supine and sitting. This could be linked with the parasympathetic predominance during supine and sitting, and a shift to sympathetic dominance while standing. During spontaneous walking, the high-frequency fluctuation was significant lower than during supine. The low- to high-frequency ratio decreased significantly from standing to spontaneous walking, which may imply an increased vagal predominance (autonomic effect) or an increased respiratory activity (mechanical effect).     

Use of visual perception in estimating static postural stresses: magnitudes and sources of errors

Very little is known about the magnitudes and sources of errors associated with the visual estimation of postural classification displayed on TV screens. This study was conducted to address this issue. Sixty-three subjects participated in the experiments. The findings indicate that: (1) subjects found it difficult to evaluate upper extremity postures (particularly the elbow and the wrist), while the postures around the lower back were the easiest to evaluate; (2) the lower extremity positions affected the ability of the subjects to accurately classify postures around the wrist, elbow, shoulder, neck, and lower back, with the estimates being > 70% for sitting and > 60% for standing (except for the elbow); and (3) in general, flexion and extension are easier to evaluate than neutral and non-neutral postures.     

Heart Rate and Small Postural Changes in Man

Heart rates of eleven normal male subjects were recorded in a number of different sitting and standing postures. Three of the sitting postures were recorded both with and without an attention-demanding task. Heart rates differed significantly among many of the postures but not between task conditions.     

Subjective response to whole-body vibration The effects of posture

In an investigation of the effects of posture on subjective responses to whole-body vibration, 20 undergraduate subjects produced equal sensation contours adopting three postures each on different occasions. The postures adopted were standing, sitting upright and sitting slouched.

The results indicated significant differences in the contour shapes from the three postures, and the level set in the sitting postures were significantly lower than in the standing posture. No difference was obtained between the two sitting postures.

Implications of these findings are discussed regarding the role of transmissibility in subjective response to vibration, and the necessity to produce different standards for different postures.     

Is active sitting as active as we think?

The aim of this study was to compare the biomechanical characteristics of sitting on a stool without a backrest (so as to encourage active sitting), sitting on a conventional office chair and standing in healthy participants. Thirteen healthy participants performed a keyboard-writing task during four (stable and unstable) sitting conditions and standing. Body segment positions and posture, postural sway and muscle activity of neck and trunk muscles were assessed with a motion capture system, a force plate and surface electromyography. The results showed that body segment positions, postural sway and trunk muscle activity were relatively similar for the stools without backrests compared with standing. All sitting conditions showed lower vertical upper body alignment, less anterior pelvic tilt and larger hip angles, compared with standing (p = 0.000). Unexpectedly, the muscle activity levels and total postural sway, sway velocity and sway in M/L and A/P directions were lower (p = 0.000) for the conditions that encouraged active sitting and standing, compared with the conventional office chair conditions.

Practitioner Summary: Thirteen healthy participants performed a keyboard-writing task during different sitting conditions and standing and were analysed regarding posture, postural sway and trunk muscle activity. Surprisingly, less postural sway and less muscle activity were observed during the conditions that encourage active sitting, compared with sitting on a conventional office chair.     

Studying the relationship between low back pain and working postures among those who stand and those who sit most of the working day

A relationship between low back pain (LBP) and prolonged standing or prolonged sitting at work has not been clearly shown, despite its biological plausibility. Because sitting and standing postures vary as to duration and freedom to alternate postures, and standing postures vary as to mobility, associations between specific working postures and LBP were explored using multiple logistic regression. Associations between work factors and self-reported LBP during the previous 12 months that interfered with usual activities were examined among 4493 standing workers and 3237 sitting workers interviewed in the 1998 Quebec Health and Social Survey; 24.5% reported significant LBP. Since the same conditions can correspond to different physiological demands for sitting compared with standing workers, analyses were performed separately for the two groups. Standing without freedom to sit was associated with LBP. Different occupational physical and psychosocial factors were associated with LBP in sitting compared with standing populations.     

