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

Spatio-Temporal Human Grip Force Analysis via Sensor Arrays

This study describes a technique for measuring human grip forces exerted on a cylindrical object via a sensor array. Standardised resistor-based pressure sensor arrays for industrial and medical applications have been available for some time. We used a special 20 mm diameter grip rod that subjects could either move actively with their fingers in the horizontal direction or exert reactive forces against opposing forces generated in the rod by a linear motor. The sensor array film was attached to the rod by adhesive tape and covered approximately 45 cm2 of the rod surface. The sensor density was 4/cm2 with each sensor having a force resolution of 0.1 N. A scan across all sensors resulted in a corresponding frame containing force values at a frame repetition rate of 150/s. The force value of a given sensor was interpreted as a pixel value resulting in a false-colour image. Based on remote sensed image analysis an algorithm was developed to distinguish significant force-representing pixels from those affected by noise. This allowed tracking of the position of identified fingers in subsequent frames such that spatio-temporal grip force profiles for individual fingers could be derived. Moreover, the algorithm allowed simultaneous measurement of forces exerted without any constraints on the number of fingers or on the position of the fingers. The system is thus well suited for basic and clinical research in human physiology as well as for studies in psychophysics

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

Does outstretching the arms improve postural stability?

We spontaneously outstretch our arms when standing upon challenging surfaces, yet the effect of stretching the arms upon postural stability is unknown. We investigated whether stretching out the arms laterally improves postural control during tandem stance on a narrow beam. Twelve healthy participants stood upon a beam, right foot in front of the left foot, for 30 s with arms outstretched or down to the side, with eyes open and closed. Mediolateral head movement was characterised by Root Mean Square amplitude (RMS), sway path, velocity during the largest excursion and power spectrum. Spectra for lateral forces from a force platform beneath the beam were also recorded. Outstretching the arms significantly reduced RMS, sway path and velocity of maximum displacement of head movement with eyes closed but not with eyes open. A similar trend was present in the power spectra of head motion and sway platform lateral forces. In conclusion, outstretching the arms helps postural stability in challenging situations such as tandem stance on a narrow beam with eyes closed. Although the exact mechanisms require further investigation, the effects are most likely mediated by changes in segmental inertia and the ability to make corrective arm movements.MRC grant to A.M.B. (MC_U950770497

