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T J Koh

Publications and source records attributed to T J Koh.

35 records · Page 2Linked to original sources
PubMed1995

Evaluation of voluntary and elicited dorsiflexor torque-angle relationships.

The purposes of this study were to determine the relative influence of mechanical (muscle force and moment arm) and neural (activation) factors on the shape of dorsiflexor torque-angle relationships and to determine the interday reliability of measurements of such relationships. Dorsiflexor torque-angle relationships produced with maximal voluntary contractions (MVCs) and with electrically elicited contractions (twitch, 20-Hz tetanic, 40-Hz tetanic) were obtained for seven male subjects on 2 separate days. MVC torque-angle relationships were reproducible between days. Elicited relationships were less reproducible than the MVC relationships, with the 40-Hz relationship the most reproducible of the elicited relationships. The shapes of the MVC and elicited twitch torque-angle relationships were significantly different. The shapes of the MVC, 20-, and 40-Hz elicited relationships were similar. The simplest explanation for this latter finding is that the shape the MVC relationship is determined primarily by mechanical factors and is not strongly dependent on neural factors. The results of this study may affect the interpretation of comparisons of torque-angle relationships between subject groups or pre- and posttraining.

Adult↗
PubMed1994

Neuromechanics of the patellofemoral joint.

Patellofemoral joint pain is one of the most common ailments associated with visits to sports medicine clinics and can be disabling, although conservative clinical treatment has a reportedly very high success rate. Patellofemoral joint pain is often associated with improper tracking of the patella within the femoral trochlear notch. Improper tracking of the patella can be associated with increased patellofemoral contact pressures that may be a mechanical stimulus underlying patellar cartilage degeneration. In those cases in which anatomic anomalies and trauma may be excluded as the basis for improper tracking, attention is directed toward possible disruptions to the central nervous system control and contractile potential of the knee joint extensor musculature that underlies proper patellofemoral mechanics. This paper presents a review of three seminal components related to the neuromechanics of patellofemoral function; patellofemoral tracking, patellofemoral contact pressures, and neuromotor control of patellofemoral agonists. It is the intent of the authors to illuminate areas requiring further basic and clinical research and provide a point of departure for this work.

Biomechanical Phenomena↗
PubMed1993

A panning DLT procedure for three-dimensional videography.

The direct linear transformation (DLT) method [Abdel-Aziz and Karara, APS Symposium on Photogrammetry. American Society of Photogrammetry, Falls Church, VA (1971)] is widely used in biomechanics to obtain three-dimensional space coordinates from film and video records. This method has some major shortcomings when used to analyze events which take place over large areas. To overcome these shortcomings, a three-dimensional data collection method based on the DLT method, and making use of panning cameras, was developed. Several small single control volumes were combined to construct a large total control volume. For each single control volume, a regression equation (calibration equation) is developed to express each of the 11 DLT parameters as a function of camera orientation, so that the DLT parameters can then be estimated from arbitrary camera orientations. Once the DLT parameters are known for at least two cameras, and the associated two-dimensional film or video coordinates of the event are obtained, the desired three-dimensional space coordinates can be computed. In a laboratory test, five single control volumes (in a total control volume of 24.40 x 2.44 x 2.44 m3) were used to test the effect of the position of the single control volume on the accuracy of the computed three dimensional space coordinates. Linear and quadratic calibration equations were used to test the effect of the order of the equation on the accuracy of the computed three dimensional space coordinates. For four of the five single control volumes tested, the mean resultant errors associated with the use of the linear calibration equation were significantly larger than those associated with the use of the quadratic calibration equation. The position of the single control volume had no significant effect on the mean resultant errors in computed three dimensional coordinates when the quadratic calibration equation was used. Under the same data collection conditions, the mean resultant errors in the computed three dimensional coordinates associated with the panning and stationary DLT methods were 17 and 22 mm, respectively. The major advantages of the panning DLT method lie in the large image sizes obtained and in the ease with which the data can be collected. The method also has potential for use in a wide variety of contexts. The major shortcoming of the method is the large amount of digitizing necessary to calibrate the total control volume. Adaptations of the method to reduce the amount of digitizing required are being explored.

