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Biomedical subjects

G M Karst

Publications and source records attributed to G M Karst.

14 recordsLinked to original sources
PubMed2001

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 specific exercises.

Abdominal Muscles↗
PubMed1999

Reliability of foot trajectory measures within and between testing sessions.

BACKGROUND: Impaired control of foot trajectory during the swing phase of gait is hypothesized to increase the risk of slipping or tripping. Before assessing the predictive validity of foot trajectory measures with respect to incidence of falls, it is necessary to establish their reliability. The purpose of this study is to assess within- and between-session reliability of foot trajectory measures and traditional temporal-distance measures in healthy elderly women during gait. METHODS: Sixteen healthy, elderly women (ages 65-79 years) completed six sets of five trials each of natural and fast cadence gait during a 3.5-hour period on each of 4 days. An optoelectric motion analysis system and heel switches were used to obtain both foot trajectory (minimum toe clearance during swing, vertical, and horizontal heel contact velocities) and temporal-distance measures (step width, cadence, velocity, stride length, and time). RESULTS: Within-session test-retest reliability of all variables at natural and fast speeds was good to excellent, with intraclass correlation coefficients (ICCs) of greater than 0.9 for all but one measure (fast cadence stride time). ICCs for between-session test-retest reliability were slightly lower, but still greater than 0.9 for all but two measures (fast cadence stride time and natural cadence vertical heel contact velocity). Heel contact velocities quantified at the instant of heel contact correlated strongly with values obtained by averaging over the last 2% of the gait cycle. DISCUSSION: The good to excellent within- and between-session reliability of these foot trajectory measures supports their use as a possible means of assessing subtle changes in gait motor control. Confirmation of an association between alterations in foot trajectory measures and incidence of falls awaits further study.

Aged↗
PubMed1998

EMG onset timing.

Explore the source record for details and available documents.

Electromyography↗
PubMed1995

Kinematic and electromyographic characteristics of children with cerebral palsy who exhibit genu recurvatum.

The objective of this article was to determine, using selected computerized gait analysis procedures, whether variation exists in the gait patterns of children with cerebral palsy who exhibit genu recurvatum. This descriptive study compared differences in kinematic, temporal-distance, and electromyographic (EMG) variables between two groups of children with cerebral palsy who exhibited genu recurvatum and age-matched controls. The setting was a motion analysis laboratory. Six children with cerebral palsy who showed genu recurvatum and a control group of four normal children participated in this study. Main outcome measures were hip-knee angle-angle diagrams, knee phase plane plots, knee angle versus time diagrams, stride length, cadence, single-limb support, and EMG data. Through use of hip-knee angle-angle diagrams, knee phase plane plots, and knee angle versus time diagrams, distinctive kinematic patterns emerged, allowing for grouping of subjects. Kinematic measures of knee angle at foot-floor contact and knee angle at greatest extension provided further support for the groups created, as did temporal-distance measures of stride length, cadence, and single-limb support. Analysis of variance procedures for the EMG data showed selected time points in the gait cycle during which differences between the groups were observed. Because differences in kinematic, temporal-distance, and EMG variables existed in this study to warrant grouping into two categories, physical therapists and orthopedic surgeons may need to vary the treatments they introduce dependent on the nature of the child's gait pattern.

Case-Control Studies↗
PubMed1995

Onset timing of electromyographic activity in the vastus medialis oblique and vastus lateralis muscles in subjects with and without patellofemoral pain syndrome.

BACKGROUND AND PURPOSE: Inappropriate neural control of the quadriceps femoris muscle group has been implicated in patellofemoral pain syndrome (PFPS). This study investigated the timing of initial electromyographic (EMG) activity of the vastus medialis oblique muscle (VMO) and the vastus lateralis muscle (VL) in asymptomatic subjects and subjects with PFPS during reflex and voluntary muscle activity. SUBJECTS: Fifteen symptomatic subjects (SYMP group) (9 with bilateral symptoms) and 12 asymptomatic subjects (ASYMP group) participated. Both knees were tested in the ASYMP group and only the symptomatic knees were tested in the SYMP group, resulting in a total of 24 data sets from each group. METHODS: Electromyographic data were recorded from the VMO and VL under three conditions: reflex knee extension (RFLX) elicited by a patellar tendon tap, and active knee extension in non-weight-bearing (NWB) and weight-bearing (WB) situations. For each condition, EMG activity onset times for the VMO and VL were determined from ensemble averages of four trials. RESULTS: There were no differences between the SYMP and ASYMP groups with respect to the relative timing of initial VMO and VL activity under any of the three conditions tested. Mean timing differences for both groups were less than 0.25 milliseconds under reflex conditions and less than 4 milliseconds for active knee extension under both WB and NWB conditions. CONCLUSION AND DISCUSSION: These findings contradict a previous report of differences in reflex timing related to PFPS. Differences in the relative timing of onset of EMG activity of the VMO and VL during voluntary knee extension were not significant between SYMP and ASYMP groups, and were not related to the relative timing differences observed during reflex testing.

