Jae Kun Shim

University of Maryland, College Park, Maryland, United States

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Publications (60)94.98 Total impact

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    ABSTRACT: Sensitivity of the myofascial trigger point (MTrP) can be inhibited by electrical stimulation of remote site. However, it remains unclear whether remote pain control of the MTrP occurs in the same spinal segment or in the supraspinal system.
    Neurorehabilitation 09/2014; · 1.42 Impact Factor
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    ABSTRACT: Carbon fiber running-specific prostheses (RSPs) have allowed individuals with lower extremity amputation (ILEA) to actively participate in sporting activities including competitive sports. In spite of this positive trait, the RSPs have not been thoroughly evaluated regarding potential injury risks due to abnormal loading during running. Vertical impact peak (VIP) and average loading rate (VALR) of the vertical ground reaction force (vGRF) have been associated with running injuries in able-bodied runners but not for ILEA. The purpose of this study was to investigate vGRF loading in ILEA runners using RSPs across a range of running speeds. Eight ILEA with unilateral transtibial amputations and eight control subjects performed overground running at three speeds (2.5, 3.0, and 3.5m/s). From vGRF, we determined VIP and VALR, which was defined as the change in force divided by the time of the interval between 20 and 80% of the VIP. We observed that VIP and VALR increased in both ILEA and control limbs with an increase in running speed. Further, the VIP and VALR in ILEA intact limbs were significantly greater than ILEA prosthetic limbs and control subject limbs for this range of running speeds. These results suggest that (1) loading variables increase with running speed not only in able-bodied runners, but also in ILEA using RSPs, and (2) the intact limb in ILEA may be exposed to a greater risk of running related injury than the prosthetic limb or able-bodied limbs.
    Gait & posture 08/2013; · 2.58 Impact Factor
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    ABSTRACT: Carbon fiber running-specific prostheses (RSPs) have allowed individuals with lower extremity amputation (ILEA) to participate in running. It has been established that as running speed increases, leg stiffness (Kleg) remains constant while vertical stiffness (Kvert) increases in able-bodied runners. The Kvert further depends on a combination of the torsional stiffnesses of the joints (joint stiffness; Kjoint) and the touchdown joint angles. Thus, an increased understanding of spring-like leg function and stiffness regulation in ILEA runners using RSPs is expected to aid in prosthetic design and rehabilitation strategies. The aim of this study was to investigate stiffness regulation to various overground running speeds in ILEA wearing RSPs. Eight ILEA performed overground running at a range of running speeds. Kleg, Kvert and Kjoint were calculated from kinetic and kinematic data in both the intact and prosthetic limbs. Kleg and Kvert in both the limbs remained constant when running speed increased, while intact limbs in ILEA running with RSPs have a higher Kleg and Kvert than residual limbs. There were no significant differences in Kankle, Kknee and touchdown knee angle between the legs at all running speeds. Hip joints in both the legs did not demonstrate spring-like function; however, distinct impact peaks were observed only in the intact leg hip extension moment at the early stance phase, indicating that differences in Kvert between limbs in ILEA are due to attenuating shock with the hip joint. Therefore, these results suggest that ILEA using RSPs has a different stiffness regulation between the intact and prosthetic limbs during running.
    Journal of biomechanics 08/2013; · 2.66 Impact Factor
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    ABSTRACT: The purpose of this study was to investigate the role of cutaneous feedback in the agonist-antagonist co-activation mechanism during maximum voluntary force (MVF) production by the fingers. Seventeen healthy male subjects (age: 23.8 ± 1.0 years) were asked to press with maximal effort at their fingertips. Finger forces at the fingertips and muscle activities of the flexor digitorum superficialis (FDS, agonist) and extensor digitorum communis (EDC, antagonist) were recorded using force sensors and electromyography, respectively. There were two experimental conditions: with and without administration of a ring block to the fingers (i.e., anesthesia and normal conditions, or AC and NC, respectively). The ring block was used to deprive cutaneous feedback. Consistent with previous studies, finger MVF decreased significantly in AC compared with NC. Moreover, the force production of non-task fingers significantly increased in AC. Muscle activity of the EDC was significantly lower in AC than in NC; no significant changes in the FDS muscle were observed. The findings of this study show that cutaneous feedback not only increases MVF and force accuracy, but facilitates agonist-antagonist co-activation by increasing antagonist muscle activation. The results of this study imply that cutaneous feedback is linked to both primary and adjacent motor neurons.
