119 research outputs found
BIOMECHANICAL BASIS OF STRENGTH TRAINING
The selection of strength exercises for qualified athletes is based on the idea of specificity. This means that training drills must be relevant to the demands of the event for which an athlete is being trained. Strength training drills must mimic the movement pattern used during the actual execution of the pertinent sport skill. However, the practical realization of this general idea is not easy Many efforts have been put forth by coaches, athletes, and scientists to find the most effective strength training drills for various sports. The main requirements of such a task are described as follows: (a) Working Muscles. The same muscle groups must be involved in the main sport event and in the training drill. (b) Type of Resistance. Tf the type of training drill resistance is changed in comparison to the resistance in the sport event itself, for which the athlete is being trained, both force production and the pattern of muscle activity are altered. (c) Time (and Rate) of Force Development. If the objective of the training is to increase maximal force production, Fm, there is no reason to use exercises in the time deficit zone, where Fm can not be developed. Tn turn, heavy resistance exercises are not a very useful training tool for enhancing the rate of force development in qualified athletes. (d) Velocity of Movement. Tf exercises are performed in the 'high force, low velocity' range of the force-velocity curve, maximal force Fm increases mainly in the trained range. On the other hand, if the 'low force, high velocity' range is used in training, the performance is improved primarily in this area. (e) Direction of Movement. Both the yielding strength and strength in reversible muscle action should be considered and trained as separate motor abilities. (f) Force - Posture Relationship. The following three approaches are used in practice and described in the presentation: Peak- Contraction Principle, Accommodating Resistance, and Accentuation. In addition several types of strength exercises are analyzed: (a) yielding exercises, (b) exercises with reversible muscle action, and (c) main sport exercises with additional resistance
MULTI-FINGER PREHENSION: BIOMECHANICS AND CONTROL
Since 1998 our group published about 20 papers in peer- reviewed journals on biomechanics and control of multi-finger tasks (see a Reference List). The research was done together with Dr. M.L. Latash in cooperation with post-doctoral fellows and graduate students Dr. F. Oanion, Z.-M. Li, S.Li, R. Gregory, F.Gao and T.Pataky. The goal of this presentation is to review some of these publications and to report on new results. Many sports-from basketball to javelin throwing and from archery to racket sports-require grasping and manipulation of hand-held objects. Study of multi-finger prehension is an imperative field of research: although human civilization has been build by hands, regrettably we know little about hand functioning. Numerous practical applications of the problem range from clinics and ergonomics to robotics. In multi-finger grasps, the fingers are statically redundant-the number of unknown forces exceeds the number of equilibrium equations-and kinematically over- onstrained, a variation in the position of a grasped object affects the position of all the fingers (likewise, a joint angle defines the length of all the muscles crossing the joint). The grasping hand is a convenient object to study the motor redundancy problem because all the involved forces can be directly measured and the sharing pattern easy documented. This is not available when the motor redundancy problem is addressed at the level of individual muscles and their contribution into the total joint torque-a most popular object for studying the sharing problem. Two considerations, a general and a specific one inspired this study. From a general perspective the idea is to study the problem of motor redundancy using the fingers as an expedient object. From a more specific standpoint, hand and finger function by itself is worthy of study
Postural Synergies and Their Development
The recent developments of a particular approach to analyzing motor synergies based on the principle of motor abundance has allowed a quantitative assessment of multieffector coordination in motor tasks involving anticipatory adjustments to self-triggered postural perturbations and in voluntary posturalsway. This approach, the uncontrolled manifold (UCM) hypothesis, is based on an assumption that the central nervous system organizes covariation of elemental variables to stabilize important performance variables in a task-specific manner. In particular, this approach has been used to demonstrate and to assess the emergence of synergies and their modification with motor practice in typical persons and persons with Down syndrome. The framework of the UCM hypothesis allows the formulation of testable hypotheses with respect to developing postural synergies in typically and atypically developing persons
Is Power Grasping Contact Continuous or Discrete?
