Children have less variable postures and muscle activities when using new electronic information technology compared with old paper-based information technology

Children now have considerable exposure to new information technologies (IT) such as desktop computers. A reported association between computer use and discomfort in children has prompted concerns about the musculoskeletal stresses associated with computer use. There were no detailed data on children reading and writing, nor any evidence on the variability of postures and muscle activity whilst children use IT.Twenty-four children (10–12 years old; 12 male) performed a reading and writing task using new IT (computer/keyboard/mouse with high display and mid height display) and old IT (book/paper/pen). Spinal and upper limb 3D posture and muscle activity were recorded and estimates of mean and variation calculated.The mean postures for children reading and writing with computers were more neutral than when they read and wrote with old IT. Similarly, mean muscle activity levels were lower during computer use than old IT use. However, new IT use also resulted in less variable, more monotonous postures and muscle activities. Moderate differences in computer display height had little effect on posture and muscle activity variation.Variation in musculoskeletal stresses is considered an important component of the risk of musculoskeletal disorders. Children should therefore be encouraged to ensure task variety when using new IT to offset the greater posture and muscle activity monotony.     

The impact of computer display height and desk design on muscle activity during information technology work by young adults.

Computer display height and desk design are believed to be important workstation features and are included in international standards and guidelines. However, the evidence base for these guidelines is lacking a comparison of neck/shoulder muscle activity during computer and paper tasks and whether forearm support can be provided by desk design. This study measured the spinal and upper limb muscle activity in 36 young adults whilst they worked in different computer display, book and desk conditions. Display height affected spinal muscle activity with paper tasks resulting in greater mean spinal and upper limb muscle activity. A curved desk resulted in increased proximal muscle activity. There was no substantial interaction between display and desk.     

Influence of memory demand and contra lateral activity on muscle activity.

Computer mouse work often includes memory demands and contra lateral activity. This study simulated video display unit (VDU) mouse-work and the focus was on forearm muscle activity as a result of standardised postural loads, memory demands and contra lateral activity. Surface and intramuscular electromyography (EMG) were recorded from the right forearm muscles during finger elevation and rest with and without memory demands and with and without contra lateral activity i.e. activity of the left hand. In most situations, memory demand increased activity in the m. extensor carpi radialis brevis and m. flexor digitalis superficialis. Also contra lateral activity increased activity in situations with and without memory demands. While surface EMG level of the m. extensor digitorum communis did not increase during memory demands, intramuscular EMG level increased when memory demands and contra lateral activity was combined. Influence of memory demands and contra lateral activity were most pronounced, in situations where activity levels were small.We presume that it is not only prolonged time of active computer mouse use that is a risk for development of musculoskeletal disorders, but also the time when people interact with the computer mentally or with the 'non-mouse hand', while resting their 'mouse-hand' on the mouse.     

Muscle activity in rapid multi-degree-of-freedom elbow movements: solutions from a musculoskeletal model

The activity of certain muscles that cross the elbow joint complex (EJC) are affected by forearm position and forearm movement during elbow flexion/extension. To investigate whether these changes are based on the musculoskeletal geometry of the joint, a three-dimensional musculotendinoskeletal computer model of the EJC was used to estimate individual muscle activity in multi-degree-of-freedom (df) rapid (ballistic) elbow movements. It is hypothesized that this model could reproduce the major features of elbow muscle activity during multi-df elbow movements using dynamic optimal control theory, given a minimum-time performance criterion. Results from the model are presented and verified with experimental kinematic and electromyographic data from movements that involved both one-df elbow flexion/extension and two-df flexion/extension with forearm pronation/supination. The model demonstrated how the activity of particular muscles is affected by both forearm position and movement, as measured in these experiments and as previously reported by others. These changes were most evident in the flexor muscles and least evident in the extensor muscles. The model also indicated that, for specific one- and two-df movements, activating a muscle that is antagonistic or noncontributory to the movement could reduce the movement time. The major features of muscle activity in multi-df elbow movements appear to be highly dependent on the joint's musculoskeletal geometry and are not strictly based on neural influences or neuroanatomical substrates. Received: 9 May 1997 / Accepted in revised form: 8 December 1998     