    Experimental Brain Research 07/2013; · 2.22 Impact Factor
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    ABSTRACT: OBJECTIVES: To (1) test the validity of a trifilar pendulum in estimating moments of inertia (MOI) for running-specific prostheses (RSPs), (2) measure inertial properties (mass, center of mass [CM] position, and MOIs) for four RSPs, (3) verify the influence of the stiffness on the inertial properties of RSPs, and (4) develop a predictive equation to estimate RSP CM positions. DESIGN: An aluminum block with known MOI was used for verifying the accuracy of the trifilar pendulum MOI measurements. MOI errors were investigated by systematically misaligning the block and pendulum principal axes across a range of 1-10 cm. Mass, CM position, and MOI were tested across four RSP designs having three stiffness categories each. Setting: University biomechanics laboratory. Specimens: Four different RSP designs and three stiffness categories per design were examined. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: MOI errors from known values and principal axis misalignments between RSPs and pendulum; mass, CM positions, RSP principal axis MOIs; predictive equation CM position errors. RESULTS: The trifilar pendulum estimated MOI within -6.21 x10(-5) kg·m² (≤ 1% error) for a block with known MOI. Misalignments of 1cm - 5cm between the RSPs' and pendulum's CM yielded errors from 0.00002 to 0.00113 kg·m(2) (0.3 - 59.2%). Each RSP's inertial properties are presented. MOI about any axis varied ≤ 0.0038 kg·m(2) across stiffness categories; MOI differed up to 0.0132 kg·m(2) between different designs. The predictive CM equation erred between 0.010-0.028 m when using average input values across an RSP design. CONCLUSIONS: Trifilar pendulums can accurately measure RSP MOI. RSP inertial properties differed slightly across stiffness categories within each design, but differed more substantially across different RSP designs. Using a predictive equation to estimate RSP CM positions can provide adequate data, but directly measuring CM positions is preferable.
    Archives of physical medicine and rehabilitation 03/2013; · 2.18 Impact Factor
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    ABSTRACT: Symmetric co-contraction of the transversus abdominis (TrA) muscle is beneficial in terms of increasing trunk stability. The aim of this study was to investigate the symmetry of lateral abdominal muscle thickness during static and dynamic conditions. Fifteen male subjects (27.13±5.51 years old) were instructed to sit on a chair and maintain upright posture. Every individual subject wore a jacket harness that could be backwardly attached to a 9-kg weight through a pulley system. An unexpected drop of the weight induced the transition from static to dynamic condition. The thickness of external oblique, internal oblique, and TrA muscles was measured with ultrasonography. Our results revealed more symmetry of TrA thickness during the dynamic condition (21% vs. 13%, p=0.019) compared with the static. The symmetric muscle thickness of TrA during the dynamic condition is considered a result of more contraction on the non-dominant side. This phenomenon could be a possible strategy of deep abdominal muscles to prevent spinal torsion during sudden trunk perturbation.
    Gait & posture 12/2012; · 2.58 Impact Factor
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    ABSTRACT: The aim of this study was to test the mechanical advantage (MA) hypothesis in multifinger torque production tasks in humans: fingers with longer moment arms produce greater force magnitudes during torque production tasks. There were eight experimental conditions: two prehension types determined by different mechanical constraints (i.e., fixed- and free-object prehension) with two torque directions (supination and pronation) and two torque magnitudes (0.24 and 0.48 N·m). The subjects were asked to produce prescribed torques during the fixed-object prehension or to maintain constant position of the free hand-held object against external torques. The index of MA was calculated for agonist and antagonist fingers, which produce torques in the same and opposite directions to the target torques, respectively. Within agonist fingers, the fingers with longer moment arms produced greater grasping forces while within antagonist fingers, the fingers with shorter moment arms produced greater forces. The MA index was greater in the fixed-object condition as compared with the free-object condition. The MA index was greater in the pronation condition than in the supination condition. This study supports the idea that the CNS utilizes the MA of agonist fingers, but not of antagonist fingers, during torque production in both fixed- and free-object conditions.
