Kinematics of the Tennis Serve Using an Optoelectronic Motion Capture System: Are There Correlations between Joint Angles and Racket Velocity?

The serve is the most important stroke in tennis. It is a complex gesture consisting of numerous rotations with a wide amplitude, which are important to manage for performance. The aim of this study was to investigate whether correlations exist between joint kinematic parameters and racket velocity. A quantitative kinematics analysis of four ranked players (two boys and two girls) was carried out using an optoelectronic system composed of 10 cameras (150 Hz). Five flat serves per player were analyzed. Eighty-two markers were located across the 15 body segments and on the racket. A descriptive statistical analysis including a correlation analysis was carried out between joint angles and racket kinematic parameters (vertical position, velocity, and acceleration) during the cocking and acceleration phases. Ten very high (0.7 < r < 0.9) and three almost perfect (r > 0.9) correlations were found. Shoulder and hip axial rotations, knee flexion, and trunk extension were correlated linearly with racket vertical position and velocity during the cocking phase. For the acceleration phase, elbow flexion, trunk flexion/extension, and trunk axial rotation were linked to racket kinematics. Some of these parameters showed differences between slow and fast serves. These parameters, which are involved in transmitting ball velocity, are important to consider for tennis players and coaches in training programs, education, and performance enhancement.

A systematic review of work related musculoskeletal disorders among physical therapists and physiotherapists.

Physical therapists and physiotherapists (PPTs) perform and repeat physical tasks that can lead to work-related musculoskeletal disorders (WMSD). The aim was to study the main research concerning this problem, i.e. the risk factors, activities that exacerbate WMSD symptoms, alterations in work habits and the proposed responses, and to estimate mean value (±standard deviation, STD) for the most studied parameters. This review was conducted according to the PRISMA guideline. Five databases (Pubmed, ScienceDirect, Google Scholar, Medeley and Science.gov) were scanned to identify works investigating the different aspects of WMSD among PPTs. Two reviewers independently selected relevant studies using inclusion/exclusion criteria, critically appraised, and extracted data. To homogenize the data, prevalence were reported to the total sample studied when necessary. Among the 9846 articles identified, 19 articles were included. The WMSD prevalence was over 50 %. The areas most affected were the lower back, neck and thumb. An exhaustive list of parameters were constructed for job risk factors (n = 19), activities that exacerbating symptoms (n = 13), altered work habits (n = 15), responses and treatments (n = 26). The mean prevalence (±STD) was calculated for the major parameters. Nine main job risk factors were extracted with an average prevalence of about 30 % and a relatively high variability. Seven activities exacerbating WMSD symptoms and five altered work habits were identified with a homogeneous rate (5-20 %). Three main responses and treatments were found with heterogeneous prevalence. This review provides useful results for the development of future protocols to prevent the occurrence of WMSD among PPTs and meta-analyses.

Work-Related Musculoskeletal Disorders Risk Assessment during Manual Lymphatic Drainage with Compressive Bands among Physiotherapists.

BACKGROUND: Complete decongestive therapy is the standard treatment for lymphedema. Manual lymphatic drainage and short-stretch multilayer compression bandaging are two daily stages of complete decongestive therapy during which physiotherapists work with patients. OBJECTIVE: The aim of this study was to assess the risks of musculoskeletal disorders to which physiotherapists are exposed during these two phases. METHOD: Five physiotherapists performed five 20 min manual lymphatic drainages, followed by the compression bandaging phase. From the video recordings, 8477 postures defined by 13 joint angles were grouped into clusters using hierarchical cluster analysis. The risk of musculoskeletal disorders in physiotherapists' postures was assessed using ergonomic tools. RESULTS: Seven clusters, called generic postures (GP), were identified and defined throughout the mean joint angle values and standard deviation. Four seated GPs were found for the drainage phase, and three standing GPs were identified for the bandaging phase. This phase corresponded to a quarter of the total duration. The GP's ergonomic scores ranged from 4.51 to 5.63 and from 5.08 to 7.12, respectively, for the Rapid Upper Limb Assessment (RULA) and Rapid Entire Body Assessment (REBA). GP1, GP3, and GP4 presented the highest ergonomic scores (RULA scores: 5.27 to 5.63; REBA scores: 6.25 to 7.12). The most affected areas were the neck (flexion > 20° for all GPs), trunk (flexion between 25 and 30° for GP2, and GP7 during the bandaging phase and GP4 during the drainage phase), and shoulder (flexion and abduction >20° for all GPs except GP5). CONCLUSIONS: These results highlighted that the two complete decongestive therapy phases could be described as a combination of GP. Ergonomic assessment showed that compression bandaging as well as drainage phases expose physiotherapists to moderate musculoskeletal disorder risks that require "further investigation and change soon".