Biomechanical Phenomena↗
PubMed1993

Evaluation of methods to minimize cross talk in surface electromyography.

The purposes of this study were to quantify the effectiveness of various methods to minimize cross talk in surface electromyography (EMG) using common recording equipment, and to compare the intra- and interday variabilities of signals recorded with these methods. Comparisons were made for signals recorded with the single differential (SD), double differential (DD) and branched electrode (BE) techniques with large and small electrodes and corresponding interelectrode distances. The amount of cross talk in tibialis anterior EMG signals during maximum voluntary effort (MVE) triceps surae excitation was estimated using a protocol involving electrical stimulation of the triceps surae via the tibial nerve. In SD signals, cross talk averaged 12.2 and 10.2 percent MVE, for the large and small interelectrode distances, respectively. DD and BE signals contained significantly less cross talk (approximately 5 percent MVE for both techniques and interelectrode distances) than SD signals. The intra- and interday variabilities associated with these methods were estimated by recording tibialis anterior EMG signals during maximum voluntary isometric dorsiflexion (3 trials on each of 2 days) and calculating coefficients of variation for average-rectified values and median frequencies. EMG signals recorded with the small interelectrode distance showed greater interday amplitude variability than those recorded with the large interelectrode distance. Intra- and interday amplitude variabilities were similar across SD, DD and BE recording techniques. Intra- and interday frequency variabilities were similar across all experimental conditions. Thus, the DD and BE techniques, in conjunction with the large interelectrode distance (and large electrodes), provide a signal which contains significantly less cross talk than the SD technique without sacrificing intra- and interday amplitude and frequency stability.

Adult↗
PubMed1993

Kinematics of recovery from a stumble.

BACKGROUND: Injuries most frequently related to accidents in elderly persons are falls during locomotion and stair ascent and descent. Although numerous risk factors have been related to falling behavior, effective strategies to predict and prevent falls have not evolved. The rationale underlying this study was that systematic experimental and analytical investigation of the effects of perturbations during locomotion and the subsequent requisites for recovery could lead to the development of clinically relevant evaluation(s) capable of identifying a predisposition to falling. The present study is the first biomechanical investigation of recovery from an anteriorly directed stumble. METHODS: Seven healthy, young males participated in this study. While the subjects walked along a walkway in the laboratory, stumbles were unexpectedly induced using a mechanical obstacle. Videotape records of these trials were analyzed and selected sagittal plane kinematics extracted. RESULTS: The perturbation caused an increase in the maximum trunk flexion angle from 4.3 degrees (control) to 18.3 degrees (p = .057), and this change was significantly associated with preperturbation walking velocity (p = .036). The maximum hip and knee flexion angles increased from 26 to 47 degrees (p = .039) and from 60 to 89 degrees (p = .009), respectively. The increases in maximum hip flexion velocity (79%) and maximum knee extension velocity (36%) were not significant. CONCLUSIONS: Identification of the principal elements of control during perturbed locomotion can contribute to understanding the relationship between specific age-related performance deficits and some types of falling behavior. The results suggest that recovery from a stumble is dependent upon lower extremity muscular power and the ability to restore control of the flexing trunk.

Accidental Falls↗
PubMed1993

Bilateral deficit is larger for step than for ramp isometric contractions.

The force produced by a muscle group during a maximum effort bilateral task has been reported to be less than that produced during a maximum effort unilateral task. It was hypothesized that if decreased activation of primarily fast motor units is responsible for this bilateral deficit, 1) the bilateral deficit would be larger in step than in ramp isometric contractions and 2) the rate of torque generation in step contractions would be slower in bilateral than in unilateral tasks. Twelve healthy male subjects performed unilateral and bilateral isometric knee joint extension tasks in which maximum torque was generated under ramp and step conditions. The maximum torques produced in the bilateral tasks were 17.0 and 24.6% less than those produced in the unilateral tasks during the ramp and step conditions, respectively (both P < 0.001). The bilateral torque deficit for the step condition was significantly greater than that for the ramp condition (P = 0.012). The rate of torque generation in the step condition was 19.7% slower in the bilateral than in the unilateral task (P = 0.010). Thus the results from these torque measurements support the notion that decreased activation of primarily fast motor units may be responsible for the bilateral deficit.

