Sit happens: Does sitting development perturb reaching development, or vice versa?

The development of reaching and of sitting during the first year of life is typically studied as separate yet related behaviors. Interestingly, very soon after learning to reach, 4-7-month-old infants start coordinating their arms with their trunk and legs for sitting. In this longitudinal study, we focused, for the first time, on how infants learn to use their arms for the dual tasks of reaching for objects while providing arm support as they learn to sit. We hypothesized that the use of arms for support during sitting development would be a temporary perturbation to reaching and result in a nonlinear progression of reaching skill. Eleven infants were studied monthly from the time they began to prop sit to the time of sitting independence (5-8 months of age). Behavioral coding, kinematics, and electromyography (EMG) characterized reaching and posture while infants sat as independently as possible. Results revealed significant changes across time in trunk movement and hand use as infants transitioned through three stages of sitting: with arm support, sitting briefly without arm support, and sitting independently. Infants used their hands more for contacting objects and less for posture support linearly across time. In contrast, changes in posture control as indicated by pelvis and trunk movement demonstrated a U-shaped curve with more movement of these two body segments during the middle stage of sitting than in the first or last stage. During the middle stage of sitting infants reached persistently even though posture control, measured by pelvis and trunk movement, appeared to be significantly challenged. Muscle activation consisted of tonic and variable combinations of muscle pairings in early sitting. As infants progressed to sitting without hand support, variable but successful strategies utilizing lower extremity muscles in a tight linkage with reach onset emerged to provide prospective control for reaching. Our findings support the contention that reaching both drives the development of sitting in infancy as well as perturbs sitting posture, factoring into the assembly of the complex dual sit-reach behavior that supports and expands flexible interaction with the environment.

Coactivation of lower leg muscles during body weight-supported treadmill walking decreases with age in adolescents.

The kinematics of children's walking are nearly adult-like by about age 3-4 years, but metabolic efficiency of walking does not reach adult values until late in adolescence or early adulthood, perhaps due to higher coactivation of agonist/antagonist muscle pairs in adolescents. Additionally, it is unknown how use of a body weight-supported treadmill device affects coactivation, but because unloading will alter the activity of anti-gravity muscles, it was hypothesized that muscle coactivation will be altered as well. Muscle coactivation during treadmill walking was evaluated for adolescents (ages 10 to 17 years, M = 13.2, SD = 2.2) and adults (ages 22 to 35 years, M = 25.2, SD = 4.3), for thigh muscles (vastus lateralis/biceps femoris) and lower leg muscles (tibialis anterior/gastrocnemius). Conditions included body weight unloadings from nearly 0% to 80% of body weight, while walking at a preferred speed (self-selected, overground speed) or a reduced speed. Unloading was accomplished using a lower body positive pressure support system. Coactivation was found to be higher in adolescents than in adults, but only for the lower leg muscles.

Influence of lower body pressure support on the walking patterns of healthy children and adults.

The purpose of this investigation was to evaluate the effect of a lower body positive pressure support system on the joint kinematics and activity of the lower extremity antigravity musculature of adults and children during walking. Adults (age = 25 ± 4 years) and children (age = 13 ± 2 years) walked at a preferred speed and a speed that was based on the Froude number, while 0-80% of their body weight was supported. Electrogoniometers were used to monitor knee and ankle joint kinematics. Surface electromyography was used to quantify the magnitude of the vastus lateralis and gastrocnemius muscle activity. There were three key findings: (1) The lower extremity joint angles and activity of the lower extremity antigravity muscles of children did not differ from those of adults. (2) The magnitude of the changes in the lower extremity joint motion and antigravity muscle activity was dependent upon an interaction between body weight support and walking speed. (3) Lower body positive pressure support resulted in reduced activation of the antigravity musculature, and reduced range of motion of the knee and ankle joints.

Individuals with total knee arthroplasty demonstrate altered anticipatory postural adjustments compared with healthy control subjects.