Postural prevalence, time of day and spent time activities during smartphone weekday use among students: A survey to prevent musculoskeletal disorders.

The long time spent on smartphones in awkward postures exposes young users to the risk of developing musculoskeletal disorders (MSDs). This study aimed to investigate 1) how the duration of smartphone use varies by the time of day and activities and 2) the risks of MSDs based on an analysis of the postures used when interacting with smartphones. A cross-sectional survey was conducted among 263 university students. The duration of smartphone use during a typical weekday was investigated over four times of the day and seven activities. After checking for normality, a nonparametric Friedman test was used to study the differences in the time spent using a smartphone according to the time of day and activity. Postural prevalence during weekdays was analyzed using a taxonomy called SmarTaxo, consisting of 41 postures. The Rapid Upper Limb Assessment (RULA) ergonomic score was chosen to assess the MSD risks associated with each posture. Smartphone use was the highest in the evening (301.1 min; 95 % confidence interval [CI]: 277.4-324.8 min, p < 0.05). Texting (170.8 min; 95 % CI: 152.0-189.6 min) and watching videos (163.6 min; 95 % CI: 146.3-180.9 min) were the most common activities. Three sitting and two walking postures were primarily used in the morning (29.3-36.9 %), afternoon (27.0-44.4 %), and evening (28.9%-38.9 %). Standing postures were preferred in the morning and afternoon (36.9 % and 42.2 %, respectively), while one lying posture was widely reported in the evening (39.2 %). The RULA scores for these postures ranged from 3 to 4. However, four lying postures, often observed during the evening (frequency between 20.5 % and 37.6 % of the time), had RULA scores of 6. In conclusion, the study identified an existing MSD risk among smartphone users, especially with long durations of daily use. Special emphasis should be placed on addressing the reclining postures adopted during evening smartphone use, as they subject students to a significantly elevated risk of MSDs.

Effect of day time on smartphone use posture and related musculoskeletal disorders risk: a survey among university students.

BACKGROUND: Musculoskeletal disorders (MSDs) are one of the most important problems among young smartphone users worldwide. Portability leads to a wide variety of postures during the different activities of the day. The objective evaluation of these postures coupled with ergonomic tools allows evaluating the level of MSD risk to which users are exposed. METHODS: The purpose was to investigate the effect of the time of day on the posture adopted during smartphone use among university students. The study was conducted through a cross-sectional survey of 263 university sports students. Four time of day, i.e. morning, afternoon, evening and night, and a taxonomy of 41 postures called SmarTaxo were considered. SmarTaxo included 18 sitting, 11 standing, 10 lying and 2 walking postures and their ergonomic score. After checking the normality of the data, a non-parametric Kruskal-Wallis test was used to study the effect of the time of day on the use duration of the different postures. RESULTS: The total mean duration use per typical weekday was 5.39 ± 2.19 h for males and 5.15 ± 1.60 h for females with maximal duration during evening. The average smartphone use durations were statistically longer in afternoon and evening for all sitting (9.44 and 9.22 min respectively, p < 0.05) and calling (3.38 and 3.33 min respectively, p < 0.05) postures. The longest duration for standing postures was recorded for afternoon (8.91 min, p < 0.05). The lying postures were significantly more present in evening (19.36 min). Some postures were more used during a time of day. The side-lying posture was used more in evening and has an ergonomic score of 6, i.e. a high MSD risk. CONCLUSIONS: The survey showed that users are exposed to MSDs regardless of posture and time of day. Sitting postures are used more in the morning and afternoon while lying postures are used more in the evening. As long as the rate of use is so high (> 5 h per day), young people will remain highly exposed to MSDs.