    Journal of applied biomechanics 07/2012; 28(3):284-90. · 1.26 Impact Factor
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    ABSTRACT: The purpose of this study was to investigate central nervous system (CNS) strategies for controlling multifinger forces during a circle-drawing task. Subjects drew 30 concentric, discontinuous clockwise and counter clockwise circles, at self and experimenter-set paces. The three-dimensional trajectory of the pen's center of mass and the three-dimensional forces and moments of force at each contact between the hand and the pen were recorded. Uncontrolled Manifold Analysis was used to quantify the synergies between pen-hand contact forces in radial, tangential and vertical directions. Results showed that synergies in the radial and tangential components were significantly stronger than in the vertical component. Synergies in the clockwise direction were significantly stronger than the counterclockwise direction in the radial and vertical components. Pace was found to be insignificant under any condition.
    Motor control 05/2012; 16(3):329-52. · 1.39 Impact Factor
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    ABSTRACT: The study investigates the effect of task uncertainty on motor synergies and movement time for a whole-body pointing task employing a Fitts' like paradigm. Thirty-three healthy, young adults were asked to hold a 1.5-m long stick and point it as quickly and accurately as possible to the unmarked center of fixed targets on the ceiling at 150% of the subject's height from the ground. Each subject performed fifteen continuous repetitions for each target size (1cm, 2cm, 3cm, 5cm, 8cm, 13cm and 21cm diameters of circles). It was assumed that the task uncertainty increased as the target size increased. Motion capture was used to collect the data for joint angles in the sagittal plane and uncontrolled manifold (UCM) analysis was used in order to investigate synergistic actions of joints. Results from the study revealed that the movement time decreased as task uncertainty increased. The variability within the uncontrolled manifold (V(UCM)) systematically increased with task uncertainty, resulting in an increase in the index of inter-joint synergies (ΔV), although the pointing task errors (V(ORT)) were consistent across different target sizes. The results suggest that the central nervous system systematically modulates the inter-joint synergies with task uncertainty in the whole-body pointing task without affecting motor performance.
    Neuroscience Letters 03/2012; 512(2):114-7. · 2.03 Impact Factor
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    ABSTRACT: This study investigates the role of cutaneous feedback on maximum voluntary force (MVF), finger force deficit (FD) and finger independence (FI). FD was calculated as the difference between the sum of maximal individual finger forces during single-finger pressing tasks and the maximal force produced by those fingers during an all-finger pressing task. FI was calculated as the average non-task finger forces normalized by the task-finger forces and subtracted from 100 percent. Twenty young healthy right-handed males participated in the study. Cutaneous feedback was removed by administering ring block digital anesthesia on the 2nd, 3rd, 4th and 5th digits of the right hands. Subjects were asked to press force sensors with maximal effort using individual digits as well as all four digits together, with and without cutaneous feedback. Results from the study showed a 25% decrease in MVF for the individual fingers as well as all the four fingers pressing together after the removal of cutaneous feedback. Additionally, more than 100% increase in FD after the removal of cutaneous feedback was observed in the middle and ring fingers. No changes in FI values were observed between the two conditions. Results of this study suggest that the central nervous system utilizes cutaneous feedback and the feedback mechanism plays a critical role in maximal voluntary force production by the hand digits.
    Journal of biomechanics 02/2012; 45(3):415-20. · 2.66 Impact Factor
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    ABSTRACT: The aim of this study was to investigate whether people change their isometric pinch grip generation depending on the surface they gripped. Specifically, the effect of grip surface friction condition on (a) maximum force produced in the direction normal to the contact surface, (b) fluctuation of normal force, and (c) the digit force's angular deviation from the direction normal to the grip surface was quantified. Isometric pinch grip has been traditionally thought to be independent from the friction condition between the finger and gripped surface, which may be questionable. For this study, 12 healthy participants performed maximum isometric pinch grip exertion on high-friction rubber and low-friction paper surfaces. Maximum normal force, normal force variance,and digit force's angular deviation from the normal direction were quantified. Pinch grip on the high-friction rubber surface was associated with 10% greater maximum normal force and 50% reduced normal force variance, compared with the low-friction paper surface (p < .05). Digit force's angular deviation was not significantly different between the two surface friction conditions. The data support that people do change their pinch grip generation (maximum normal force and normal force variance) depending on the surface they gripped, potentially by using sensory feedback. The results of this study demonstrate that even a simple isometric pinch grip (no lifting associated) is affected by grip surface friction. Grip surface condition should be considered for clinical assessments, biomechanical investigation, and motor control studies to ensure consistency in measurements and validity of comparisons.
