Alignment of the human body in standing

Alignment of the body in typical symmetrical standing was studied by photographing fifteen subjects in profile on a reaction board. Two aspects of alignment were studied: (1) the anteroposterior position of the body landmarks of knee joint, hip joint, shoulder joint, and ear, compared to the ankle joint; and (2) the positions of the partial centers of gravity above the knee and hip, as a measure of how the body is balanced above these joints. The knee, hip, shoulder, and ear were forward of the ankle in all subjects. On average, the knee was 3.8 (+/- 2.0), the hip 6.2 (+/- 1.3) the shoulder 3.8 (+/- 1.9), and the ear 5.9 (+/- 1.6) cm (+/-S.D.) anterior to the ankle. The positions of landmarks were positively correlated with one another but not highly. The position of the center of gravity could be predicted well from the positions of the landmarks within individual subjects' data, but not across subjects. The centers of gravity above the knee and hip were calculated by subtracting the mass and position of the segments below the joint from the whole-body center of gravity. The center of gravity above the knee was located on average 1.4 (+/- 1.1) cm in front of the joint, and that of the hip 1.0 (+/- 1.6) cm behind the trochanter. Thus, at both knee and hip in typical standing, there exist slight gravitational torques tending to extend the joints.     

The influence of foot position on standing balance

To test the hypothesis that variations in foot position would significantly affect standing balance, we studied ten normal subjects on a Kistler force platform which measured the travel and center of pressure displacement. With the feet together there was substantially more mediolateral (ML) travel than with the axes of the feet 15, 30 or 45 cm apart and the mean ML position of the center of pressure was displaced toward the right; there was no consistent effect on anteroposterior (AP) travel or position. As the right foot was placed 10 and 30 cm forward or back, the least amount of ML and AP travel occurred with the feet even or at 10 cm either direction; the mean AP and ML position moved toward the foot which was placed more posteriorly. Of the five foot angles ranging from toes-out 45 degrees to toes-in 45 degrees, the extent of ML and AP travel was lowest in the toes-out 25 degrees position and greatest in the toes-in 45 degrees position; the mean AP and ML position was farthest forward and to the right with toes-in 45 degrees. These findings have implications for the prosthetic replacement of the lower limbs, sports, ergonomics and postural sway studies.     

Large postural fluctuations but unchanged postural sway dynamics during tiptoe standing compared to quiet standing

The purpose of this study was to detect the characteristics of center of pressure (COP) movement during tiptoe standing (TS) compared to quiet standing (QS). Eight healthy subjects were asked to perform QS and TS on a force platform. During standing, surface electromyograms (EMGs) were recorded from the soleus (SOL), flexor hallucis brevis (FHB), medial gastrocnemius (MG), lateral gastrocnemius (LG), and tibialis anterior (TA) muscles. The path length and rectangular area of the COP trajectory were significantly larger during TS than during QS. In contrast, irrespective of standing condition, the scaling coefficients in the short and long regions were above and below 0.5, respectively. The coherence spectrum between the COP and EMG from the SOL and FHB muscles was statistically significant during TS at frequencies up to 17 Hz, while that for the QS was only significant below 1 Hz. In conclusion, the control of COP movement during TS was similar to that during QS despite large COP fluctuations during TS. Our results suggest that unstable posture during TS is compensated for by the activities of the SOL and FHB muscles, which enhance postural control.     

Assessment of neuromuscular activity during maximal isometric contraction in supine vs standing body positions

BackgroundWhen comparing neuromuscular activity between different individuals or different conditions by use of surface electromyography (sEMG) it is necessary to apply standardized assessment protocol. Most frequently used method is the maximum voluntary isometric contraction (MVIC). However, the influence of body posture on sEMG activity during MVIC testing remains largely unknown.AimTo evaluate the MVIC method for sEMG normalization in supine versus standing positions for selected muscles of the lower extremity and trunk.MethodsTwelve healthy individuals participated; five females and seven males (age 22–51 yrs). sEMG signals were recorded bilaterally from mm tibialis anterior, gluteus medius, adductor longus, rectus abdominus, external oblique and internal oblique/transversus abdominus according to standardized test protocol. Two different body positions were used: supine and standing position.ResultsMVIC peak sEMG signal amplitudes did not differ systematically between supine and standing test positions. Pronounced inter-subject variability in MVIC reference sEMG activity were observed between participants, during both supine and standing test positions.ConclusionPresent data demonstrate that MVIC EMG normalization is a biomechanically stable procedure that can be performed in a reproducible manner for the major leg and trunk muscles when comparing supine vs. standing test positions.     