During power grasp, the number of local force maxima reflects either the central nervous system's preferential use of particular hand regions, or anatomical constraints, or both. Previously, both bimodal and trimodal force maxima have been hypothesized for power grasp of a cylindrical handle. Here we measure the number of local force maxima, with a resolution of 4.8 degrees, when performing pushing and pulling efforts in the plane perpendicular to the cylinder's long axis. Twelve participants produced external forces to eight targets. The number of contacts was defined as the number of local maxima exceeding background variance. A minimum of four and a maximum of five discrete contacts were observed in all subjects at the distal phalanges and metacarpal heads. We thus reject previous hypotheses of bimodal or trimodal force control for cylindrical power grasping. Since we presently observed only 4-5 contacts, which is rather low considering the hand's kinematic flexibility in the flexion plane, we also reject hypotheses of continuous contact, which are inherent to current grasping taxonomy. A modification to current grasping taxonomy is proposed wherein power grasp contains separate branches for continuous and discrete contacts, and where power and precision grasps are distinguished only by grasp manipulability.ArticleJOURNAL OF APPLIED BIOMECHANICS. 29(5):554-562 (2013)journal articl
Hijerarhije sinergija u ljudskim pokretima
This brief review addresses the problem of motor redundancy, which exists at many levels of the neuro-motor hierarchies involved in the production of voluntary movements. An approach to this problem is described based on the principle of abundance. This approach offers an operational definition for motor synergies using the framework of the uncontrolled manifold hypothesis. It is shown that hierarchical systems have inherent trade-offs between synergies at different control levels. These trade-offs have been demonstrated in experimental studies of human multi-finger pressing and prehension. They are likely to be present in other hierarchical systems, for example, those involved in the control of large groups of muscles. The framework of the equilibrium-point hypothesis offers a physiologically based mechanism, which may form the basis for hierarchies of synergies.Ovaj se pregledni rad bavi problemom motoriÄke redundancije (zalihosti) koja postoji na viÅ”e razina neuromotoriÄkih hijerarhija ukljuÄenih u realizaciju voljnih pokreta. Opisan je pristup utemeljen na principu obilja (brojnosti). Pristup nudi operativnu definiciju za motoriÄke sinergije koriÅ”tenjem okvira Å”to ga pruža hipoteza neupravljanih slojeva (ljusaka). Pokazuje se da hijerarhijski sustavi posjeduju inherentne kompromise izmeÄu sinergija na razliÄitim razinama upravljanja. Ti se kompromisi mogu pokazati pomoÄu eksperimentalnih studija pritiska prstima ljudske Å”ake. Vjerojatno je da su isti prisutni i u drugim hijerarhijskim sustavima, npr. onima ukljuÄenima u upravljanje velikim skupinama miÅ”iÄa. Okvir hipoteze ravnotežne toÄke nudi fizioloÅ”ki utemeljen mehanizam koji može predstavljati osnovu za hijerarhije sinergija.
Problem motoriÄke redundancije
Svi neuromotoriÄki procesi unutar ljudskoga ti-jela povezani s izvoÄenjem prirodnih voljnih pokreta ukljuÄuju nekoliko preslikavanja (mapiranja) tipa āod nekoliko na viÅ”eā, kakva se uobiÄajeno smatraju problemom redundancije. Drugim rijeÄima, ograniÄenja definirana ulazom (npr. zadatkom) ne definiraju jednoznaÄno uzorak izlaza (npr. uzorci rotacije zglobova, miÅ”iÄne sile, aktivacije motoriÄkih neurona itd.) na naÄin da postoji viÅ”e (beskonaÄan broj, uobiÄajeno) rjeÅ”enja. Problem je uoÄio Bernstein (1935, 1967), smatrajuÄi ga srediÅ”njim problemom motoriÄkog upravljanja: āNa koji naÄin srediÅ”nji živÄani sustav (SŽS) odabire jednoznaÄna rjeÅ”enja iz brojnih, naizgled jednakih moguÄnosti?ā
Tradicionalni naÄin shvaÄanja problema motoriÄke redundancije pretpostavljao je da SŽS rabi skup kriterija da bi pronaÅ”ao jednoznaÄna rjeÅ”enja takvih problema. Konkretno, mnoÅ”tvo optimizacijskih tehnika uporabljeno je za pristup tim problemima ukljuÄujuÄi optimizaciju funkcija troÅ”kovi-korist, temeljenu na mehaniÄkim, psihologijskim i neuropsihologijskim varijablama (vidjeti pregled u Prilutsky, 2000; Osenbaum i sur., 1993; Latash, 1993).
Princip obilja
Gelfand i Tselin (1966) su usporedili mnoge ele-mente ukljuÄene na bilo kojem koraku generiranja pokreta s razredom studenata koji žele sa Å”to manje rada izvrÅ”iti zadatak. Uveli su princip minimalnog meÄudjelovanja da bi opisali takve oblike velikih skupina elemenata. Prema tom naÄelu svaki element nastoji minimizirati svoje meÄudjelovanje s ostalima, s upravljaÄkim dijelom te s okolinom. Drugim rijeÄima, svaki element nastoji minimizirati ulaz koji prima iz svih spomenutih izvora.