    Human Factors The Journal of the Human Factors and Ergonomics Society 12/2011; 53(6):740-8. · 1.18 Impact Factor
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    ABSTRACT: The purpose of this study was to test the principle of motor abundance, which has been hypothesized as the principle by which the central nervous system controls the excessive degrees of freedom of the human movements, in contrast to the traditional negative view of motor redundancy. This study investigated the changes in force stabilizing and moment stabilizing synergies for multi-finger pressing tasks involving different number of fingers. Twelve healthy subjects produced a constant pressing force while watching visual feedback of the total pressing force for the fingers involved in each task. Based on the principle of motor abundance, it was hypothesized that the multi-finger synergies for the total force stabilizing synergy and the total moment stabilizing synergy would be greater as the number of task finger increases. Force stabilizing and moment stabilizing synergies were quantified using the framework of the uncontrolled manifold analysis. It was found that strong force stabilizing synergies existed for all the finger combinations. The index of force stabilizing synergies was greater when the task involved more number of fingers. The index of moment stabilizing synergies was negative for the two-finger combination, representing moment destabilizing synergies. However, the index of moment stabilizing synergies was positive for three-finger and four-finger combinations, representing strong moment stabilizing synergies for these finger combinations. We interpret the findings as an evidence for the principle of abundance for stabilization of both, total force as well as total moment.
    Experimental Brain Research 02/2011; 208(3):359-67. · 2.22 Impact Factor
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    ABSTRACT: The purpose of this study was to examine the relative importance of physiological characteristics during firefighting performance, as assessed by the Candidate Physical Ability Test (CPAT). Subjects included career and volunteer firefighters aged 18-39 (N = 33). Upper- and lower-body strength, muscle endurance, lower body muscle power, body composition analysis, aerobic capacity, anaerobic fitness, and the heart rate (HR) and blood pressure response to stair climbing were assessed to determine the physiological characteristics of the subjects. To quantify firefighting performance, the CPAT was administered by members of the fire service. Absolute and relative mean power during the Wingate anaerobic cycling test (WAnT), relative peak power during the WAnT, and absolute maximal oxygen uptake (VO2max) were significantly higher in those who passed the CPAT (N = 18), compared to those who failed (N = 15; p < 0.01). Mean power during the WAnT, fatigue index during WAnT, absolute VO2max, upper body strength, grip strength, and the HR response to stair climbing were significantly related to CPAT performance time (p < 0.01). Absolute VO2max and anaerobic fatigue resistance during WAnT best predicted CPAT performance (Adj. R2 = 0.817; p < 0.001). Performance on the ceiling breach and pull was the only CPAT task that was not significantly related to the physiological characteristics assessed. Measures of anaerobic and cardiovascular fitness best predict overall CPAT performance, and individual task performance. Remedial programs aimed at improving firefighting performance should target anaerobic and aerobic fitness qualities.
    The Journal of Strength and Conditioning Research 10/2010; 24(11):3112-22. · 1.80 Impact Factor
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    ABSTRACT: To determine the effects of lumbar extension exercise on strength, disability index, and pain scores in patients after lumbar discectomy surgery. Prospective comparative study. Forty patients experiencing a herniated disc at lumbar levels were divided into 4 subgroups for different training frequencies: twice/week (group 1), once/week (group 2), once/2 weeks (group 3), and no training (control). After completing the initial 12-week training, all subjects participated in a 12-week follow-up training. Groups 1 and 2 showed significant increases in lumbar extension strength (26 Nm and 7 Nm, respectively), while group 3 and the control group showed significant decreases in lumbar extension strength. Groups 1 and 2 showed significant decreases in disability index (1.4 and 0.8 Oswestry Disability Index points, respectively), and group 1 showed significant decreases in back and leg pain scores (both 0.5 units on a 10-cm visual analog scale). Lumbar extension strength and disability index improve with training frequencies of once and twice per week, while back and leg pain improve with a training frequency of twice per week. The clinical importance of these improvements is questionable, as the scores were already very low after the discectomy and the magnitude of absolute improvements were small.