A mass-length scaling law for modeling muscle strength in the lower limb

Musculoskeletal computer models are often used to study muscle function in children with and without impaired mobility. Calculations of muscle forces depend in part on the assumed strength of each muscle, represented by the peak isometric force parameter, which is usually based on measurements obtained from cadavers of adult donors. The aim of the present study was twofold: first, to develop a method for scaling lower-limb peak isometric muscle forces in typically-developing children; and second, to determine the effect of this scaling method on model calculations of muscle forces obtained for normal gait. Muscle volumes were determined from magnetic resonance (MR) images obtained from ten children aged from 7 to 13yr. A new mass-length scaling law was developed based on the assumption that muscle volume and body mass are linearly related, which was confirmed by the obtained volume and body mass data. Two musculoskeletal models were developed for each subject: one in which peak isometric muscle forces were estimated using the mass-length scaling law; and another in which these parameters were determined directly from the MR-derived muscle volumes. Musculoskeletal modeling and quantitative gait analysis were then used to calculate lower-limb muscle forces in normal walking. The patterns of muscle forces predicted by the model with scaled peak isometric force values were similar to those predicted by the MR-based model, implying that assessments of muscle function obtained from these two methods are practically equivalent. These results support the use of mass-length scaling in the development of subject-specific musculoskeletal models of children.     

Thumb postures and physical loads during mobile phone use – A comparison of young adults with and without musculoskeletal symptoms

The aim of this study was to evaluate thumb postures, thumb movements and muscle activity when using mobile phones for SMS messaging and to determine whether there were differences in these exposures (a) across various mobile phone tasks, (b) between gender and (c) between subjects with and without musculoskeletal symptoms in shoulders and upper extremities. Fifty-six young adults (15 healthy and 41 with musculoskeletal symptoms) performed a series of distinct tasks on a mobile phone. Muscular load in four forearm/hand muscles in the right arm and the right and left trapezius muscles were measured using electromyography (EMG). Thumb movements were registered using an electrogoniometer. The results showed that postures (sitting or standing) and the type of mobile phone task (holding the phone versus texting) affected muscle activity and thumb positions. Females compared to males had higher muscle activity in the extensor digitorum and the abductor pollicis longus when entering SMS messages and tended to have greater thumb abduction, higher thumb movement velocities and fewer pauses in the thumb movements. Subjects with symptoms had lower muscle activity levels in the abductor pollicis longus and tended to have higher thumb movement velocities and fewer pauses in the thumb movements compared to those without symptoms.     

Upper extremity biomechanics in computer tasks differ by gender

ObjectivesThis laboratory study examined gender differences in upper extremity postures, applied forces, and muscle activity when a computer workstation was adjusted to individual anthropometry according to current guidelines.MethodsFifteen men and 15 women completed five standardized computer tasks: touch-typing, completing a form, editing text, sorting and resizing graphical objects and navigating intranet pages. Subjects worked at a height-adjustable workstation with the keyboard on top of the work surface and the mouse to the right. Subjects repeated the text editing task with the mouse in two other locations: a “high” mouse position, which simulated using a keyboard drawer with the mouse on the primary work surface, and “center” mouse position with the mouse between the keyboard and the body, centered with the body’s center line. Surface electromyography measured muscle activity; electrogoniometric and magnetic motion analysis system measured wrist, forearm and upper arm postures; load-cells measured typing forces; and a force-sensing mouse measured applied forces.ResultsRelative forces applied to the keyboard, normalized muscle activity of two forearm muscles, range of motion for the wrist and shoulder joints and external rotation of the shoulder were higher for women (p < 0.05). When subjects were dichotomized instead by anthropometry (either large/small shoulder width or arm length), the differences in forces, muscle activity of the shoulder and wrist posture and shoulder posture became more pronounced with smaller subjects having higher values. Postural differences between the genders increased in the high mouse position and decreased in the center mouse location.ConclusionsWhen a workstation is adjusted per current guidelines differences in upper extremity force, muscle activity and postural factors still exist between genders. However, these were often stronger when subjects were grouped by anthropometry suggesting that perhaps the computer input devices themselves should be scaled to be more in proportion with the anthropometry and strength of the user.     