Influences of illusionary position perception on anticipatory postural control associated with arm flexion

We examined the effect of illusionary perception on anticipatory postural control associated with arm flexion with subjects in a standing position, using vibration stimulation of the Achilles’ tendon. Arm flexion was performed five times under each of the following conditions: (1) quiet standing, (2) vibration of the Achilles’ tendon at 100 Hz frequency and 1.5 mm amplitude with the trunk fixed by a stopper during quiet standing, and (3) a perceived standing position during vibration. The reproduced positions were located forward by about 20% of the foot length compared with the quiet standing position; these positions showed no significant differences among the five trials. In the first trial of arm flexion during vibration, the biceps femoris began activating approximately 40 ms before the anterior deltoid. The same time difference between activation of the two muscles was observed in the reproduced condition. As the vibration trials were repeated, this activation timing approached the value in the quiet standing condition. In both the biceps femoris and erector spinae, the mean amplitude of electromyogram for the first 50 ms after the start of activation did not differ significantly among the three conditions.     

Internal reference frame for the representation and storage of visual information during standing

The roles of different afferent systems in the organization of an internal reference frame was studied. The task of visual comparison was performed by subjects under different experimental conditions: in the upright standing position and with the body or head inclined in the frontal plane and with the visual information about an external environment available or not available. It was shown that the dominant orientation of a referent stimulus (the minimum value of the mean error in the reproduction of the stimulus and the minimal variability of the error) was correlated with the body position, mainly the position of the head, more than with the gravitational or visual vertical, even when the visual information was available. This means that the proprioceptive information about the longitudinal axis of body, rather than gravity, is mainly used by the central nervous system for creating the internal representing of vertical during standing.     

Analysis of center-of-pressure data during unipedal and bipedal standing using fractional Brownian motion modeling

To question the relation between uni-and bipedal postural skills, 21 subjects were required to stand on a force platform through uni- and bipedal conditions. These two protocols are commonly used paradigms to assess the balance capacities of healthy and disabled patients. The recorded displacements of the center of pressure (CP) were decomposed along mediolateral and anteroposterior axes and assessed through variance positions and parameters obtained from fractional Brownian motion (fBm) modeling to determine the nature and the spatiotemporal organization of the successive controlling mechanisms. The variances underline the relative independence of the two tasks. Nevertheless, as highlighted by the fBm framework, postural correction is initiated for the unipedal stance after shorter time delays and longer covered distances. When compared to bipedal standing, one of the main characteristics of unipedal standing is to induce better-controlled CP trajectories, as deduced from the scaling regimes computed from the fBm modeling. Lastly, the control of the CP trajectories during the shortest time intervals along the anteroposterior axis appears identical for both uni- and bipedal conditions. Unipedal and bipedal standing controls should thus be viewed as two complementary tasks, each providing specific and complementary insights into the postural control organization.     

Effect of arm motion on standing lateral jumps

The role of arm swing in jumping has been examined in numerous studies of standing jumps for height and forward distance, but no prior studies have explored its effect on lateral jumping. The purpose of the present study was to investigate the effect of arm motion on standing lateral jump performance and to examine the biomechanical mechanisms that may explain differences in jump distance. Six participants executed a series of jumps for maximum lateral distance from two in-ground force platforms for two jump cases (free and restricted arms) while an eight-camera, passive-reflector, motion capture system collected 3D position data throughout the movements. Inverse kinematics and dynamics analyses were performed for all jumps using three-dimensional (3D) link models to calculate segment angular velocities, joint moments, joint powers, and joint work. Free arm motion improved standing lateral jump performance by 29% on average. This improvement was due to increased takeoff velocity and improved lateral and vertical positions of the center of gravity (CG) at takeoff and touchdown. Improved velocity and position of the CG at takeoff resulted from a 33% increase in the work done by the body. This increase in work in free arm jumps compared to restricted arm jumps was found in both upper and lower body joints with the largest improvements (>30 J) occurring at the lower back, right hip, and right shoulder.     

Validity of hand-to-foot measurement of bioimpedance: standing compared with lying position

Objective: To assess the reliability of the standing measurement of hand‐to‐foot bioimpedance compared with measurements made in the lying position. Research Methods and Procedures: In 205 volunteers 6 to 89 years of age, 111 males and 94 females from six ethnic groups, effects of posture, time, and age on hand‐to‐foot resistance were studied over a range of body size. The effect of time in a position on resistance was also recorded in a small subset (n = 10), and repeat measurements over 3 days at the same time of the day were recorded in another subset (n = 12). Results: Lying impedance was consistently higher than standing, with the relationship (resistance lying/resistance standing) for the children (5 to 14 years) being 1.031, progressing to a ratio of 1.016 in those >60 years. The time spent static in either position did change resistance measurements—a decrease of up to 9 Ω (mean 5 Ω, 1.0%) over 10 minutes of standing and an increase of up to 7 Ω (mean 3 Ω, 0.7%) with lying. Discussion: In the field, measurements of hand‐to‐foot bioimpedance can be made in the standing position, and, with appropriate adjustment, previously validated recumbent equations can be used. Given that errors in the measurement of height and weight also affect the reliability of the derivation of body fat from bioelectrical conductance, the errors that may arise from a more practical standing measurement rather than lying are minimal.     