BACKGROUND: Persons with total knee arthroplasty (TKA) have many impairments that may compromise postural control. Most work examining postural control following TKA has focused on static and reactive postural control. PURPOSE: The purpose of this study was to (1) compare anticipatory postural adjustments (APAs) between individuals with TKA and healthy controls; and (2) identify possible pre- to postoperative changes in APAs in those undergoing TKA. METHODS: Ten individuals planning TKA and 10 healthy age- and sex-matched controls were recruited. During a standing reaching task, lower extremity muscle activity was measured using electromyography (EMG) onsets and normalized EMG amplitudes, and center of pressure (COP) excursion was measured via a force platform. Other outcome measures included isometric strength of the knee flexors and extensors. Individuals in the TKA group were tested preoperatively, and at 3 and 6 months postoperatively. Controls were also measured 3 times over 6 months. RESULTS: There were no pre- to postoperative differences in lower extremity EMG onsets, normalized EMG amplitudes, or COP excursion in those with TKA. When compared to controls, individuals with TKA demonstrated lower EMG amplitudes of the vastus lateralis and biceps femoris, whereas EMG onsets and COP excursion did not differ. Individuals with TKA demonstrated lower knee extension torque. DISCUSSION AND CONCLUSIONS: It seems that the surgery itself did not alter APAs among individuals with TKA. Potential contributors to the differences in EMG amplitudes in those with TKA compared to controls, such as impaired neural activation or efforts to reduce stress on the involved knee joint, need further investigation.

The effects of shoe traction and obstacle height on lower extremity coordination dynamics during walking.

This study aims to investigate the effects of shoe traction and obstacle height on lower extremity relative phase dynamics (analysis of intralimb coordination) during walking to better understand the mechanisms employed to avoid slippage following obstacle clearance. Ten participants walked at a self-selected pace during eight conditions: four obstacle heights (0%, 10%, 20%, and 40% of limb length) while wearing two pairs of shoes (low and high traction). A coordination analysis was used and phasing relationships between lower extremity segments were examined. The results demonstrated that significant behavioral changes were elicited under varied obstacle heights and frictional conditions. Both decreasing shoe traction and increasing obstacle height resulted in a more in-phase relationship between the interacting lower limb segments. The higher the obstacle and the lower the shoe traction, the more unstable the system became. These changes in phasing relationship and variability are indicators of alterations in coordinative behavior, which if pushed further may have lead to falling.

Phase-dependent and task-dependent modulation of stretch reflexes during rhythmical hand tasks in humans.

Phase-dependent and task-dependent modulation of reflexes has been extensively demonstrated in leg muscles during locomotory activity. In contrast, the modulation of reflex responses of hand muscles during rhythmic movement is poorly documented. The objective of this study was to determine whether comparable reflex modulation occurs in muscles controlling finger motions during rhythmic, fine-motor tasks akin to handwriting. Twelve healthy subjects performed two rhythmic tasks while reflexes were evoked by mechanical perturbations applied at various phases of each task. Electromyograms (EMGs) were recorded from four hand muscles, and reflexes were averaged during each task relative to the movement phase. Stretch reflexes in all four muscles were found to be modulated in amplitude with respect to the phase of the rhythmic tasks, and also to vary distinctly with the tasks being conducted. The extent and pattern of reflex modulation differed between muscles in the same task, and between tasks for the same muscle. Muscles with a primary role in each task showed a higher correlation between reflex response and background EMG than other muscles. The results suggest that the modulation patterns observed may reflect optimal strategies of central-peripheral interactions in controlling the performance of fine-motor tasks. As with comparable studies on locomotion, the phase-dependency of the stretch reflexes implies a dynamically fluctuating role of proprioceptive feedback in the control of the hand muscles. The clear task-dependency is also consistent with a dynamic interaction of sensory feedback and central programming, presumably adapted to facilitate the successful performance of the different fine-motor tasks.

Center of pressure measures during standing tasks in minimally impaired persons with multiple sclerosis.