Electromyographic patterns of individuals suffering from lateral tennis elbow.

This study investigated the applicability of using surface electromyography (EMG) as a tool for differentiating between persons suffering from lateral tennis elbow and the healthy age-matched adults. Temporal muscle activation patterns of the tennis elbow group were evaluated to determine if they varied between subject groups and if noted variations might be interpreted as arresting or exacerbating the injury. Sixteen subjects (Healthy Controls, n = 6; Tennis Elbow, n = 10) were tested under simulated tennis playing conditions. All subjects were males (Healthy group (CON) 38.8 +/- 13.1, Injured group (INJ) 40.8 +/- 10.8 yrs). EMG response data, temporal and spatial muscle activities, of the forearm extensors (Ext), the forearm flexors (Flex) and the triceps (Tri) were recorded for each subject during a single test session using all combinations of three different velocities on three different racket head impact locations. Data were collected at a frequency of 1000 Hz. Statistical analysis was performed using a 2 x 3 x 3 (Health status x Impact velocity x Impact location) ANOVA with repeated measures. Results indicated statistically significant differences (p < 0.05) between the CON and INJ subject groups for the response variables associated with forearm extensor muscle activation. During simulated play, the INJ group employed an earlier, longer, and greater activation of Ext than the CON group, such changes may be considered detrimental to the healing process. These results support the use of surface EMG to quantify differences in muscle activation strategies employed by individuals suffering from soft tissue muscle microtrauma injuries and healthy controls.     

Residual Upper Arm Motor Function Primes Innervation of Paretic Forearm Muscles in Chronic Stroke after Brain-Machine Interface (BMI) Training

Background

Abnormal upper arm-forearm muscle synergies after stroke are poorly understood. We investigated whether upper arm function primes paralyzed forearm muscles in chronic stroke patients after Brain-Machine Interface (BMI)-based rehabilitation. Shaping upper arm-forearm muscle synergies may support individualized motor rehabilitation strategies.

Methods

Thirty-two chronic stroke patients with no active finger extensions were randomly assigned to experimental or sham groups and underwent daily BMI training followed by physiotherapy during four weeks. BMI sessions included desynchronization of ipsilesional brain activity and a robotic orthosis to move the paretic limb (experimental group, n = 16). In the sham group (n = 16) orthosis movements were random. Motor function was evaluated with electromyography (EMG) of forearm extensors, and upper arm and hand Fugl-Meyer assessment (FMA) scores. Patients performed distinct upper arm (e.g., shoulder flexion) and hand movements (finger extensions). Forearm EMG activity significantly higher during upper arm movements as compared to finger extensions was considered facilitation of forearm EMG activity. Intraclass correlation coefficient (ICC) was used to test inter-session reliability of facilitation of forearm EMG activity.

Results

Facilitation of forearm EMG activity ICC ranges from 0.52 to 0.83, indicating fair to high reliability before intervention in both limbs. Facilitation of forearm muscles is higher in the paretic as compared to the healthy limb (p<0.001). Upper arm FMA scores predict facilitation of forearm muscles after intervention in both groups (significant correlations ranged from R = 0.752, p = 0.002 to R = 0.779, p = 0.001), but only in the experimental group upper arm FMA scores predict changes in facilitation of forearm muscles after intervention (R = 0.709, p = 0.002; R = 0.827, p<0.001).