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 motoričke redundancije (zalihosti) koja postoji na više razina neuromotoričkih hijerarhija uključenih u realizaciju voljnih pokreta. Opisan je pristup utemeljen na principu obilja (brojnosti). Pristup nudi operativnu definiciju za motoričke sinergije korištenjem okvira što ga pruža hipoteza neupravljanih slojeva (ljusaka). Pokazuje se da hijerarhijski sustavi posjeduju inherentne kompromise između sinergija na različitim razinama upravljanja. Ti se kompromisi mogu pokazati pomoću eksperimentalnih studija pritiska prstima ljudske šake. Vjerojatno je da su isti prisutni i u drugim hijerarhijskim sustavima, npr. onima uključenima u upravljanje velikim skupinama mišića. Okvir hipoteze ravnotežne točke nudi fiziološki utemeljen mehanizam koji može predstavljati osnovu za hijerarhije sinergija. Problem motoričke redundancije Svi neuromotorički procesi unutar ljudskoga ti-jela povezani s izvođenjem prirodnih voljnih pokreta uključuju nekoliko preslikavanja (mapiranja) tipa “od nekoliko na više”, kakva se uobičajeno smatraju problemom redundancije. Drugim riječima, ograničenja definirana ulazom (npr. zadatkom) ne definiraju jednoznačno uzorak izlaza (npr. uzorci rotacije zglobova, mišićne sile, aktivacije motoričkih neurona itd.) na način da postoji više (beskonačan broj, uobičajeno) rješenja. Problem je uočio Bernstein (1935, 1967), smatrajući ga središnjim problemom motoričkog upravljanja: “Na koji način središnji živčani sustav (SŽS) odabire jednoznačna rješenja iz brojnih, naizgled jednakih mogućnosti?” Tradicionalni način shvaćanja problema motoričke redundancije pretpostavljao je da SŽS rabi skup kriterija da bi pronašao jednoznačna rješenja takvih problema. Konkretno, mnoštvo optimizacijskih tehnika uporabljeno je za pristup tim problemima uključujući optimizaciju funkcija troškovi-korist, temeljenu na mehaničkim, 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 uključene na bilo kojem koraku generiranja pokreta s razredom studenata koji žele sa što manje rada izvršiti zadatak. Uveli su princip minimalnog međudjelovanja da bi opisali takve oblike velikih skupina elemenata. Prema tom načelu svaki element nastoji minimizirati svoje međudjelovanje s ostalima, s upravljačkim dijelom te s okolinom. Drugim riječima, 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 motoričke redundancije pogrešno formulirani. Preslikavanja tipa “od nekoliko na više”, tipična za takve probleme, ne bi trebalo gledati kao problem računalne naravi za upravljački sustav, nego pak više kao svojevrsni luksuz koji dozvoljava kombiniranje stabilnog funkcioniranja zadatka uz obavljanje ostalih zadataka i reagiranje na moguće ometajuće utjecaje okoline. Rješavanje problema motoričke redundancije ne uključuje izbor jednoznačnog, optimalnog rješenja, nego prije olakša-vanje čitave obitelji rješenja koje mogu biti jedna-ko uspješne u rješavanju problema. Broj tih obitelji rješenja puno je manji od ukupnog broja mogućih rješenja, što znači da se ipak događa neka vrsta selekcije. Taj pomak od traženja jedinstvenog rješenja prema definiranju pravila kojima se organiziraju obitelji rješenja rezultirao je novim pogledom na motoričke sinergije, paradigmatskim pomakom koji je doveo do izvedbene definicije sinergija i do stvaranja novog računalnog pristupa identifikaciji i kvantifikaciji sinergija. Sinergija - radna definicija Riječ “sinergija” rabila se u studijima ljudskog kretanja, kao i za opis motoričkih poremećaja više od stotinu godina. Općenito, definicija je bila sukladna s grčkim prijevodom “raditi zajedno”. U posljednje vrijeme, međutim, ta je riječ poprimila određenije značenje ukorijenjeno u principu obilja (detalj-no vidjeti u Latash, 2008). Postoje, naime, tri vrste sinergija. Prvo, kada je u zadatak uključen privi-dno redundantni skup elemenata, odabire se srednji uzorak raspodjele koji će karakterizirati prosječni doprinos svakog elementa. Drugo, kada se analizira nekoliko pokušaja izvedbe zadatka, izlazi elemenata mogu kovarirati, što je za zadatak korisno, tj. smanjuje se varijabilnost važne varijable u usporedbi sa situacijom koja bi se mogla očekivati kada kovarijacije ne bi bilo. To se svojstvo ponekad naziva kompenzacijom pogreške ili stabilnošću. Treće, isti skup elemenata može se rabiti za formiranje različitih sinergija, tj. različitih uzoraka kovarijacije koji su povoljni za različite varijable cjelokupnog sustava. To se svojstvo može nazvati stabilnošću. Samo sustavi koji mogu pokazati sva tri svojstva nazivat će se sinergijama. Nema apstraktnih sinergija – one uvijek nešto čine. 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 – 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 višeslojnih hipoteza. Ona pretpostavlja da neuralni kontroler djeluje u prostoru elementarnih varijabla i u tom prostoru izabire potprostore koji odgovaraju željenoj vrijednosti uspješno izvedene varijable. Nadalje, kontroler organizira interakcije među elementima tako da je varijanca među elementarnim varijablama uglavnom ograničena UCM-om. Bilo je nekoliko pokušaja da se ponude mehanizmi koji mogu organizirati takvu vrstu kontrole – feedback perifernih senzora, feedback koji koristi uparivanje centralnih i povratnih neuralnih petlji, kontrolni anticipacijski program. Pojam referentne konfiguracije pruža privlačan okvir za analizu motoričkih sinergija. Taj okvir pretpostavlja hijerarhijski kontrolni sustav u kojemu je , na svakom stupnju hijerarhije, taj sustav redundantan, tj. proizvodi puno više izlaznih varijabli od broja ograničenja specificiranih ulaznim varijablama (kao na slici 3). Ostale karakteristike akcije mogu varirati na temelju sekundarnih zakonitosti, koje vjerojatno odražavaju optimizaciju nekih osobina izvedbe. Zato što je sustav redundantan, referentna konfiguracija na višem hijerarhijskom stupnju ne specificira sasvim nedvojbeno sve referentne konfiguracije na nižim stupnjevima. Izranjanje određenih 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-čelu kontrole ravnotežne točke, čini vrlo vjerojatnom strukturom koja podržava motoričke sinergije