Three-month work-related musculoskeletal disorders assessment during manual lymphatic drainage in physiotherapists using Generic Postures notion.

OBJECTIVES: The aim of this work was to quantify the postures and to assess the musculoskeletal disorders (MSDs) risk in physiotherapists repeating a manual lymphatic drainage (MLD) over a three-month period. The underlying hypothesis was that there would be Generic Postures (GP) that would be repeated and could be used to more simply describe repetitive and long-duration complex activities. METHODS: The posture of five physiotherapists performing five 20-min MLD at their workplace was captured by two cameras. From the recordings, the adopted postures were extracted every 5 s and quantified through 13 joint angles, that is, 6594 analyzed postures. Rapid Upper Limb (RULA) and Rapid Entire Body Assessment (REBA) were used to assess MSDs risks. A hierarchical analysis was used to define GP. RESULTS: Seven GP were identified through mean values and standard deviation. GP ergonomic assessment showed a low to moderate MSD risk (RULA between 3 and 6 and REBA between 2 and 7). High neck (>20°) and trunk (>15°) flexion were observed for all GP. High shoulder abduction and flexion (>40°) were evidenced for GP3 to GP5. GP1 was the most used (34%) and presented the lowest ergonomic scores (RULA: 4.46 ± 0.84; REBA: 5.06 ± 1.75). GP3 to GP6 had frequency of between 10 and 20%. GP5, GP6, and GP7 obtained the highest ergonomic scores (RULA>5; REBA>7). All physiotherapists use different GP combinations to perform MLD. CONCLUSION: MLD could be described as a combination of GP. Ergonomic analysis showed that MLD exposes physiotherapists to low at moderate MSD risks.

Effect of Assisted Surgery on Work-Related Musculoskeletal Disorder Prevalence by Body Area among Surgeons: Systematic Review and Meta-Analysis.

Surgeons are highly exposed to work-related musculoskeletal disorders (WMSDs). The objective of this review was to summarize the WMSD prevalence by body area with and without assistive devices. The underlying question was whether there is an effect of assistive device use (robot, video, or other) during surgery on WMSD prevalence by body area among surgeons, regardless of their specialty. The systematic review was conducted according to the PRISMA guidelines. The Google Scholar, Pubmed/Medline, and ScienceDirect databases were scanned to identify relevant studies. The article selection, review, critical appraisal, and data extraction were performed by two authors independently. Among the 34,854 unique identified records, 77 studies were included. They were divided into two groups: 35 focused on robotic- and video-assisted surgery (RVAS) and 48 concerning surgery without video/robotic assistance (WAS) (6 studies evaluated the prevalence for both groups). WMSD prevalence was reported for 13 body areas: the neck, back, upper back, mid-back, lower back, shoulders, elbows, wrists, fingers, thumbs, hips, knees, and ankles. The results showed that WMSD prevalence was significantly higher (unpaired t-test, p < 0.05) for RVAS in the shoulders (WAS: 28.3% vs. RVAS: 41.9%), wrists (WAS: 20.9% vs. RVAS: 31.5%), and thumbs (WAS: 9.9% vs. RVAS: 21.8%). A meta-analysis was performed for 10 body areas (with 4 areas including more than 25 studies). No sufficient data were available for the mid-back, thumbs, or hips. A high heterogeneity (Cochran's Q test and I2 statistic) was observed. A random-effects model revealed that the highest worldwide prevalence was in the neck (WAS: 41% and RVAS: 45.3%), back (WAS: 37.7% and RVAS: 49.9%), lower back (WAS: 40.0% and RVAS: 37.8%), and shoulders (WAS: 27.3% and RVAS: 41.4%). Future work could focus on work environment design, particularly the positioning and adjustment of equipment, and on postural analysis to reduce the appearance of WMSDs. Recommendations are proposed for future reviews and meta-analyses.