Adult↗
PubMed1992

Cross talk in surface electromyograms of human hamstring muscles.

Signals generated from muscles other than the muscle(s) of interest (cross talk) can confound the interpretation of surface electromyograms (EMGs). In this study, the amount of cross talk in surface EMGs of human hamstring muscles was estimated using a protocol in which the quadriceps femoris was electrically stimulated via the femoral nerve. EMGs were recorded from the vastus lateralis and the medial and lateral hamstring muscle groups. The amplitude of the EMG response of the vastus lateralis to electrical stimulation was adjusted to match that of its maximum voluntary effort (MVE) under isometric conditions. Subsequent power density spectrum analysis showed that the median frequencies of the signals generated by electrical stimulation and MVE were not significantly different. In conventional bipolar recordings, cross talk in lateral hamstring EMGs averaged 17.1% MVE and in medial hamstring EMGs 11.3% MVE (average-rectified values). The double differential technique significantly reduced cross talk to 7.6% MVE for the lateral hamstrings, and to 4.2% MVE for the medial hamstrings. The double differential technique appears to be more selective than the bipolar technique when recording EMGs from muscles with highly active neighbors and thus should be used in such situations. Software simulations of the double differential technique also appear to be more selective than the bipolar technique and may be used when the number of amplifiers available is limited.

Adult↗
PubMed1992

In vivo tracking of the human patella.

The purpose of this study was to describe the dynamic, in vivo, three-dimensional tracking pattern of the patella for one normal male subject. Intracortical pins were inserted into the patella, tibia, and femur. The subject performed seated and squatting knee flexion/extension, and maximum voluntary quadriceps contractions. In addition, the vastus medialis oblique was subjected to maximal electrical stimulation. Motions of the markers attached to the intracortical pins were analyzed using an automated video system. Patellar and tibial motions were determined relative to a femoral reference system. While the tibia flexed 50 degrees from full extension (seated condition), the patella flexed 30.3 degrees, tilted laterally 10.3 degrees, and shifted laterally 8.6 mm. In general, these results show qualitative agreement with the data collected from cadaveric specimens [van Kampen and Huiskes, J. orthop. Res. 8, 372-382 (1990)]. The differences present may reflect different passive constraints to patellar motions, and different relative loading of the individual quadriceps components, in our study compared to the cadaveric study. Only small differences were found between patellar motions in the seated and squatting conditions. Differences in patellar displacements produced by (1) maximal electrical stimulation of the vastus medialis oblique, and (2) maximum voluntary quadriceps contraction, at 30 degrees knee flexion and full extension, may reflect the dominant influence of passive constraints, and the vastus lateralis, on normal patellar motions. Further in vivo study of patellar tracking seems warranted to evaluate surgical and conservative interventions for patellofemoral disorders.

Adult↗
PubMed1992

Decoupling of bilateral paraspinal excitation in subjects with low back pain.

This study compared bilateral paraspinal excitation in normal subjects and subjects with low back pain. Comparison was made between six control subjects and seven low back pain subjects who performed maximum-effort isometric trunk extension in minimum elapsed time at two trunk angles. Electromyographic signals were collected bilaterally from the paraspinal musculature. The time- and amplitude-normalized electromyographic data were analyzed using a repeatability criterion sensitive to temporal and amplitude differences. This analysis showed that low back pain subjects demonstrated temporal and amplitude decoupling of the paraspinal musculature bilaterally. Low back pain subjects also demonstrated clinically meaningful disruptions between paraspinal excitation and isometric trunk extension moment. This method may be useful in quantifying neuromotor control in low back pain for initial and follow-up clinical evaluation for static and dynamic functional tests.

Adult↗
PubMed1992

Further evidence against a direct automatic neuromotor link between the ACL and hamstrings.