Validity of self-reported duration of work postures obtained by interview

The aim of the study was to validate interview data concerning the duration of four work postures (1) sitting, (2) standing/walking with hands above shoulder level, (3) standing/walking with hands between shoulder and knuckle level, and (4) standing/walking with hands below knuckle level. The self-reported time spent in each posture was tested in relation to observations and technical measurements in 20 subjects during two full working days. The linear relationships between self-reports and observations were strong for the three postures; sitting (r²=0.55), hands above shoulder level (r²=0.58) and hands below knuckle level (r²=0.69). Thus, using this interview technique, self-reports concerning time spent in (1) sitting, (2) standing/walking with hands above shoulder level and, (3) standing/walking with hands below knuckle level may be accurate enough for studying these work postures in epidemiological studies.     

Validity of self-reported duration of work postures obtained by interview. MUSIC-Norrtälje Study Group

The aim of the study was to validate interview data concerning the duration of four work postures (1) sitting, (2) standing/walking with hands above shoulder level, (3) standing/walking with hands between shoulder and knuckle level, and (4) standing/walking with hands below knuckle level. The self-reported time spent in each posture was tested in relation to observations and technical measurements in 20 subjects during two full working days. The linear relationships between self-reports and observations were strong for the three postures; sitting (r2 = 0.55), hands above shoulder level (r2 = 0.58) and hands below knuckle level (r2 = 0.69). Thus, using this interview technique, self-reports concerning time spent in (1) sitting, (2) standing/walking with hands above shoulder level and, (3) standing/walking with hands below knuckle level may be accurate enough for studying these work postures in epidemiological studies.     

Forward sloping chair effects on spinal shape in the Hong Kong Chinese and Indian populations

Forward sloping seats are universally accepted based on their increased trunk-thigh angle during sitting. However, these seats are not preferred by some individuals due to reasons such as excessive pressure on knees, difficulties during ingress and egress, and postural fixity during sitting. Some researchers have claimed that forward sloped sitting preserves the lumbar lordosis, thereby making it more comfortable for the sitter. This claim has not been validated across all populations and, therefore, appears to have some disagreement among researchers. In this study, spinal shape during standing and sitting in forward sloping chairs is measured and quantified using a three-dimensional sonic digitizer. Twenty subjects (ten Hong Kong Chinese and ten Indian) have participated in the experiment. Fifteen points on the spine are digitized during standing and sitting in a forward-sloping seat with trunk–thigh angles of 70°, 80°, 90°, 100°, 110°, and 120°. Different measures are used to analyze and differentiate the spinal shape. The correlation between the length of spine during standing and a subject's height is low, but significant. The behavior of the spinal shape change during sitting differs between the populations as shown by the maximum lumbar and maximum thoracic deviations. The Indian subjects seem to approach the standing curvatures in the thoracic region during 30° forward sloping sitting. The Hong Kong Chinese subjects, on the other hand, do not show any resemblance to the standing curvatures during forward sloping sitting. One possible reason could be the differences in arch angle between the two populations. The variations in spinal shape among subjects appear to be similar within a population.Relevance to industryForward sloping seats may not be appropriate for all populations since changes in the spinal shape differ between populations. The arch angle may be an indicator and possibly a predictor of the appropriateness of forward sloping seats for different populations, if spinal shape is related to sitter discomfort.     

Hip joint mobility and spinal angles in standing and in different sitting postures

The spinal angles of 25 female subjects were measured in standing and in four different sitting postures with various trunk-thigh angles. Additionally, measurements of hip and lumbar mobility were made in an attempt to relate mobility to spinal curvature in the different sitting postures. Lumbar curvature was observed to decrease as the trunk-thigh angle decreased across the different sitting postures, and reduction in curvature was associated with hip mobility. An exploratory analysis of interrelationships among hip mobility, spinal mobility, and spinal curvature was also carried out. A significant correlation between lumbar and thoracic angular deviations in standing and hip flexion/extension range was obtained. The findings are discussed with reference to theories of sitting posture and the influence of hip joint mobility on postural adaptations to furniture. Further investigations, including studies of male subjects, are indicated.     

An electromyographic analysis of seated and standing lifting tasks.