    Journal of rehabilitation medicine: official journal of the UEMS European Board of Physical and Rehabilitation Medicine 10/2010; 42(9):839-45. · 1.88 Impact Factor
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    ABSTRACT: This study investigated synergistic actions of hand-pen contact forces during circle drawing tasks in three-dimensional (3D) space. Twenty-four right-handed participants drew thirty concentric circles in the counterclockwise (CCW) and clockwise (CW) directions. Three-dimensional forces acting on an instrumented pen as well as 3D linear and angular positions of the pen were recorded. These contact forces were then transformed into the 3D radial, tangential, and normal force components specific to circle drawing. Uncontrolled manifold (UCM) analysis was employed to calculate the magnitude of the hand-pen contact force synergy. Three hypotheses were tested. First, hand-pen contact force synergies during circle drawing are dependent on the angular position of the pen tip. Second, hand-pen contact force synergies are dependent on force components in circle drawing. Third, hand-pen contact force synergies are greater in CCW direction than CW direction. The results showed that the strength of the hand-pen contact force synergy increased during the initial phase of circle drawing and decreased during the final phase. The synergy strength was greater for the radial and tangential components as compared to the normal component. Also, the circle drawing in CW direction was associated with greater hand-pen contact force synergy than the CCW direction. The results of this study suggest that the central nervous system (CNS) prioritizes hand-pen contact force synergies for the force components (i.e., radial and tangential) that are critical for circle drawing. The CNS modulates hand-pen contact force synergies for preparation and conclusion of circle drawing, respectively.
    Journal of biomechanics 08/2010; 43(12):2249-53. · 2.66 Impact Factor
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    ABSTRACT: The purpose of this study is to develop and validate a model with optimal set-up of reflective markers, producing minimal errors in inverse dynamics calculations. The Statement of Work for this project indicated two specific aims. Specific Aim 1 proposed to develop and validate a model with unique optimal marker placements for specific running prosthesis designs. The proposed timeline indicated that preparation for the experimental setting and formulation of the program for data analysis would occur during Months 1-8. These milestones were reached on schedule. During Months 8-16, we proposed to complete MTS testing, begin validating the general model, and begin analyzing the MTS data to determine the final marker model for each running-specific prosthesis. We are currently in Month 13 of the project since its formal approval on August 1, 2009, and we are on schedule of the proposed timeline for these milestones. The general analysis model is in the process of validation and raw experimental data has begun being analyzed for the final marker model for each prosthesis design. Upon completion of Specific Aim 1 goals, we will complete goals set for Specific Aim 2, to determine the resultant optimal marker placement for all tested running prosthetic designs.
    07/2010;
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    You-Sin Kim, Jaebum Park, Jae Kun Shim
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    ABSTRACT: To compare the effects of aquatic backward locomotion exercise and progressive resistance exercise with a machine on lumbar extension strength in patients who have undergone diskectomy for a lumbar disk herniation. Prospective comparative study. Department of Kinesiology at a state university. Male patients (N=30) with disk herniation at spinal levels L3 to S1 completed this study as subjects. After the diskectomy for a lumbar disk herniation, all patients had 6 weeks of rest time. At the end of the rest period, the aquatic backward locomotion exercise and progressive resistance exercise groups, respectively, started first 6 weeks of underwater training and lumbar extension training twice per week. After completion of the first 6-week training, subjects participated in a second 6-week training. After the whole 12-week training, subjects had no training for 6 weeks (detraining) and a follow-up 6-week training (retraining). The control (CON) group did not undergo any training. For each test, maximum voluntary isometric lumbar extension strength was measured in 7 trunk positions (72 degrees , 60 degrees , 48 degrees , 36 degrees , 24 degrees , 12 degrees , and 0 degrees of the trunk angle). The progressive resistance exercise and aquatic backward locomotion exercise groups showed increases in lumbar extension strength after the first 6-week training, although they were not statistically different from the CON group. After a second 6-week training, the progressive resistance exercise and aquatic backward locomotion exercise groups showed statistically significant increases in their strength levels as compared with the CON group. After the detraining period, the strength levels of the progressive resistance exercise and aquatic backward locomotion exercise groups did not statistically differ from the CON group. After the retraining period, the progressive resistance exercise and aquatic backward locomotion exercise groups showed increases in their strength levels, which were different from that of the CON group. The results obtained suggested that the aquatic backward locomotion exercise is as beneficial as progressive resistance exercise for improving lumbar extension strength in patients after lumbar diskectomy surgery.
