Gregory M. Karst

Muscle activity for initiation of planar, two-joint arm movements in different directions

SummaryWe studied planar, point-to-point arm movements to various target positions, starting from a fixed initial position. The movements involved coordinated rotations about the shoulder and elbow joints. One of our aims was to determine for each joint whether flexion was always initiated by flexor muscle activity and, similarly, whether extension was always initiated by extensor activity. Based on electromyographic records we found for all subjects that this was not the case for movements to certain areas of the workspace. An alternative hypothesis, that the initial muscle activity should be appropriate for exerting an isometric force in the direction of the final position, was also contradicted by our observations. Nevertheless, the data for each joint revealed that flexor-initiated movements occurred for final positions in a certain region of the workspace that was distinct from the region of final positions for extensor-initiated movements.

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 gender-matched 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.

Antagonist muscle activity during human forearm movements under varying kinematic and loading conditions

SummaryDuring the performance of unidirectional, single-joint movements it is known that muscle activation is not confined to the agonist, but is generally seen in the antagonist as well, appearing as a burst of antagonist activity if the movement is quite rapid. We have studied the integral over time of antagonist electromyographic activity (Eant) during forearm movements encompassing a wide range of movement speeds, amplitudes and inertial loads, with two intents: first, to provide an empirical description of the dependence of Eant on kinematic and loading parameters which would be valid over a several hundred-fold range of Eant; and second, to test the hypothesis that Eant is related to the torque necessary for braking the movement. With respect to the first aim, we found that for all subjects Eant was correlated with a simple algebraic expression dependent upon peak velocity, movement amplitude and total moment of inertia, when each of these movement parameters was varied either singly or in combination. Although a more complex algebraic expression, in which exponents for each parameter were optimized for a given subject, provided marginally better correlations with Eant, we prefer the simpler expression on the grounds that it provides similar correlations without requiring a different form for each subject. With respect to the second aim of the study, the braking hypothesis was supported by the fact that the simple expression could be interpreted as representing the average net torque required for braking. However, in experiments in which an external torque was provided to assist in braking, antagonist muscle activity was not reduced as much as would be expected if provision of braking torque was the sole function of antagonist activity. We conclude that: (1) antagonist activity varies with kinematic parameters and inertial load in accordance with the requirements for braking the limb, but (2) the activity may in fact provide a multiple of the torque that is required for braking alone, with excess activity presumably offset by concurrent agonist activity. Possible roles of such cocontraction are discussed.

Model for Ability-Based Assessment in Physical Therapy Education

ABSTRACT: An ability‐based assessment program was developed to facilitate the transition of physical therapy students from classroom to clinic. Generic abilities critically important to physical therapy practice were identified by surveying selected clinical educators using the delphi technique. Evaluation criteria then were developed to define behaviors representing competence in each generic ability. Formal introduction of the program into the curriculum occurred in the fall of 1993. Exercises were developed to assess progress and to provide immediate feedback to students throughout the professional curriculum. Based on feedback from students and clinicians, we believe that we have developed an assessment program critically important to physical therapy practice. Ability‐based assessment does not replace knowledge and skill‐acquisition assessment. It complements these more traditional assessment forms in helping students develop the repertoire of behaviors essential for clinical success.

Effects of an aquatics exercise program on quality of life measures for individuals with progressive multiple sclerosis

Background and Purpose For persons with Multiple Sclerosis (MS), aquatic exercise has been advocated to allow for aerobic, strength, balance, and flexibility exercises in a safe environment while avoiding potentially detrimental increases in body temperature. This pilot study describes the effects of an aquatic exercise program on the health-related quality of life (QoL) for individuals with progressive MS, characterizes individuals able to successfully participate, and identifies common barriers to participation. Subjects and Methods Thirty-one adults with progressive MS initially agreed to participate in a 12-week aquatic exercise program. The 19 individuals who participated in at least 25% of the exercise sessions are characterized in terms of initial scores on the Expanded Disability Status Scale, as well as pre- and postexercise scores on the Medical Outcome Study Short Form-36 (SF-36) and the Multiple Sclerosis Quality of Life Inventory. Results Significant improvements in the QoL domains of social functioning and fatigue were found for the exercise participants. Barriers limiting exercise adherence included physical and psychological symptoms, transportation difficulties, and availability of a significant other to assist during the program. Discussion and Conclusion Persons with progressive MS may benefit from aquatic exercise programs. Interventions that promote general health, improve energy levels and mental health, and foster social interaction in the presence of physical disability are beneficial for individuals with progressive MS. Barriers to exercise should be anticipated and adequately addressed to maximize potential benefits from such programs.

Force development and relaxation in single motor units of adult cats during a standard fatigue test.

1. The purpose of this study was to investigate tetanic force development and relaxation in single motor units that were subjected to a standard fatigue test. 2. Motor units of tibialis posterior, a hindlimb muscle in the adult cat, were assigned to four categories (i.e. types S, FR, FI, FF) using conventional criteria. 3. Based on the first tetanus of the fatigue test, type S units took significantly longer to develop force and to relax than the fast‐twitch units. Within the fast‐twitch subpopulations, type FR and FI units were significantly slower to develop force and to relax than were type FF units, but there were no significant differences between type FR and FI units. 4. After 120 s of the fatigue test, the rates of force development were faster than initial values in type S and FR units, but were largely unchanged for the type FI and FF units. Most relaxation parameters were unaffected by stimulation in type S and FR units, but all parameters became significantly slower in type FI and FF units. 5. The average time courses of force development and relaxation showed that during 240 s of the fatigue test, type S units exhibited either a progressive increase in a parameter or no change at all. In contrast, fast‐twitch units displayed profiles that included initial increases in a force development or relaxation parameter followed by variable amounts of decline that corresponded to fatigability. 6. It is concluded that repetitive activation affects the development and relaxation of tetanic force in all motor‐unit types. Average changes in these parameters tended to parallel the conventional classification of motor units into four categories.

Relative Activity of Abdominal Muscles During Commonly Prescribed Strengthening Exercises

The purpose of this study was to determine the relative electromyographic (EMG) activity of the upper and lower rectus abdominis and the external oblique muscles during 5 commonly performed abdominal strengthening exercises. Twenty-five healthy subjects participated in the study. EMG data were collected under isometric and dynamic conditions. The reverse curl resulted in the greatest amount of lower rectus activity, the v-sit and reverse curl exercises resulted in the greatest amount of external oblique activity, and the trunk curl, reverse curl, trunk curl with a twist, and v-sit all resulted in similar amounts of upper rectus EMG activity. The vacuum exercise resulted in moderate levels of external oblique EMG activity but very low levels of activity in the rectus abdominis. Our findings support the concept that abdominal strengthening exercises can differentially activate various abdominal muscle groups, but contradict some traditionally held assumptions regarding the effects of pecific exercises.

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.

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.