The objective of this project was to compare the muscular effort exerted during manual lifting tasks performed in standing versus seated posture. Six male undergraduate and graduate students performed 12 different static and dynamic lifts in both sitting and standing positions. During each effort electromyographic (EMG) data were collected on four muscles groups (low back, upper back, shoulder, and abdominals). Four contractions were designed to elicit maximum muscular effort in the four groups being monitored. The remaining data were then expressed as a percentage of maximum EMG. Each subject performed the following: maximum static lift when sitting; maximum static lift when standing; sitting, static lift with 15.9 kg; standing, static lift with 15.9 kg; dynamic sit-forward lift with 15.9 kg, dynamic stand-forward lift with 15.9 kg, dynamic sit-twist with 15.9 kg, dynamic stand-vertical lift with 15.9 kg. Each of the lifts was performed with a wooden tray with slotted handles. Root mean square (RMS) values of the EMG data were calculated for three second periods. EMG activity in the low back, upper back, and shoulder was greater during sitting lifting than during standing lifting. The sit-twist lift resulted in the highest EMG in the abdominal muscles. Dynamic lifts resulted in more muscle activity than did static lifts. From these data it was concluded that sitting-lifting results in greater stress in the low back, upper back, and shoulders than does lifting while standing.     

The effects of knee angles on subjective discomfort ratings, heart rates, and muscle fatigue of lower extremities in static-sustaining tasks

The purpose of this study was to investigate the effects of knee-flexion angles on subjective discomfort ratings, heart rates, and muscle fatigue using median frequency (MDF) in a static-sustaining task. Thirty healthy participants maintained 13 postures including standing, squatting, sitting, and kneeling postures and then MDFs from the erector spinae, biceps femoris, vastus medialis, gastrocnemius, and tibialis anterior muscles, subjective discomfort, and heart rates were collected every 3 min during a sustained 15 min task. Results showed that the discomfort, heart rate and muscle fatigue were significantly influenced by the body postures. In general, standing and sitting postures showed less discomfort as well as lower heart rates, whereas squatting postures (KF120, KF90, KF60) had higher discomfort and heart rates. Three MDF change trends were reported associated with postures in this study. First, there were less changes of MDFs for standing and sitting postures; second, all patterns of MDFs for KF 150 and KF120 decreased, and lastly some MDFs had increasing trends and others showed decreasing trends for KF30, KF30T, and kneeling postures.     

An electromyographic analysis of seated and standing lifting tasks

The objective of this project was to compare the muscular effort exerted during manual lifting tasks performed in standing versus seated posture. Six male undergraduate and graduate students performed 12 different static and dynamic lifts in both sitting and standing positions. During each effort electromyographic (EMG) data were collected on four muscles groups (low back, upper back, shoulder, and abdominals). Four contractions were designed to elicit maximum muscular effort in the four groups being monitored. The remaining data were then expressed as a percentage of maximum EMG. Each subject performed the following: maximum static lift when sitting; maximum static lift when standing; sitting, static lift with 15·9 kg; standing, static lift with 15·9 kg; dynamic sit-forward lift with 15·9 kg, dynamic stand-forward lift with 15·9 kg, dynamic sit-twist with 15·9 kg, dynamic stand-vertical lift with 15·9 kg. Each of the lifts was performed with a wooden tray with slotted handles. Root mean square (RMS) values of the EMG data were calculated for three second periods. EMG activity in the low back, upper back, and shoulder was greater during sitting lifting than during standing lifting. The sit-twist lift resulted in the highest EMG in the abdominal muscles. Dynamic lifts resulted in more muscle activity than did static lifts. From these data it was concluded that sitting-lifting results in greater stress in the low back, upper back, and shoulders than does lifting while standing.     

The effects of knee angles on subjective discomfort ratings,heart rates,and muscle fatigue of lower extremities in static-sustaining tasks

The purpose of this study was to investigate the effects of knee-flexion angles on subjective discomfort ratings, heart rates, and muscle fatigue using median frequency (MDF) in a static-sustaining task. Thirty healthy participants maintained 13 postures including standing, squatting, sitting, and kneeling postures and then MDFs from the erector spinae, biceps femoris, vastus medialis, gastrocnemius, and tibialis anterior muscles, subjective discomfort, and heart rates were collected every 3 min during a sustained 15 min task. Results showed that the discomfort, heart rate and muscle fatigue were significantly influenced by the body postures. In general, standing and sitting postures showed less discomfort as well as lower heart rates, whereas squatting postures (KF120, KF90, KF60) had higher discomfort and heart rates. Three MDF change trends were reported associated with postures in this study. First, there were less changes of MDFs for standing and sitting postures; second, all patterns of MDFs for KF 150 and KF120 decreased, and lastly some MDFs had increasing trends and others showed decreasing trends for KF30, KF30T, and kneeling postures.     