Age-related changes and sex differences in postural control adaptability in children during periodic floor oscillation with eyes closed

We investigated age-related changes and sex differences in adaptability of anticipatory postural control in children. Subjects comprised 449 children (4-12 years old) and 109 young adults (18-29 years old). Subjects stood with eyes closed on a force-platform fixed to a floor oscillator. We conducted five trials of 1-minute oscillation (0.5 Hz frequency, 2.5 cm amplitude) in the anteroposterior direction. Postural steadiness was quantified as the mean speed of the center of pressure in the anteroposterior direction (CoPy). In young adults, CoPy speed decreased rapidly until the third trial for both sexes. Adaptability was evaluated by changes in steadiness. The adaptability of children was categorized as "good," "moderate," and "poor," compared with a standard variation of the mean CoPy speed regression line between the first and fifth trials in young adults. Results were as follows: (1) anticipatory postural control adaptability starts to develop from age 6 in boys and 5 in girls, and greatly improves at age 7-8 in boys and 6 in girls; (2) the adaptability of children at age 11-12 (74% of boys and 63% of girls were categorized as "good") has not yet reached the same level as for young adults; (3) the adaptability at age 11-12 for girls is temporarily disturbed due to early puberty.     

Sway-dependent modulation of the triceps surae H-reflex during standing.

Previous research has shown that changes in spinal excitability occur during the postural sway of quiet standing. In the present study, it was of interest to examine the independent effects of sway position and sway direction on the efficacy of the triceps surae Ia pathway, as reflected by the Hoffman (H)-reflex amplitude, during standing. Eighteen participants, tested under two different experimental protocols, stood quietly on a force platform. Percutaneous electrical stimulation was applied to the posterior tibial nerve when the position and direction of anteroposterior (A-P) center of pressure (COP) signal satisfied the criteria for the various experimental conditions. It was found that, regardless of sway position, a larger amplitude of the triceps surae H-reflex (difference of 9-14%; P = 0.005) occurred when subjects were swaying in the forward compared with the backward direction. The effects of sway position, independent of the sway direction, on spinal excitability exhibited a trend (P = 0.075), with an 8.9 +/- 3.7% increase in the H-reflex amplitude occurring when subjects were in a more forward position. The observed changes to the efficacy of the Ia pathway cannot be attributed to changes in stimulus intensity, as indicated by a constant M-wave amplitude, or to the small changes in the level of background electromyographic activity. One explanation for the changes in reflex excitability with respect to the postural sway of standing is that the neural modulation may be related to the small lengthening and shortening contractions occurring in the muscles of the triceps surae.     

Suppression of cardiocirculatory responses to orthostatic stress by passive walking-like leg movement in healthy young men

ABSTRACT: BACKGROUND: Although passive walking-like leg movement in the standing posture (PWM) has been used in the clinical field, the safety of PWM has not been fully determined despite the risks of orthostatic intolerance due to standing posture. The aim of the present study was to examine cardiocirculatory response during PWM in healthy young men. METHODS: The subjects (n = 13) spent 5 min in a sitting position and then 5 min in a quiet standing position to determine baseline levels. Thereafter, they underwent 25-min rhythmic PWM at 1 Hz while standing. In another bout, subjects experienced the same protocol except that they underwent 25-min quiet standing (QS) instead of 25-min PWM. Two subjects dropped out of the 25-min QS due to feeling of discomfort. Thus, data obtained in the remaining eleven subjects are presented. RESULTS: In the PWM trial, systolic arterial blood pressure (SAP) decreased from 112 +/- 8 mmHg during the sitting baseline period to 107 +/- 8 mmHg during the standing baseline period (p <0.05), while heart rate (HR) increased from 73 +/- 9 bpm during the sitting baseline period to 84 +/- 10 bpm during the standing baseline period (p <0.001). After the imposition of PWM, SAP increased from 107 +/- 8 mmHg in the standing baseline period to 120 +/- 6 mmHg (p <0.001), while HR decreased from 84 +/- 10 bpm in the standing baseline period to 76 +/- 9 bpm (p <0.05). In the QS trial, SAP, which had decreased during the standing baseline period compared to that during the sitting baseline period, remained lowered during the 25-min QS period, while HR, which had increased during the standing baseline period compared to that during the sitting baseline period, remained elevated during the 25-min QS period. In both bouts, HR showed almost mirror-image changes in the high-frequency component of HR variability, suggesting that the changes in HR were due to change in parasympathetic activation. Double product (HR x SAP), as a predictor of myocardial oxygen consumption, during the 25-min QS period tended to increase with time, but double product remained almost constant during the 25-min PWM period. CONCLUSIONS: The results suggest that PWM is effective for suppressing cardiocirculatory responses to orthostatic stress.     