Global prevalence of musculoskeletal disorders among physiotherapists: a systematic review and meta-analysis.

BACKGROUND: Musculoskeletal disorders (MSD) are one of the most important problems among physiotherapists worldwide. However, there is no meta-analysis of the MSD prevalence in all body areas among physiotherapists. OBJECTIVES: The purpose was to investigate and estimate the worldwide prevalence of MSD among physiotherapists using a systematic review-, meta-analysis and meta-regression. METHODS: The systematic review, meta-analysis and meta-regression were performed in 2022 using the PRISMA guidelines. DATA SOURCES: The search was performed on PubMed/Medline, ScienceDirect, Google Scholar, Medeley and Science.gov databases. STUDY APPRAISAL: The quality appraisal of the included articles was assessed using the critical appraisal tool for cross-sectional studies AXIS. RESULTS: A total of 722 articles were found. After screening and comparison with the inclusion criteria, 26 studies were retained. Based on the random-effects model, the worldwide MSD prevalence in neck, upper back, mid back, lower back, shoulders, elbows, wrists/hands, thumb, hips/thighs, knees/legs, and ankles/feet was 26.4% (CI 95%: 21.0-31.9%), 17.7% (CI 95%: 13.2-22.2%), 14.9% (CI 95%: 7.7-22.1%), 40.1% (CI 95%: 32.2-48.0%), 20.8% (CI 95%: 16.5-25.1), 7.0% (CI 95%: 5.2-8.9), 18.1% (CI 95%: 14.7-21.5%), 35.4% (CI 95%: 23.0-47.8), 7.0% (CI 95%: 5.2-8.8), 13.0% (CI 95%: 10.3-15.8), and 5% (CI 95%: 4.0-6.9) respectively. The neck and shoulder prevalence of four continents were close to the world prevalence. No effect of continent was found on MSD prevalence. The heterogeneity of the results obtained in the meta-analysis and meta-regression was discussed. CONCLUSIONS: Based on the random effects model, the results of the worldwide meta-analysis showed that lower back pain, thumb, neck and shoulder were the area most at risk for MSD and were therefore those to be monitored as a priority. Recommendations were proposed for future reviews and meta-analyses.

Ergonomic risk assessment during massage among physiotherapists: Introduction of generic postures notion.

BACKGROUND: Musculoskeletal disorders are prevalent among physiotherapists. They perform many repetitive tasks in awkward postures that require a high muscle load. This issue was often addressed through subjective analysis (questionnaire) but rarely using postural evaluation combined with an ergonomic assessment. OBJECTIVE: The aim of this article was to decompose draining lymphatic massage performed by physiotherapists in generic postures (GP) to quickly assess the musculoskeletal disorder risk. METHODS: Based on numeric video recorded in frontal and sagittal planes, 3360 postures were defined from 20-minutes lymphatic drainage massage performed by 7 physiotherapists over a period of 6 months. A hierarchical cluster analysis was applied to define the GP. RESULTS: Seven GP were identified, which were described by their average joint angles, variability and relative frequency. GP6, GP4 and GP2 had the highest frequencies (17.6%, 16.9% and 16.8% respectively). Trunk and neck were essentially flexed (neck: 17.0±10.0° for GP1; trunk: 20.4±8.7° for GP4). Shoulder flexion and abduction ranged from 15° (GP7) to 60° (GP4). Mean and variability of RULA score completed the analysis. The highest scores were computed for GP2 and GP3 (respectively 5.5±1.3 and 5.8±1.2, p < 0.05). GP4 had the lowest (4.0±1.1, p < 0.05). The results suggest that massages have to be monitored. Attention should be paid to the postures used to prevent the occurrence of MSDs. CONCLUSION: The massages could be described as a combination of generic posture for a rapid ergonomic assessment.