The purpose of this experiment was to determine whether reflexive, or more broadly, automatic hamstrings excitation could be elicited during isometric, maximum effort, step increases in knee extension torque. Eight healthy subjects without lower extremity dysfunction or injury performed maximum effort isometric knee extension at 15 and 85 degrees of knee flexion in minimum elapsed time. Surface electromyography was used to record medial (semimembranosis and semitendinosis) and lateral (biceps femoris--long head) hamstrings excitation that was subsequently normalized to the excitation during maximum isometric knee flexion. To assess whether automatic hamstrings excitation was elicited, the amplitude of the EMG signals was analyzed and compared for 50 ms prior to peak knee extension torque, and three consecutive 50 ms windows following peak knee extension torque. The amplitude of the EMG subsequent to the peak knee extension torque failed to support the contention of an automatic hamstrings excitation mediated by neural circuitry from the ACL. The present results, in conjunction with previous work, suggest that a protective automatic hamstrings contraction is not normally elicited in response to conditions subjecting the anterior cruciate ligament to strain.

Adult↗
PubMed1991

Fatigue rates of vastus medialis oblique and vastus lateralis during static and dynamic knee extension.

Fatigue rates of the vastus medialis oblique and vastus lateralis muscles were compared from static and dynamic exercise. Based upon clinical observations, it was expected that the vastus medialis oblique would demonstrate greater fatigability than the vastus lateralis. However, based upon physiological and morphological considerations, it was expected that the vastus lateralis would exhibit greater fatigability than the vastus medialis oblique. In the static exercise condition, nine subjects--having no history of knee problems--maintained an isometric knee extension torque at 30 and 60% of their maximum value until exhaustion, during which time electromyography (EMG) data were collected every 10 s. The knee angle was 20 degrees flexion. In the dynamic exercise condition, a different group of seven subjects performed exercise sets consisting of eight cycles of concentric-eccentric knee extension, with a resistance equal to 40% of the maximum isometric value. The range of motion was restricted to the terminal 30 degrees of knee extension. Each set of eight repetitions was followed by a 1 s 50% maximum isometric knee extension, during which time EMG data were collected. The raw EMG data were numerically processed to extract the median frequency of the power density spectrum, which has been shown to reflect the metabolic processes associated with fatigue. Linear regression generated a slope coefficient representing the rate of change of the median frequency, with respect to contraction duration for each subject, muscle, and condition. Analyses of variance, with repeated measures from both exercise conditions suggest that "short-arc" quadriceps exercise did not selectively fatigue either the vastus medialis oblique or vastus lateralis, thereby supporting neither clinical nor physiological expected outcomes. Therefore, the clinical contention that short-arc quadriceps exercises selectively strengthen the vastus medialis oblique is questioned.

Adult↗
PubMed1987

Molecular cloning of chromosome I DNA from Saccharomyces cerevisiae: isolation of the MAK16 gene and analysis of an adjacent gene essential for growth at low temperatures.

MAK16 is an essential gene on chromosome I defined by the thermosensitive lethal mak16-1 mutation. MAK16 is also necessary for M double-stranded RNA replication at the permissive temperature for cell growth. As part of an effort to clone all the DNA from chromosome I, plasmids that complemented both the temperature-sensitive growth defect, and the M1 replication defects of mak16-1 strains were isolated from a plasmid YCp50: Saccharomyces cerevisiae recombinant DNA library. The two plasmids analysed contained overlapping inserts that hybridized proportionally to strains carrying different dosages of chromosome I. Furthermore, integration of a fragment of one of these clones occurred at a site linked to ade 1, confirming that this clone was derived from the appropriate region of chromosome I. An open reading frame adjacent to MAK16 potentially coding for a 468 amino acid protein was defined by sequence analysis. 185 amino acids of this open reading frame were replaced with a 1.2 kb fragment carrying the S. cerevisiae URA3 gene by a one-step gene disruption. The resulting strains grew at a rate indistinguishable from the wild type at 20 degrees C, 30 degrees C, or 37 degrees C, but could not grow at 8 degrees C. The deleted region is thus essential only at 8 degrees C, and we name this gene LTE1 (low temperature essential).

Base Sequence↗
PubMed1975

Effect of low doses of alpha-chlorohydrin on fertility and semen characteristics and binding of the drug of spermatozoa in swine.