Adolescent↗
PubMed1993

Electromyographic analysis of exercises proposed for differential activation of medial and lateral quadriceps femoris muscle components.

BACKGROUND AND PURPOSE: The purpose of this study was to determine whether active exercises combining hip adduction with knee extension activate medial components of the quadriceps femoris muscle (QF) more than does knee extension alone. SUBJECTS: Twelve healthy adults (6 men, 6 women), aged 20 to 36 years (mean = 24.8, SD = 5.8), participated in the study. METHODS: The subjects performed quadriceps femoris setting (QS), straight leg raising (SLR), straight leg raising with the hip laterally rotated (SLR/LR), and straight leg raising combined with isometric hip adduction (SLR/ADD). Electromyographic (EMG) activity was recorded from the oblique (VMO) and longitudinal (VML) portions of the vastus medialis, vastus lateralis (VL), and rectus femoris muscles. RESULTS: Comparison of normalized mean EMG magnitudes revealed that the single-joint QF components (VMO, VML, and VL) demonstrated significantly greater activity during QS than during any of the three SLR variations and that SLR/LR and SLR/ADD did not elicit greater relative activity of medial QF components than did QS or SLR. CONCLUSION AND DISCUSSION: These findings do not support the notion that concurrent use of the hip adductors during knee extensor exercises results in preferential strengthening of the VMO.

Adult↗
PubMed1991

Initiation rules for planar, two-joint arm movements: agonist selection for movements throughout the work space.

1. The question we addressed was the following: what rules does the CNS employ, given the initial and final positions of the arm for a pointing movement, to decide which shoulder and elbow muscles ("agonists") to activate for initiating movement? 2. Widely varying initial and final positions were used, so that the movements studied encompassed much of the reachable work space within the horizontal plane. For each movement, the initial electromyographic (EMG) activity at each joint was classified qualitatively in terms of the "sign," i.e., flexor or extensor muscle activity, and quantitatively in terms of the integral of the rectified EMG. 3. The sign of initial muscle activity at each joint was found to be related to the angular excursions at both joints during movement to the final position. 4. Two different hypothesized rules, derived from previously proposed strategies for control of multijoint limb movements, were tested for their ability to predict correctly the sign of initial muscle activity at each joint. Although four variables are needed to describe the initial and final positions of a two-segment arm, only two combinations of these four were relevant for testing the rules. The two positional variables were the spatial direction of the final tip position with respect to the initial forearm orientation (psi) and the initial elbow angle (theta Einit). 5. According to one of the rules tested, which was based on statics, the initial muscle activity at each joint should be such that the distal tip of the limb exerts an initial force in the direction of the final tip position. Our data concerning the sign of initial shoulder muscle activity clearly contradicted this rule in two distinct regions of the (psi, theta Einit) plane. 6. According to the other rule tested, which was based on dynamics, the initial muscle activity at each joint should be such that the initial acceleration of the distal tip is in the direction of the final tip position. The data contradicted the predicted sign of initial shoulder muscle activity for a certain range of psi. This shows that the activation of muscles is not always appropriate even qualitatively for a straight-line path. Furthermore, the effects of added inertial loads predicted by this rule were not observed for trials in which a 1.8-kg mass was attached to the distal portion of the limb.(ABSTRACT TRUNCATED AT 400 WORDS)

Arm↗
PubMed1991

Timing and magnitude of electromyographic activity for two-joint arm movements in different directions.