Prevalence of Body Area Work-Related Musculoskeletal Disorders among Healthcare Professionals: A Systematic Review.

Healthcare professionals perform daily activities that can lead to musculoskeletal disorders (MSDs). The objective of this review was to summarize these MSDs by body areas in relation to healthcare professions. The underlying question is, worldwide, whether there are areas that are more exposed depending on the occupation or whether there are common areas that are highly exposed to MSDs. This issue has been extended to risk factors and responses to reduce MSDs. The review was conducted according to the PRISMA guidelines between February and May 2022. Google scholar and Science Direct databases were scanned to identify relevant studies. Two authors independently reviewed, critically appraised, and extracted data from these studies. Overall and body area prevalence, risk factors, and responses to MSDs were synthetized by occupational activity. Among the 21,766 records identified, 36 covering six healthcare professions were included. The lower back, neck, shoulder and hand/wrist were the most exposed areas for all healthcare professionals. Surgeons and dentists presented the highest prevalence of lower back (>60%), shoulder and upper extremity (35-55%) MSDs. The highest prevalence of MSDs in the lower limbs was found for nurses (>25%). The main causes reported for all healthcare professionals were maintenance and repetition of awkward postures, and the main responses were to modify these postures. Trends by continent seem to emerge regarding the prevalence of MSDs by healthcare profession. Africa and Europe showed prevalence three times higher than Asia and America for lower back MSDs among physiotherapists. African and Asian nurses presented rates three times higher for elbow MSDs than Oceanians. It becomes necessary to objectively evaluate postures and their level of risk using ergonomic tools, as well as to adapt the work environment to reduce exposure to MSDs with regard to the specificities of each profession.

A predictive model to quantify joint torques and support reaction forces when using a smartphone while standing with support.

The present study had a dual objective: (1) to present and validate a predictive model of standing posture in the sagittal plane, joint torques and support forces for a smartphone user built from biomechanical principles; (2) propose risk scales for joint torques and reaction forces based on simulations in order to use them into the musculoskeletal disorders prevention. Comparison of the modelled data with experimental measurements (400 tested postures with sample size verification) for calling and texting tasks highlights the model's ability to correctly estimate posture and reaction forces on the ground. The model was able to provide estimates of the range of variation of each parameter for a wide range of environmental conditions as a function of the user body mass index (setting between 12.5 and 50). Joint torques risk scales have been constructed, especially for shoulder and elbow, to characterise the risks incurred by the users. Practitioner summary: The proposed model enables the postures, joint torques and reaction forces to be estimated from subject's body mass index and environmental configuration without resorting to experimentation, which is relevant in industry. This approach allows the proposition of new scales based on joint torques to reinforce the recommendations for MSDs prevention. Abbreviations: BMI: body mass index; LUBA: postural loading on the upper body assessment; MSDs: musculoskeletal disorders; RULA: rapid upper limb assessment; WHO: World Health Organization.

Identification of generic drilling task phases and height effect: Coupling of kinematic and kinetic data.

BACKGROUND: The prevention of work-related musculoskeletal disorders (MSDs) is an important issue for the health of operators, especially when the workload is heavy, such as in drilling activities. RESEARCH QUESTION: This study aimed to propose an objective identification and description of the different phases that comprise a drilling task as well as a biomechanical and ergonomic evaluation of the subjects' overall posture during its effective part. METHODS: Fourteen healthy subjects performed three successive drillings at two different heights, 130 (H130) and 170 cm (H170) from the ground, of a 3 mm thick steel plate. Kinematic and kinetic data were simultaneously acquired using a Qualisys motion capture system and a force platform. The speed profiles of each body segments and the evolution of the force exerted were analysed to identify the different phases. Then, the average joint angles of the whole body were calculated during the drilling phase for the two heights considered and compared. Finally, the measured postures were qualified through the Rapid Upper Limb Assessment (RULA) score in order to assess the risk of developing MSDs for each condition. RESULTS: The drilling task could be divided into five phases whatever the condition: transport, aiming, repositioning, drilling, and return. The increase in height has doubled the duration of the drilling phase while reducing the force exerted by about 30 %. Similarly, height variation significantly influenced the subjects' posture, mainly in the head, trunk and upper limbs, and the RULA scores obtained for conditions H130 and H170 were 6 and 7 respectively. SIGNIFICANCE: The results indicate, on the one hand, the relevance of coupling kinematic and kinetic data in order to analyse the drilling task and, on the other hand, that operators are exposed to considerable risk of developing MSDs, even at intermediate heights, which increase as the height increases.