Conclusions

Residual upper arm motor function primes recruitment of paralyzed forearm muscles in chronic stroke patients and predicts changes in their recruitment after BMI training. This study suggests that changes in upper arm-forearm synergies contribute to stroke motor recovery, and provides candidacy guidelines for similar BMI-based clinical practice.     

Frequency and severity of musculoskeletal symptoms in humans during an outbreak of trichinellosis caused by Trichinella britovi

Musculoskeletal symptoms such as myalgia are well-known features in the course of trichinellosis; however, the characteristics of musculoskeletal findings have been described in detail in only 1 study. The present study was aimed to determine the joint and muscle symptoms in subjects diagnosed with acute trichinellosis at our rheumatology unit during a Trichinella britovi outbreak that occurred in Izmir, Turkey, in 2004. In total, 98 patients (55 females, 43 males; mean age 32.3 +/- 10.9 yr) were included in the study. A detailed history and full musculoskeletal examination were obtained in each patient. A self-administered questionnaire developed for recording the musculoskeletal symptoms was completed monthly until all the symptoms were resolved. Pain at the joints, restriction of movements (in shoulders, elbows, wrists, knees, ankles, and temporomandibular joints), myalgia, and muscle weakness (neck and shoulder girdle, muscles of the upper and forearm, back, thigh, and calf muscles) were assessed in every patient. Eosinophil counts, serum levels of creatine kinase, and lactate dehydrogenase also were analyzed. The most frequent musculoskeletal symptoms were muscle pain (86 cases [87.8%]), joint pain (83 [84.7%]), subjective muscle weakness (75 [76.5%]), and restriction of joint movements (63 [64.3%]). Calves, upper arm, neck and shoulder girdle, and forearms were the most affected muscle groups. Muscle pain was reported more frequently in the upper than in the lower extremities and during activity. The most frequent painful joints were shoulders, knees, wrists, and ankles. Upper extremity joints were affected more frequently than the lower extremity joints (77.6 vs. 70.4%). Joint pain occurred more frequently at rest. Both muscle weakness and restriction of joint movements were reported in and around the most frequently affected regions. No evidence of arthritis and objective muscle weakness was noted on physical examination in any patient. Musculoskeletal symptoms in the course of T. britovi infection are frequent but with an excellent prognosis. Joint pain in people suffering from acute trichinellosis may occur more frequently than reported previously.     

A method to quantify frequency and duration of sustained low-level muscle activity as a risk factor for musculoskeletal discomfort

AimThe purpose of this paper was to describe and evaluate different aspects of muscle activity patterns associated with musculoskeletal discomfort/pain.MethodSurface electromyography (sEMG) of the right upper trapezius and the right extensor digitorum muscles was conducted continuously during one working day in 19 male forest machine operators driving harvesters, 20 driving forwarders and 20 researchers at the Forest Research Institute.Perceived discomfort/pain in the right side of the neck and the right forearm was rated morning, noon and afternoon with Borg’s CR-10 scale. Static, median and peak levels of muscle activity were analyzed and the number and total duration of EMG gaps (muscular rest) were calculated. Sustained low-level muscle activity (SULMA) was defined as continuous muscle activity above 0.5% of the maximal EMG activity quantified into 10 periods of predetermined duration intervals from 1.6 to 5 s up to above 20 min. The number of SULMA periods is presented within each interval and as cumulative periods above the already determined levels. The operators handled control levers seated in a fixed position while the researchers performed mainly PC work and other varied tasks.ResultsA positive correlation was found between discomfort/pain in the right upper trapezius muscle region in the afternoon and cumulative SULMA periods above 10 min duration, and a negative correlation to cumulative SULMA periods also including the short durations. No specified patterns were found for discomfort/pain in the right extensor digitorum or for the other EMG measurements. All EMG measurements distinguished to some extent between the occupational groups, especially between machine operators driving harvesters and researchers.ConclusionsNumber of SULMA periods longer than 10 min per hour was positively correlated, and predominantly short periods were negatively correlated, to complaints in the neck region. This seems promising in order to find duration limits for sustained low-level muscle activity as a risk factor for musculoskeletal disorders.     