    Archives of physical medicine and rehabilitation 02/2010; 91(2):208-14. · 2.18 Impact Factor
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    Jaebum Park, You-Sin Kim, Jae Kun Shim
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    ABSTRACT: Previous studies have shown that the interactions of human hand digits are influenced by internal constraints, i.e., biomechanical and central constraints. However, little is currently known about the influence of externally imposed mechanical constraints on multi-finger behavior. This study investigates maximal digit force production during fixed object and free object prehension in statics. The results from the fixed object prehension indicated that the closer the non-task finger was positioned to the task finger, the greater the force produced by the non-task finger, which supports the proximity hypothesis. Conversely, the non-task fingers with longer moment arms showed greater force production during free object prehension, which supports the mechanical advantage hypothesis. During the free object prehension, equal and opposite torques were produced by the digit normal forces and tangential forces, while this phenomenon was not observed in the fixed object prehension. The results also showed that the thumb normal force had a positive linear relationship with task-finger normal forces during fixed object prehension while the thumb normal force remained constant during free object prehension tasks. We concluded that the CNS employed different strategies when different sets of internal and external constraints are provided during multi-digit prehension tasks.
    Human movement science 02/2010; 29(1):19-34. · 2.15 Impact Factor
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    ABSTRACT: The authors examined the kinematic qualities of the aiming trajectory as related to expertise. In all, 2 phases of the trajectory were discriminated. The first phase was regular approximation to the target accompanied by substantial fluctuations obeying the Weber-Fechner law. During the first phase, shooters did not initiate the triggering despite any random closeness of the aiming point (AP) to the target. In the second phase, beginning at 0.6-0.8 s before the trigger pull, shooters applied a different control strategy: They waited until the following random fluctuation brought the AP closer to the target and then initiated triggering. This strategy is tenable when sensitivity of perception is greater than precision of the motor action, and could be considered a case of stochastic resonance. The strategies that novices and experts used distinguished only in the values of parameters. The authors present an analytical model explaining the main properties of shooting.
    Journal of Motor Behavior 08/2009; 41(4):367-82. · 1.04 Impact Factor
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    ABSTRACT: This study investigated the phenomena of finger enslaving, involuntary finger actions by non-intended fingers, and force deficit, smaller maximum force by all four fingers than the sum of individual finger maximum forces in individuals with cervical spinal cord injuries (SCI). A total of 16 subjects participated in this study: 8 with a cervical spinal cord injury and 8 controls. Each of the injured subjects had one paralyzed finger. The results showed that the efforts to produce force using any individual finger induced force production in all other fingers, suggesting finger force enslaving. The maximum force during the four-finger task was greater than the sum of the individual finger forces during single-finger tasks in the SCI group, which was reflected by positive force deficit, "force surplus". One may utilize these findings for rehabilitation of paralyzed fingers caused by cervical spinal injuries.
    Experimental Brain Research 06/2009; 195(4):627-33. · 2.22 Impact Factor

Publication Stats

692 Citations
94.98 Total Impact Points

Institutions

  • 2006–2014
    • University of Maryland, College Park
      • Department of Kinesiology
      Maryland, United States
    • William Penn University
      Worcester, Massachusetts, United States
  • 2013
    • Japan Society for the Promotion of Science
      Edo, Tōkyō, Japan
    • Kyung Hee University
      Sŏul, Seoul, South Korea
  • 2012
    • Seoul National University
      • Department of Physics Education
      Seoul, Seoul, South Korea
  • 2008
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 2003–2008
    • Pennsylvania State University
      • Department of Kinesiology
      University Park, MD, United States