The need to accommodate monitor height changes between sitting and standing

Abstract

Recently, the rate at which sit-to-stand workstations are being introduced into the workplace has seen a dramatic increase. Aside from adjusting the height of the desk when transitioning from sitting to standing, there is a lack of literature regarding the necessary adjustments to other equipment associated with the workstation. To better understand some of these limitations, 16 participants stood and sat at a sit-to-stand workstation while adhering to current Canadian Standards Association (CSA) Guidelines for Office Ergonomics. Transitioning between sitting and standing while adhering to CSA guidelines resulted in a 3.9?cm difference in monitor height relative to work surface between sitting and standing. Results from this investigation support the notion that monitor height adjustments relative to the work surface are necessary when utilising sit-to-stand workstations – with the implementation to practice message being that both sitting and standing configurations need to be assessed and accommodated in the workstation configuration.

Practitioner summary: Limited guidelines exist in the necessary adjustments to equipment associated with sit-to-stand workstations. Transitioning between sitting and standing resulted in a 3.9?cm difference in monitor height relative to work surface between sitting and standing. This supports that monitor height adjustments relative to the workstation are necessary when using sit-to-stand workstations.     

A comparative investigation of the influence of certain arm positions on hand pinch grips in the standing and sitting positions of dentists

Hand pinch grips in the standing and sitting positions on a group of 46 healthy males of 20 to 26 years old were measured. The results were as follows: 1. Hand pinch grip forces are higher when the subject is standing than when he is sitting. 2. Hand pinch grip forces, depending on the position of the arm in the working space, are higher when the arm is supported than the corresponding forces of the unsupported arm. 3. There is an effect due to the position of the arm in relation to the frontal position of the subject's thorax. In the standing position, the forces are maximum when the forearm has a 60 degrees angle towards the frontal position, while in the sitting position pinch grip forces are maximum when the forearm is perpendicular (90 degrees ) to the frontal position. 4. A handle which permits all fingers to be spread in a pinch grip is capable of having an applied force 50% greater than if the thumb and either forefinger or middlefinger grips the handle. In such a handle each finger is required to apply less force to contribute to the total needed for the task, and therefore there is a diminished likelihood of the onset of fatigue.     

Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible -- particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p<0.001) low back compressive loads (mean +/- SD 1698 +/- 467 N) than those experienced by the lumbar spine during standing (1076 +/- 243 N). Subjects were equally divided into adopting one of two sitting strategies: a single 'static' or a 'dynamic' multiple posture approach. Within each individual, standing produced a distinctly different spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.     

Where Does the Mouse Go? An Investigation into the Placement of a Body-Attached TouchPad Mouse for Wearable Computers

We investigated the effects of placement of a TouchPad input device on a user's body for the control of a wearable computer. This study involved 25 subjects performing selection tasks with a TouchPad mouse while wearing a wearable computer on their back and using a head-mounted display. Each subject performed the tasks in 27 different combinations of four postures (sitting, kneeling, standing and prone) and seven different placements of the TouchPad mouse on the subject's body (forearm, thigh by 2, torso by 2, and upper arm by 2). We measured the time and error rate to complete the selection of a circular target. The results for the effects due to posture showed that there were similar time effects for sitting, standing and kneeling. When examining the effects resulting from mouse position, the front of the thigh was shown to be the best position of the mouse. When the posturing and mouse position conditions were combined, the results indicated that the thigh front mouse position would be most appropriate for sitting, kneeling and standing postures, and the forearm mouse position would be best for the prone position.     

Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible—particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p< 0.001) low back compressive loads (mean±SD 1698±467 N) than those experienced by the lumbar spine during standing (1076±243 N). Subjects were equally divided into adopting one of two sitting strategies: a single ‘static’ or a ‘dynamic’ multiple posture approach. Within each individual, standing produced a distinctly diVerent spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level diVerences in the individuals who adopted diVerent sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.