1. We studied electromyographic (EMG) and kinematic features of self-paced human arm movements involving rotations about the shoulder and elbow joints. Movements were initiated from various positions and covered much of the reachable work space in the horizontal plane. The attempt was to characterize robust features of the relative timing and magnitude of the EMG activity at the two joints, and to correlate them with variables related to the initial and final positions. 2. The pattern of muscle activity at each joint was typically characterized by bursts of alternating agonist and antagonist activity, comparable with the three-burst pattern associated with single-joint movements. As the spatial direction of the target was altered, the magnitude of each burst was modulated over a continuous range. Modulation down to zero activity was observed, not only for later bursts, as has been shown in some cases of single-joint movements, but for the first agonist burst as well. 3. In the preceding paper we showed that the choice of agonists (i.e., flexors or extensors) at each joint is predictable on the basis of the target direction relative to the distal segment (psi). Here, we present quantitative analyses of initial agonist EMG activity at the shoulder and elbow, which reveal that the onset-time difference between agonists at the two joints also varied systematically with psi, and so did their relative magnitude. 4. For most target directions, initial EMG activity at the shoulder preceded that at the elbow by 5-40 ms. Exceptions were observed mainly for target directions near the transitions between initial flexor and initial extensor activity at the shoulder. In these cases the initial agonist activity at the shoulder was greatly reduced or, in some cases, appeared entirely suppressed, although the later bursts were present in their usual temporal alignment with the corresponding bursts at the elbow. 5. Antagonist onset at the elbow tended to precede antagonist onset at the shoulder, but the difference in timing did not vary consistently with psi. 6. Despite the consistency of initial agonist timing between the two joints, the agonist onset-time difference was poorly correlated with the apparent difference in the onset times of shoulder and elbow joint rotations. The latter difference, which is affected by mechanics, cannot therefore be imputed directly to the CNS.(ABSTRACT TRUNCATED AT 400 WORDS)

Arm↗
PubMed1990

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.

Animals↗
PubMed1989

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

We 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.

Adult↗
PubMed1987

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

During 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.

Adolescent↗
PubMed1986

Effect of simulated weightlessness on exercise-induced anaerobic threshold.

Ventilation (VE), CO2 output (VCO2), oxygen uptake (VO2), respiratory exchange ratio (R), and the ventilatory equivalents for VO2 and VCO2 were measured during graded exercise before and after 10 d of continuous bed rest (BR) in the -6 degrees head-down position to determine the effect of deconditioning on the anaerobic threshold (AT), i.e., the highest workrate or VO2 which was achieved without evidence of lactic acidosis, as judged from the profile of ventilatory and gas exchange responses. Ten healthy male subjects performed a supine graded cycle ergometer test before (pre) and after (post) BR which consisted of 4 min of unloaded pedaling at 60 rpm followed by an increased workrate of 15 W X min-1 until volitional fatigue (max). VE, VCO2, VO2, R, VE/VO2 and VE/VCO2 were measured every 30 s and used collectively to identify the AT. Plasma (PV) and blood (BV) volumes were measured pre- and post-BR by T-1824. Following BR, VO2max decreased from 2.42 +/- 0.17 to 2.25 +/- 0.13 L X min-1 (7.0%, p less than 0.05). BR significantly (p less than 0.05) reduced the AT from 1.26 +/- 0.09 to 0.95 +/- 0.05 L X min-1 VO2; from 52.2 +/- 2.0 to 42.6 +/- 1.6% VO2max; and from 93 +/- 9 to 65 +/- 6 W. A correlation coefficient (r) of -0.11 (NS) was found between the change in VO2max and change in AT. A decrease in BV of 8.8% (p less than 0.05) was due to the 11.0% reduction in PV; red cell volume remained constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Acidosis↗
PubMed1985

Response to muscular exercise following repeated simulated weightlessness.

The effect of repeated weightlessness exposures on maximal aerobic capacity was determined when seven healthy men (36-48 yr) underwent two 10-d bedrest (BR) periods in the -6 degrees headdown position, which were separated by a 14-d recovery period. No prescribed exercise was performed by the subjects during the course of the experiment. A graded supine cycle ergometer test consisting of 4 min of unloaded pedaling at 60 rpm followed by increased work rate of 15 W X min-1 until volitional fatigue (max) was performed before (pre) and after (post) the first and second BR periods, i.e., BR1 and BR2, and again 14 d after BR2 (REC). During exercise, submaximal and maximal oxygen uptake (VO2), ventilation (VE), heart rate (HR), systolic (SBP) and diastolic (DBP) blood pressures were measured and the gas exchange anaerobic threshold (AT) was determined. Plasma volume (Vp, T-1824) and body composition were measured pre- and post-BR1 and BR2 and following REC. Compared to the respective pre-BR control values, VO2max decreased (p less than 0.05) by 8.7% after BR1 and 5.2% after BR2 but returned to pre-BR values following 14 d REC. Submaximal and maximal HR increased (p less than 0.05) post-BR1 and BR2 but returned to pre-BR levels after REC. The AT and Vp decreased (p less than 0.05) post-BR1 and BR2 but returned to pre-BR levels after REC. Body weight increased (p less than 0.05) gradually during the experiment and did not return to control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