Evaluation of a method to scale muscle strength for gait simulations of children with cerebral palsy

Cerebral palsy (CP) is a neurological disorder that results in life-long mobility impairments. Musculoskeletal models used to investigate mobility deficits for children with CP often lack subject-specific characteristics such as altered muscle strength, despite a high prevalence of muscle weakness in this population. We hypothesized that incorporating subject-specific strength scaling within musculoskeletal models of children with CP would improve accuracy of muscle excitation predictions in walking simulations. Ten children (13.5 ± 3.3 years; GMFCS level II) with spastic CP participated in a gait analysis session where lower-limb kinematics, ground reaction forces, and bilateral electromyography (EMG) of five lower-limb muscles were collected. Isometric strength was measured for each child using handheld dynamometry. Three musculoskeletal models were generated for each child including a ‘Default’ model with the generic musculoskeletal model’s muscle strength, a ‘Uniform’ model with muscle strength scaled allometrically, and a ‘Custom’ model with muscle strength scaled based on handheld dynamometry strength measures. Muscle-driven gait simulations were generated using each model for each child. Simulation accuracy was evaluated by comparing predicted muscle excitations and measured EMG signals, both in the duration of muscle activity and the root-mean-square difference (RMSD) between signals. Improved agreement with EMG were found in both the ‘Custom’ and ‘Uniform’ models compared to the ‘Default’ model indicated by improvement in RMSD summed across all muscles, as well as RMSD and duration of activity for individual muscles. Incorporating strength scaling into musculoskeletal models can improve the accuracy of walking simulations for children with CP.     

Upper trapezius muscle activity patterns during repetitive manual material handling and work with with a computer mouse.

Firstly, upper trapezius EMG activity patterns were recorded on the dominant side of 6 industrial production workers and on the side operating a computer mouse of 14 computer-aided design (CAD) operators to study differences in acute muscular response related to the repetitiveness of the exposure. The work tasks were performed with median arm movement frequencies ranging from 5 min(-1) to 13 min(-1) and were characterized by work cycle times ranging from less than 30 sec to several days. However, the static and median EMG levels and EMG gap frequencies were similar for all work tasks indicating that shoulder muscle loads may be unaffected by large variations in arm movement frequencies and work cycle times. An exposure variation analyses (EVA) showed that the EMG activity patterns recorded during production work were more repetitive than during CAD work, whereas CAD work was associated with more static muscle activity patterns, both may be associated with a risk of developing musculoskeletal symptoms. Secondly, upper trapezius EMG activity patterns recorded on the mouse side of the CAD operators were compared with those recorded on the non-mouse side to study differences in muscular responses potentially related to the risk of developing shoulder symptoms which were more prevalent on the mouse side. The number of EMG gaps on the mouse side were significantly lower than the values for the upper trapezius on the non-mouse side indicating that more continuous activity was present in the upper trapezius muscle on the mouse side and EVA analyses showed a more repetitive muscle activity pattern on the mouse side. These findings may be of importance to explain differences in the prevalence of shoulder symptoms.     

Electromyographic analysis of trapezius and sternocleidomastoideus muscles during nasal and oral inspiration in nasal- and mouth-breathing children.

The purpose of this study was to evaluate sternocleidomastoideus (SCM) and trapezius (superior fibers) muscle activity patterns in mouth-breathing children, and to compare them with nasal-breathing children. Forty-six children, of both sexes, ranging from 8 to 12 years old, were evaluated through electromyography. The selected children were divided into two groups; Group I, was made up of 26 mouth-breathing children and Group II of 20 nasal-breathing children. Electromyographic recordings were obtained through surface electrodes in the SCM and trapezius muscles, bilaterally, during oral and nasal inspiration. Root-mean-square (RMS) data expressed in microvolts (microV), were analyzed using the Kruskall-Wallis statistical test. From the results obtained, we concluded that there was a significant difference in the muscle activity between the groups, with higher activity during nasal inspiration in the mouth-breathing group. During oral inspiration, there was no significant difference between groups. Within the groups, only the mouth-breathing group showed higher activity during nasal inspiration.     