Daily feeding of 1 mg of alpha-chlorohydrin per kg body weight to boars prevented fertility completely when the ejaculate was used for insemination. The semen charactreated than in untreated boars, but the sperm morphology was otherwise normal. In vitro addition of 5 mg/epididymal contents from the treated boars revealed normal Na+, K+ and glycerylphosphorylcholine concentrations. The movement of sperm cytoplasmic droplets was completed on all spermatozoa more distally in treated than in untreated boars, but the sperm morphology was otherwise normal. In vitro addition of 5 mg/100 ml of alpha-chlorohydrin to ejaculate boar semen completely inhibited and 2.5 mg/100 ml decreased fertility. Removal of the alpha-chlorohydrin prior to insemination partially restored fertility. 14C-alpha-chlorohydrin was shown to be more firmly bound to boar spermatozoa than 14C-carboxyinulin and could not be removed from the spermatozoa with 3 washings. The contraceptive mechanism of the drug is suggested to be alkylation of the sperm membrane by free alpha-chlorohydrin in the epididymis.

Animals↗
PubMedDate not supplied

Technique and ground reaction forces in the back handspring.

The ground reaction forces at the hand that produced compression forces and varus/valgus moments at the elbow joint during the double-arm support phase of the back handspring were studied. The relationship of technique, namely elbow joint flexion, to these forces were also studied. Compression forces and forces producing valgus moments have been implicated in overuse injuries to the elbow joint. Video and force plate analysis of six young female gymnasts showed that 1) the elbow joint flexed during the double-arm support phase, and 2) the reaction forces at the hand produced large compression forces (an average of 2.37 times body weight) and sizable valgus moments at the elbow (an average of 0.03 times body weight times body height). The combination of these forces may contribute to the occurrence of lateral compression injuries of the elbow joint (e.g., osteochondritis dissecans of the capitellum). Correlations of measures of elbow angle and measures of reaction force showed that large elbow flexions may protect the elbow from large valgus moments.

Adolescent↗
PubMedDate not supplied

Effect of an ankle orthosis and ankle ligament anesthesia on ankle joint proprioception.

The purposes of this study were to determine the effect of a rigid ankle orthosis (Aircast Air-Stirrup) and lateral ankle ligament anesthesia on ankle joint proprioception. Twelve noninjured subjects attempted to match nine reference ankle joint positions with their eyes closed before and after application of the ankle brace and before and after one or two of the lateral ankle ligaments (anterior talofibular and calcaneofibular) were anesthetized. Three-dimensional ankle joint orientations were recorded with a Motion Analysis system. No significant differences in the constant, variable, or absolute error were seen between subjects in the non-anesthetized and anesthetized conditions (P > 0.05), regardless of whether one or two ligaments were anesthetized. Thus, it appears that ligament mechanoreceptors contributed little to ankle joint proprioception, and that the afferent feedback from skin, muscle, and other joint receptors was adequate for the positioning task of the present study. Both the variable and absolute error in matching the reference positions were significantly less with the orthosis than without (P < 0.05). Application of an orthosis may increase the afferent feedback from cutaneous receptors in the foot and shank, which may in turn lead to an improved ankle joint position sense.

Adult↗
PubMedDate not supplied

The regulation of histidine decarboxylase gene expression.

Histamine is a biogenic amine, which is involved in a variety of biologic processes comprising inflammation, allergic responses, neurotransmission and regulation of gastric acid secretion. The key enzyme for the generation of histamine is histidine decarboxylase (HDC), which converts the amino acid L-histidine to histamine. In this article, we review the history, biochemistry and molecular biology of this enzyme. Northern blot studies in rats demonstrated that HDC gene expression in the stomach and liver are developmentally regulated with highest levels of expression in the late fetal state, indicating a role of the gene in growth and development. In the stomach of adult rats, HDC mRNA levels are elevated after omeprazole-induced hypergastrinemia, and in situ hybridization showed that expression of HDC is restricted to the glandular area in which ECL cells are located. Since no permanent ECL cell line is at hand for in vitro studies, we established a suitable cell system by stable transfection of a human gastric adenocarcinoma cell line (AGS) with the CCK-B/gastrin receptor. Transfection of this AGS-B cell line with reporter gene constructs comprising 5'-flanking DNA sequence of the HDC gene joined to the firefly luciferase gene revealed transcriptional regulation of the HDC promoter by gastrin through a protein-kinase C-dependent pathway. Taken together, these studies are consistent with the concept of HDC transcriptional regulation as at least one phase of the overall response to gastrin.

Animals↗