Subject-specific musculoskeletal model of the lower limb in a lying and standing position

Accurate estimation of joint loads implies using subject-specific musculoskeletal models. Moreover, as the lines of action of the muscles are dictated by the soft tissues, which are in turn influenced by gravitational forces, we developed a method to build subject-specific models of the lower limb in a functional standing position. Bones and skin envelope were obtained in a standing position, whereas muscles and a set of bony landmarks were obtained from conventional magnetic resonance images in a lying position. These muscles were merged with the subject-specific skeletal model using a nonlinear transformation, taking into account soft tissue movements and gravitational effects. Seven asymptomatic lower limbs were modelled using this method, and results showed realistic deformations. Comparing the subject-specific skeletal model to a scaled reference model rendered differences in terms of muscle length up to 4% and in terms of moment arm for adductor muscles up to 30%. These preliminary findings enlightened the importance of subject-specific modelling in a functional position.     

Middle cerebral artery blood velocity during a valsalva maneuver in the standing position.

Occasionally, lifting of a heavy weight leads to dizziness and even to fainting, suggesting that, especially in the standing position, expiratory straining compromises cerebral perfusion. In 10 subjects, the middle cerebral artery mean blood velocity (V(mean)) was evaluated during a Valsalva maneuver (mouth pressure 40 mmHg for 15 s) both in the supine and in the standing position. During standing, cardiac output decreased by 16 +/- 4 (SE) % (P < 0.05), and at the level of the brain mean arterial pressure (MAP) decreased from 89 +/- 2 to 78 +/- 3 mmHg (P < 0.05), as did V(mean) from 73 +/- 4 to 62 +/- 5 cm/s (P < 0.05). In both postures, the Valsalva maneuver increased central venous pressure by approximately 40 mmHg with a nadir in MAP and cardiac output that was most pronounced during standing (MAP: 65 +/- 6 vs. 87 +/- 3 mmHg; cardiac output: 37 +/- 3 vs. 57 +/- 4% of the resting value; P < 0.05). Also, V(mean) was lowest during the standing Valsalva maneuver (39 +/- 5 vs. 47 +/- 4 cm/s; P < 0.05). In healthy individuals, orthostasis induces an approximately 15% reduction in middle cerebral artery V(mean) that is exaggerated by a Valsalva maneuver performed with 40-mmHg mouth pressure to approximately 50% of supine rest.     

Effect of long-term bedrest on lower leg muscle activation patterns during quiet standing

It has been well known that balance instabilities after long-term exposure to microgravity (e.g., Anderson et al. 1986) or bedrest (BR) can be related to alterations and/or adaptations to postural control strategies. Little is known, however, how the reduced muscular activity affects the activation pattern of the lower limb muscles during quiet standing (QS). The purpose of this study was to investigate whether or not any changes in the lower limb muscle activation patterns during QS would occur after BR.     

Internal reference frame for representation and storage of visual information during standing

The participation of different afferent systems in organization of internal reference frame was studied. For this was chosen the task of visual comparison that executed in different experimental conditions: in upright standing, with inclined body or head in frontal plane and availability or non-availability visual information about external environment. Results showed that dominant orientation of referent stimulus (minimal value of mean error and minimal variability of responses) was connected with body position, mainly head position but not with gravity and visual vertical even when visual environment was available. It means that for creating the internal representing of vertical CNS mainly uses proprioceptive information about longitudinal axis of body.     