Flexion–relaxation ratio in computer workers with and without chronic neck pain

This study evaluated the flexion–relaxation phenomenon (FRP) and flexion–relaxation ratios (FR-ratios) using surface electromyography (sEMG) of the cervical extensor muscles of computer workers with and without chronic neck pain, as well as of healthy subjects who were not computer users. This study comprised 60 subjects 20–45 years of age, of which 20 were computer workers with chronic neck pain (CPG), 20 were computer workers without neck pain (NPG), and 20 were control individuals who do not use computers for work and use them less than 4 h/day for other purposes (CG). FRP and FR-ratios were analyzed using sEMG of the cervical extensors. Analysis of FR-ratios showed smaller values in the semispinalis capitis muscles of the two groups of workers compared to the control group. The reference FR-ratio (flexion relaxation ratio [FRR], defined as the maximum activity in 1 s of the re-extension/full flexion sEMG activity) was significantly higher in the computer workers with neck pain compared to the CG (CPG: 3.10, 95% confidence interval [CI95%] 2.50–3.70; NPG: 2.33, CI95% 1.93–2.74; CG: 1.99, CI95% 1.81–2.17; p < 0.001). The FR-ratios and FRR of sEMG in this study suggested that computer use could increase recruitment of the semispinalis capitis during neck extension (concentric and eccentric phases), which could explain our results. These results also suggest that the FR-ratios of the semispinalis may be a potential functional predictive neuromuscular marker of asymptomatic neck musculoskeletal disorders since even asymptomatic computer workers showed altered values. On the other hand, the FRR values of the semispinalis capitis demonstrated a good discriminative ability to detect neck pain, and such results suggested that each FR-ratio could have a different application.     

Children with cerebral palsy have larger Achilles tendon moment arms than typically developing children

The effectiveness of the plantarflexor muscle group to generate desired plantarflexion moments is modulated by the geometry of the Achilles tendon moment arm (ATMA). Children with cerebral palsy (CP) frequently have reduced plantarflexion function, which is commonly attributed to impaired muscle structure and function, however little attention has been paid to the potential contribution of ATMA geometry. The use of musculoskeletal modelling for the simulation of gait and understanding of gait mechanics, rely on accuracy of ATMA estimates. This study aimed to compare 3D in-vivo estimates of ATMA of adults, children with CP and typically developing (TD) children, as well as compare 3D in-vivo estimates to linearly scaled musculoskeletal model estimates. MRI scans for eight children with CP, 11 TD children and nine healthy adults were used to estimate in-vivo 3D ATMA using a validated method. A lower limb musculoskeletal model was linearly scaled to individual tibia length to provide a scaled ATMA estimate. Normalised in-vivo 3D ATMA for children with CP was 17.2% ± 2.0 tibia length, which was significantly larger than for TD children (15.2% ± 1.2, p = 0.013) and adults (12.5% ± 0.8, p < 0.001). Scaled ATMA estimates from musculoskeletal models significantly underestimated in-vivo estimates for all groups, by up to 34.7%. The results of this study show children with CP have larger normalised 3D ATMA compared to their TD counterparts, which may have implications in understanding reduced plantarflexor function and the efficacy of surgical interventions whose aim is to modify the musculoskeletal geometry of this muscle group.     