Effects of environmental illumination and screen brightness settings on upper limb and axial skeleton parameters: how do users adapt postures?

79% of smartphone users carry their phone 22 hours a day. In this context, the main task worldwide, texting, is performed under a wide range of light and position conditions. The aim of this study was to test the effects of environmental illumination and screen brightness settings on upper limb and axial skeleton parameters. Twelve subjects performed three trials of texting under three experimental positions, two screen luminance settings and three environmental light conditions. 3 D axial skeleton and upper limb angles, smartphone orientation and face-to-smartphone distance were used as dependent variables. High environmental illumination and/or low screen brightness resulted in an increase in interaction time and a reduction in the face-to-smartphone distance by approximately 10%. Subjects attempted to compensate for the unfavourable effects of such light conditions by adopting postures rated 5 in the Rapid Upper Limb Assessment, indicating an increased risk of developing musculoskeletal disorders. Practitioner's summary: The purpose of the study was to quantify the joint angles of the upper body in experimental conditions that represent daily life. Postures were influenced by ambient illumination and display brightness. The most harmful postures were observed when the display brightness was minimum, and the ambient light was similar to a sunny day. Abbreviations: ST: seated with table; SWT: seated without any support; STA: standing; 0L: in the dark, no light; AL: ambient light; SL: strong light; BrightMin: minimum display brightness; BrightMax: maximum display brightness (BrightMax); ISB: International Society of Biomechanics; RULA: rapid upper limb assessment; MSDs: musculoskeletal disorders.

Biomechanical analysis of upper limb during the use of touch screen: motion strategies identification.

Nowadays touch technology is growing and developers try to make it ever more intuitive and easier to use. This present work focused on the upper limb joint coordination during the achievement of puzzles on touch screen. A 5-inch and 10-inch devices were used to perform 9 and 16 pieces puzzles dragged with digits. The conclusions showed an increase in joint solicitation with the number of piece and the touch screen size. Moreover, three interactions strategies proved to be an evidence: the 'wrist strategy' preferentially implying wrist flexion/extension, the 'elbow strategy' preferentially implying the elbow flexion/extension and the 'neutral strategy' mobilising equally the two joints. From an ergonomic point of view, the data about how the upper limb segments are mobilised while interacting with the screen could be relevant to increase the adaptability of the devices to the user, including users with motor impairments. Practitioner Summary: Information about the biomechanical organisation of movement during interaction with touch devices appears relevant in order to develop applications adapted to the motor capacities of users. From the analysis of joint angles when performing several times a puzzle with healthy subjects, three motor strategies were highlighted.

Human upper-limb force capacities evaluation with robotic models for ergonomic applications: effect of elbow flexion.

The aim of this study was to apply models derived from the robotics field to evaluate the human upper-limb force generation capacity. Four models were compared: the force ellipsoid (FE) and force polytope (FP) based on unit joint torques and the scaled FE (SFE) and scaled FP (SFP) based on maximum isometric joint torques. The four models were assessed from four upper-limb postures with varying elbow flexion (40°, 60°, 80° and 100°) measured by an optoelectronic system and their corresponding isometric joint torques. Ten subjects were recruited. Three specific ellipsoids and polytopes parameters were compared: isotropy, principal force orientation and volume. Isotropy showed that the ellipsoids and polytopes were elongated. The angle between the two ellipsoids main axis and the two polytopes remained low but increased with the elbow flexion. The FE and FP volumes increased and those of SFE and SFP decreased with the elbow flexion. The interest and limits of such models are discussed in the framework of ergonomics and rehabilitation.