Influence of high ambient temperatures on the physiological responses and body sway in healthy young adults after quickly standing

This study was aimed to compare the variations in cerebral oxygenation, blood pressure and center-of-foot pressure after standing from sitting and supine positions at normal (22 degrees C) and high (32 degrees C) room temperatures. Thirty young adults stood up from a resting posture (sitting or supine position) and kept the static standing posture for 90 sec. Meanwhile, their center-of-foot pressure (COP), blood pressure, and cerebral oxygenation kinetics were measured in continuity. The change of the frequency domain low-to-high frequency (LF/HF) ratio of the R-R interval before and after standing from a supine position was significantly higher than that from a sitting position under both temperature conditions. Blood pressure as well as total and oxygenated hemoglobin levels decreased immediately after standing up and the ratio of blood pressure change when moving from a supine position to standing at high room temperature was the largest as compared with the other conditions. Total hemoglobin (Hb) volume was found to temporarily decrease after standing and required 22-24 sec to recover when the subject started from the sitting position and 33-36 sec when the subject started from the supine position. Cerebral oxygenation kinetics tended to be larger under high, rather than normal, temperature conditions. All COP parameters after standing were significantly larger in the high temperature condition than in the normal temperature condition. Body sway after standing was larger in the high temperature condition than in the normal temperature condition and after standing from a supine position than from a sitting position. In conclusion, cerebral oxygenation kinetics and blood pressure measured after the subject moved to the standing position changed dramatically under high temperature conditions, and variations in this parameter may influence body sway.     

The effects of the load mass and load position on body sway in supporting a load on the back

We examined the effects of a load's mass and position on body sway during standing with a load on the back. Three healthy male subjects participated in this experiment. The subjects supported loads of 23kg, 33kg, and 43kg on their backs using a carrier frame. They were asked to stand for 75s on a force platform with their eyes open while being as quiet as possible. Time series data of center-of-pressure (COP) were collected at a sampling rate of 50Hz during the last 60s of the 75s standing period. The COP was measured under three conditions in terms of the load position on the frame: lower (close to the hip), middle, and upper (close to the shoulder). All subjects showed that the lower the position of the load, the more anteriorly the mean COP coordinate was located in the anteroposterior (AP) direction, and the smaller the total distance of the COP trajectories became. Regarding carrying the heavier loads, each subject showed a specific tendency in the mean AP coordinate. The three subjects had different physical characteristics in terms of body height and experience at carrying heavy loads. These results suggest that the examintion of the COP in a static posture can help our understanding of individual information on the posture supporting loads and the general positioning of the body.     

Effects of window size on ankle joint stiffness calculation during quiet standing: How the rule changes the result

Measuring ankle joint stiffness (AJS) during quiet standing QS using an inverted pendulum model typically involves a single calculation covering the entire period of QS. This study compared AJS using the same 20.0s set of QS postural sway data but employing seven different calculation windows (0.25s, 0.5s, 1.0s, 2.0s, 5.0s, 10.0s and 20.0s). AJS was calculated for both anterio-posterior AP and medio-lateral ML directions of sway. Postural sway data from 19 subjects were used to calculate mean±SD and time-normalized AJS over the same 20s period of QS. Statistical power of this study was 0.99. The AJS had ICCs ranging from 0.47 to 0.85 with coefficient of variations ranging from 11.1% to 31.8%. There were significant differences in AJS between window sizes (P<0.0001) for both directions of sway. Specifically, AJS calculated by 1.0s windows was significantly larger (P<0.01) than others, except 0.5s, while the AJS of the largest two windows 10.0s and 20.0s were significantly smaller (P<0.01) than all others in both directions of sway. In conclusion, it is recommended that 1.0s windows be used to calculate AJS and that stiffness analyzed as a continuous signal offers a more complete picture of how AJS behaves during QS.     

Effects of room temperature and body position change on cerebral blood volume and center-of-foot pressure in healthy young adults

This study aimed to examine the effects of room temperature and body position changes on cerebral blood volume, blood pressure and center-of-foot pressure (COP). Cerebral oxygenation kinetics and blood pressure were measured by near infrared spectroscopy (NIRS) and volume-compensation, respectively, in 9 males and 9 females after rapid standing from sitting and supine positions in low (12 degrees C) or normal (22 degrees C) room temperatures. COP was also measured in a static standing posture for 90 s after rapid standing. The total hemoglobin (Hb) decreased just after standing. Blood pressure after standing at normal temperature tended to decrease immediately but at low temperature tended to decrease slightly and then to increase greatly. The decreasing ratio of total Hb and blood pressure upon standing from a supine position at normal room temperatures was the largest of any condition. Total Hb recovered to a fixed level approximately 25 sec after standing from a sitting position and approximately 35 sec after standing from a supine position. All COP parameters after standing tended to change markedly in the supine position compared to the sitting position, especially at normal temperatures. The COP parameters after standing in any condition were not significantly related to the decreasing ratio of total Hb but were related to the recovery time of total Hb after standing. In conclusion, decreasing ratios of total Hb and blood pressure after standing from a supine position at normal temperatures were large and may affect body sway.