Taj je princip u novije vrijeme razvijen u princip obilja (Gelfand i Latash, 1998). Prema njemu su problemi motoriÄke redundancije pogreÅ”no formulirani. Preslikavanja tipa āod nekoliko na viÅ”eā, tipiÄna za takve probleme, ne bi trebalo gledati kao problem raÄunalne naravi za upravljaÄki sustav, nego pak viÅ”e kao svojevrsni luksuz koji dozvoljava kombiniranje stabilnog funkcioniranja zadatka uz obavljanje ostalih zadataka i reagiranje na moguÄe ometajuÄe utjecaje okoline. RjeÅ”avanje problema motoriÄke redundancije ne ukljuÄuje izbor jednoznaÄnog, optimalnog rjeÅ”enja, nego prije olakÅ”a-vanje Äitave obitelji rjeÅ”enja koje mogu biti jedna-ko uspjeÅ”ne u rjeÅ”avanju problema. Broj tih obitelji rjeÅ”enja puno je manji od ukupnog broja moguÄih rjeÅ”enja, Å”to znaÄi da se ipak dogaÄa neka vrsta selekcije. Taj pomak od traženja jedinstvenog rjeÅ”enja prema definiranju pravila kojima se organiziraju obitelji rjeÅ”enja rezultirao je novim pogledom na motoriÄke sinergije, paradigmatskim pomakom koji je doveo do izvedbene definicije sinergija i do stvaranja novog raÄunalnog pristupa identifikaciji i kvantifikaciji sinergija.
Sinergija - radna definicija
RijeÄ āsinergijaā rabila se u studijima ljudskog kretanja, kao i za opis motoriÄkih poremeÄaja viÅ”e od stotinu godina. OpÄenito, definicija je bila sukladna s grÄkim prijevodom āraditi zajednoā. U posljednje vrijeme, meÄutim, ta je rijeÄ poprimila odreÄenije znaÄenje ukorijenjeno u principu obilja (detalj-no vidjeti u Latash, 2008). Postoje, naime, tri vrste sinergija. Prvo, kada je u zadatak ukljuÄen privi-dno redundantni skup elemenata, odabire se srednji uzorak raspodjele koji Äe karakterizirati prosjeÄni doprinos svakog elementa. Drugo, kada se analizira nekoliko pokuÅ”aja izvedbe zadatka, izlazi elemenata mogu kovarirati, Å”to je za zadatak korisno, tj. smanjuje se varijabilnost važne varijable u usporedbi sa situacijom koja bi se mogla oÄekivati kada kovarijacije ne bi bilo. To se svojstvo ponekad naziva kompenzacijom pogreÅ”ke ili stabilnoÅ”Äu. TreÄe, isti skup elemenata može se rabiti za formiranje razliÄitih sinergija, tj. razliÄitih uzoraka kovarijacije koji su povoljni za razliÄite varijable cjelokupnog sustava. To se svojstvo može nazvati stabilnoÅ”Äu. Samo sustavi koji mogu pokazati sva tri svojstva nazivat Äe se sinergijama. Nema apstraktnih sinergija ā one uvijek neÅ”to Äine.
Sinergija se, prema tomu, definira kao neuralna organizacija skupa elementarnih varijabla s ciljem osiguranja svojevrsnih svojstava stabilnosti (stabilizirati ili destabilizirati) varijable koja je izlaz sustava kao cjeline.
Hipoteza neupravljanih ljusaka (UCM ā uncontrolled manifold hypothesis) i hijerarhijsko upravljanje
Uvedena definicija sinergije zahtijeva kvantitativnu metodu koja bi mogla razlikovati sinergiju od nesinergije, kao i kvantificirati sinergije. Takva je metoda razvijena u sklopu nekontroliranih viŔeslojnih hipoteza. Ona pretpostavlja da neuralni kontroler djeluje u prostoru elementarnih varijabla
i u tom prostoru izabire potprostore koji odgovaraju željenoj vrijednosti uspjeÅ”no izvedene varijable. Nadalje, kontroler organizira interakcije meÄu elementima tako da je varijanca meÄu elementarnim varijablama uglavnom ograniÄena UCM-om. Bilo je nekoliko pokuÅ”aja da se ponude mehanizmi koji mogu organizirati takvu vrstu kontrole ā feedback perifernih senzora, feedback koji koristi uparivanje centralnih i povratnih neuralnih petlji, kontrolni anticipacijski program.
Pojam referentne konfiguracije pruža privlaÄan okvir za analizu motoriÄkih sinergija. Taj okvir pretpostavlja hijerarhijski kontrolni sustav u kojemu je , na svakom stupnju hijerarhije, taj sustav redundantan, tj. proizvodi puno viÅ”e izlaznih varijabli od broja ograniÄenja specificiranih ulaznim varijablama (kao na slici 3). Ostale karakteristike akcije mogu varirati na temelju sekundarnih zakonitosti, koje vjerojatno odražavaju optimizaciju nekih osobina izvedbe. Zato Å”to je sustav redundantan, referentna konfiguracija na viÅ”em hijerarhijskom stupnju ne specificira sasvim nedvojbeno sve referentne konfiguracije na nižim stupnjevima. Izranjanje odreÄenih nižerazinskih referentnih trajektorija može se temeljiti na mehanizmu povratne sprege ili na mehanizmu anticipacije (feed-forward). Stoga se hijerarhija kontrolnih razina, gdje svaka razina funkcionira na na-Äelu kontrole ravnotežne toÄke, Äini vrlo vjerojatnom strukturom koja podržava motoriÄke sinergije
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