A new postural force production index to assess propulsion effectiveness during handcycling.

The aim of this study was to propose a new index called Postural Force Production Index (PFPI) for evaluating the force production during handcycling. For a given posture, it assesses the force generation capacity in all Cartesian directions by linking the joint configuration to the effective force applied on the handgrips. Its purpose is to give insight into the force pattern of handcycling users, and could be used as ergonomic index. The PFPI is based on the force ellipsoid, which belongs to the class of manipulability indices and represents the overall force production capabilities at the hand in all Cartesian directions from unit joint torques. The kinematics and kinetics of the arm were recorded during a 1-min exercise test on a handcycle at 70 revolutions per minute performed by one paraplegic expert in handcycling. The PFPI values were compared with the Fraction Effective Force (FEF), which is classically associated with the effectiveness of force application. The results showed a correspondence in the propulsion cycle between FEF peaks and the most favorable postures to produce a force tangential to the crank rotation (PFPI). This preliminary study opens a promising way to study patterns of force production in the framework of handcycling movement analysis.

The manipulability: a new index for quantifying movement capacities of upper extremity.

In this work, it is proposed to evaluate the upper-limb movements through a global index of performance borrowed from the field of robotics: the manipulability. For a given posture, this index quantifies the set of velocities that can be achieved at the wrist in all the Cartesian directions. The manipulability can be represented by an ellipsoid from which the volume and shape related parameters can be derived. During a reach to grasp movement, the ellipsoid obtained from experiment presented a flattened shape along the forearm longitudinal axis and an increased volume as the arm was extended. From this study, it is concluded that: (1) the ellipsoid volume reflects well the ability to generate speed at the wrist which is effectively maximal for an extended posture; (2) if maximal velocity is an important parameter it might be advisable to primarily move the hand perpendicularly to the forearm longitudinal axis. PRACTITIONER SUMMARY: The interest of manipulability indices is that they evaluate globally a posture of the upper-limb in relation to a given task. This original parameter could help to design environments or devices in order that the adopted postures maximise one particular aspect of the performance, i.e. the velocity of the hand.

Adaptation of joint flexibility during a reach-to-grasp movement.

In the presence of motor redundancy, recent studies have shown that goal equivalent configurations of the body segments might be used by the central nervous system (CNS) instead of stereotypical movement patterns. In particular, some authors have shown that the CNS might choose a subset of joint configurations (termed the uncontrolled manifold or UCM) such that variability (goal equivalent variance or GEV) in this subset does not affect the value of a particular performance variable while variability in the orthogonal subset ORT (non-goal equivalent variance or NGEV) does. This hypothesis has been used successfully to test whether specific performance variables such as endpoint trajectory or segment global orientation are stabilized by the CNS or to study the influence of constraints on the organization of the movement. Few studies have examined the redundancy problem when considering obstacle avoidance during a grasping task. Indeed, the majority of the works on this topic considers non redundant arm models or do not take into account the movement variability. In the present work, we sought to study the coordination of the trunk and the arm during a reaching task involving an obstacle and to test whether such a spatial constraint in extrinsic space may induce particular adaptations in term of joint flexibility when considering the shoulder, elbow, and wrist joint center positions. In this framework, the upper limb three-dimensional kinematics was recorded. From the calculated joint angles, the variability in joint space related to the three joint center positions was computed and decomposed into GEV and NGEV. In agreement with the UCM hypothesis, results showed higher values of GEV than NGEV for all the experimental conditions. The main finding of the study is that joints' synergy is strengthened for the stabilization of the elbow joint center position during the late phases of the movement. This strengthening seems to be due mainly to an increase of GEV. Therefore, our results suggest that an increase of joint flexibility may be a mechanism by which the CNS takes into account a spatial constraint in extrinsic space represented by an obstacle.