BACKGROUND AND PURPOSE: Balance impairments are common in persons with multiple sclerosis (MS), but clinical balance tests may not detect subtle deficits in adults with MS who are not yet experiencing functional limitations or disability. The purpose of this study was to determine if center of pressure (COP) displacement during standing tasks could be a useful performance-based evaluative measure for adults with MS who have minimal or no balance deficits on clinical examination using the Berg Balance Scale (BBS). SUBJECTS AND METHODS: Twenty-one adults with MS were compared with 21 age- and gendermatched healthy adults. Subjects with MS were tested with the BBS, Mini-mental State Exam, Expanded Disability Status Scale (EDSS), and Multiple Sclerosis Functional Composite (MSFC). They also performed voluntary leaning and reaching movements while kinematic and kinetic data were collected. Control subjects performed the same tasks with the exception of the EDSS and MSFC. RESULTS: COP displacement during reaching and leaning was less in adults with MS when compared to control subjects. There were no differences in anthropometric, kinematic, or foot position variables that could account for this difference. Furthermore, there was no difference between groups when COP displacement during reaching was expressed as a percentage of the maximum COP displacement during leaning. DISCUSSION AND CONCLUSION: COP measures show clear differences when comparing healthy adults with minimally impaired adults with MS. The lack of between-group differences when COP displacement during reaching was expressed as a percentage of the maximum COP displacement during leaning suggests that the subjects with MS adopt a reaching strategy that allows them to stay within their reduced limits of stability. COP measures during standing tasks appear well-suited to quantifying changes in postural control over time or in response to intervention for minimally impaired persons with MS.

Timing of muscle activity during reaching while standing: systematic changes with target distance.

We examined the effects of changing target distance from within arm's length (AL) to beyond arm's length on the onsets of electromyographic (EMG) activity of non-focal muscles for a reaching task performed while standing. Two questions were addressed. First, do changes in target distance result in consistent changes in the onsets of non-focal anticipatory muscle activity of the trunk and legs in healthy subjects? Second, do changes in onsets of all non-focal muscles vary in a similar fashion in response to varying target distance? Thirteen young, healthy adults performed rapid, bilateral reaching movements to targets placed at shoulder height at four distances while electromyographic activity was recorded from muscles of the arm, trunk and legs. Ground reaction forces and arm kinematics were also recorded. The onsets of most non-focal muscles occurred prior to the onset of arm movement, and occurred progressively earlier as target distance was increased. An exception to this trend was the onset of the erector spinae muscle, which occurred progressively later as target distance was increased. These data support the notion that reaches to targets beyond arm's length involve anticipatory non-focal muscle activity that acts to transport the arm to the target rather than simply to resist the perturbation caused by the arm movement. The consistent patterns of anticipatory muscle activity observed in healthy subjects provide a template against which to compare activity patterns of non-focal muscles for individuals with potential deficits in the control of standing balance.

Effects of specific exercise instructions on abdominal muscle activity during trunk curl exercises.

STUDY DESIGN: A repeated-measures, counterbalanced design. OBJECTIVES: To test whether subjects could learn and retain the ability to alter the relative activity of abdominal muscle groups when performing trunk curl exercises. BACKGROUND: Although trunk curl exercises are widely prescribed, a disadvantage of trunk curls is that they primarily activate rectus abdominis, while the internal and external oblique abdominis muscles are considered to be more important contributors to lumbar stability. METHODS AND MEASURES: A convenience sample of 25 subjects performed trunk curl exercises in accordance with 3 different sets of instructions: nonspecific instructions (NS), instructions intended to emphasize rectus abdominis activity (RE), and instructions intended to emphasize oblique abdominis activity (OE). Electromyographic (EMG) activity was recorded from the upper and lower rectus and the internal and external oblique abdominis muscles while a physical target was used to insure that the trunk was raised to the same height for all conditions. Normalized root-mean-square EMG amplitude measures were used to test for instruction-dependent changes in the relative EMG activity of the rectus and oblique muscle groups. RESULTS: Following a single, brief, instruction session, subjects performing trunk curls had significantly greater normalized oblique:rectus EMG ratios when following OE instructions (mean [+/- SD] oblique-rectus ratio, 1.45 +/- 0.34) than when following RE (mean [+/- SD] oblique-rectus ratio, 0.76 +/- 0.24) or NS (mean [ISD] oblique-rectus ratio, 0.63 +/- 0.23) instructions. Retesting 1 week later indicated that subjects retained this skill. CONCLUSIONS: With minimal instruction, subjects are able to volitionally alter the relative activity of the oblique and rectus abdominis muscles when performing trunk curls. Incorporating instructions emphasizing oblique abdominis activity into lumbar stabilization programs appears promising and has potential advantages over other approaches to altering abdominal muscle activity during trunk