The influence of an elastic tendon on the force producing capabilities of a muscle during dynamic movements

With increasing computer power, computer simulation of human movement has become a popular research tool. However, time to complete simulations can still be long even on powerful computers. One possibility for reducing simulation time, with models of musculo-skeletal system, is to simulate the muscle using a rigid tendon rather than the more realistic compliant tendon. This study examines the effect of tendon elasticity on muscle force output under different dynamic conditions. A single muscle, point mass model was used and simulations were performed varying the mass, the tendon length, the initial position, and the task. For simulations for relatively slow motion, as experienced for example in upper limb reaching motions or rising from a chair, tendon properties had little influence on muscle force, in contrast simulations of an explosive task similar to jumping or throwing tendon had a much larger effect.     

Regional variations of in vivo surface stiffness of soft tissue layers of musculoskeletal extremities

Surface stiffness of bulk soft tissue in musculoskeletal extremities is important to consider in the design of prosthetics, exoskeletons, and protective gear. This knowledge is also foundational for surgical simulation and clinical interventions leveraging manipulation of the musculoskeletal surfaces. Injuries to musculoskeletal extremities are common and surgical and preventive interventions require interactions between various objects such as surgical tools and support surfaces with tissue boundaries. While a handful of investigations examined the variations in indentation mechanics due to pathology or injury specific sites, a comprehensive analysis across the surfaces of musculoskeletal extremities has not been completed. In this study we examine variations of surface stiffness across 8 sites of the upper and lower arms and legs for 95 subjects using an instrumented ultrasound device. Differences in surface stiffness were observed between gender, activity level, and indentation location groups. The lower arm posterior location had the highest average stiffness (3.89 × 10⁻³ MPa/mm), while the lowest stiffness was observed at the upper leg posterior location (0.98 × 10⁻³ MPa/mm). The differences between indentation sites were larger in magnitude when compared to differences due to demographics (gender and activity level). However the large ranges of the 95% confidence intervals suggest that an aggregated metric based on population or sub-group may not capture individual variations. This study implicates the motivation to explore tissue composition variations within the indentation sites as well as the potential importance to include variations in surface stiffness during surgical simulations.     

The influence of an elastic tendon on the force producing capabilities of a muscle during dynamic movements

With increasing computer power, computer simulation of human movement has become a popular research tool. However, time to complete simulations can still be long even on powerful computers. One possibility for reducing simulation time, with models of musculo-skeletal system, is to simulate the muscle using a rigid tendon rather than the more realistic compliant tendon. This study examines the effect of tendon elasticity on muscle force output under different dynamic conditions. A single muscle, point mass model was used and simulations were performed varying the mass, the tendon length, the initial position, and the task. For simulations for relatively slow motion, as experienced for example in upper limb reaching motions or rising from a chair, tendon properties had little influence on muscle force, in contrast simulations of an explosive task similar to jumping or throwing tendon had a much larger effect.     

The development of rhythm regularity, neuromuscular strategies, and movement smoothness during repetitive reaching in typically developing children

IntroductionThis study examined the development of paced coordinated reaching characterized by the successful entrainment of the movement to an external pacer, synchronous muscle activations and movement smoothness.MethodsThirty children, 5–10 years of age, and ten adults were instructed to repeatedly reach for and move an object from a lower shelf to an upper shelf in time to a metronome. Surface electromyography data were recorded. Amplitude and cross-correlations were calculated on three muscle pairs crossing the shoulder and elbow. A motion capture system captured the space curve accelerations of hand, forearm and upper arm segments to quantify movement smoothness.ResultsThe 5–6 year old children showed the greatest amount of temporal variability, followed by 7–10 year olds and then the adults. Correlations between muscle pairs stabilizing the shoulder girdle were higher in each group as compared to the other two muscle pairs but the correlations for all pairs were consistently higher for adults. Movement smoothness for children 9–10 years of age was closer to an adult-like pattern with respect to control of the upper arm, but the hand segment had the greatest variability across groups.ConclusionsThe increased temporal variability and decreased movement smoothness of the hand and forearm segments suggest that control of more distal musculature may be more difficult in children. The neuromuscular strategies adopted by adults were more optimal than those adopted by children as reflected by smoother and more consistent reaching.