“One should beware of m a them aticians and all who m a ke em pty prophecies. T he danger already exists that the m a them aticians have m a de a covenant with the devil to darken the spirit and confine m a n in the bonds of Hell” St Augustine, Bishop of Hippo, circa 400 A.D “To m ove things is all that Mankind can do…For such the sole exe cu t a n t is m u scle , whether in whisper i ng a sy llab le o r in felling a f orest” Charles Sher ington, 1924 O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 Optimization Principles in Motor Control Prof. Dava J. Newm an Joseph H. Saleh Depa rt m e nt of Aer o nau t i c s a n d Ast r o n aut i c s Ma s s a ch us e t ts I n s titu t e of T e ch no log y O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 Outline ? R eview of M u scle Cont racti o n – F rom AP generation to con t raction of fib e rs – M uscle proprio ceptors (spindles and Golgi tendo ns) – A fferent and eff e rent axons ? T h e M u scle Si m u lin k m o d e l ? R eflecti o ns on Mode ls ? O p tim ization Pri n ci p l es in Motor Con t rol – U nderstanding the fundamental question in biom echanics – A re all motor b e havior op timal in some sense? – K inem atic vers u s d y n a m i c obj ect ive fun c tions ? P r e - P ro gr am med M u scle Resp on se Du ring D o wn w a r d Jum p s O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 3 Muscles: Effectors of the Motor System ? The m ajor out p ut of the ela b orate i n form ation proces si ng t h at ta ke s place in o u r brai n i s t h e g e n e rati o n of a con t ractile force i n our sk eletal m u scles. ? M uscl e fasci c u l us – M us cle fib e r ?M y o f i b r i l – S arcom e re ? E ach m u scle f i b e r is i n n e rv at ed b y on ly on e m o to r neuro n , althou gh each m o to r n e ur on i n n e rv ates a num b e r of m u scl e f i b e r s ? T he m o t o r ne u r o n an d al l t h e fi ber s i t i nne r v at es i s cal l e d a m o t o r u n i t (t he sm al l e st f u nct i ona l uni t c ont r o l l e d by t h e m o t o r sy st em ) O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 4 Muscles: Effectors of the Motor System ? T he num ber of m uscle fibers innervated by one m otor neuron is called the innervation ratio. The innervation ratio can vary between 10 and 2000 ? A low innervation ratio indicates a g r eater cap acity for f i nely g r ad ing the m u scle to tal f o rc e O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 5 Muscles: Effectors of the Motor System A sim plified sequence from AP gene ration to m uscular contraction ? M otor neuron f i r e s an action po tential ? I t propag a tes do wn the m o tor ax on unti l i t r each e s the n e uro-m u s c ular junc tion ? I t tr iggers an AP in the m u scle fi ber ? T his AP is prop agated rapid l y ov er th e surface of the f i ber and con ducted into the my ofi b ri l by mea n of t h e T - t ubule sy st e m ? T his in turn rel e ases Ca ++ from the Sarcop lasmic Reticulum (SR) -the SR serv es as a store of C a ++ ? T his in turn trigg e rs the c y c lic m o tion of M y osin h eads, att aching a nd det aching on the Ac tin f ilam e nts, thus form ing cross-bridges an d gener a ting the pulling force ? C a ++ are pumped back to the SR O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 6 Muscles: Effectors of the Motor System ? T he fo rce o f c o nt ract i o n de pe nd s on t h e l e n g t h o f t h e m u scl e (l e n gt h- t e n s i o n relati on sh i p ) ? T he fo rce o f c o nt ract i o n al s o depe n d s o n t h e rel a t i v e rat e s o f m ovem e nt o f t h e Actin and My osin filam e n t s (t en si on -v el o c it y relati on sh i p , Hill’s curv e) ? M ot or u n i t s a r e rec r ui t e d i n a fi xe d or de r f r o m t h e weake s t t o t h e st ro n g est (Hen n e m a n size pri n ci p le): Th e weak est i n pu ts recru it th e slo w un its wh ich gene rate t h e s m allest f o rc e a n d a r e m o st re s i stan t t o fati g u e . The f a st fat i gue - resi st a n t a r e re cr ui t e d next , f o l l o we d by t h e f a st fat i ga bl e un i t s w h i c h ge ner a t e the st rongest force. O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 7 Muscles: Effectors of the Motor System ? M uscl e P r op ri ocep t o rs (s pi nd l e s a n d G o l g i t e nd o n s ) There are differ e nt t y p e s of r ecep tors which r e spond to light , soun d, odor , he at , touch, pain , etc . The r ecep tors which lead to cons cious s e ns at ions are cal led exteroc e p t ors , those which are n ot responsible fo r conscious sens ation are called- prim ar y in m o to r funct i ons- a re c a lled p r op rio c e p tors – S pindle org a ns Those a r e s t ret c h r ecepto rs s cat te r e d deep within al l m u scles . The y a r e usu a ll y at tach ed in parallel with a m u scle f i ber , and ther efore experience the sa m e r e lative length change. Spindles give inform ation about its length and r a te of ch ange of its length – G olgi tendon They are found ver y close to th e junction b e tween tendon and muscle f i bers. They are p l aced in series with the muscle fib e rs an d respond to th e tendon str e tch which a ccom p an ies a m u scle tens ion. Thus th e y ar e for ce t r an s d ucers for the musc l e . O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 8 Muscles: Effectors of the Motor System ? The nerve axons whic h run out of t h e spi n al cord a r e call ed efferent , the ones that c arry inform ation t o t h e c o rd a r e afferent ? G r o u p I a f fe re nt fi be rs ha ve l a r g e di am et ers t h e r e f o r e rel a t i vel y hi g h co ndu cti o n v e l o cities. Th ey bri n g inform ation fro m th e sp i n d le (Ia) a n d th e g o l g i (Ib ) to th e co r d ? T he e f fere nt whic h i nne rvate the m a in m u scl e m a ss are the α , and those t h at se rve the intra f usa l fi be rs wit h i n t h e s p i n dles are called γ ? T he s t re tch re fl ex, c o -acti v ati o n o f α -m n a n d γ -m n O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 9 Muscles: Effectors of the Motor System St ret c h reflex st i ffness ? U ntil recent l y, it was s u ppose d t h at the tendon orga n se rve d as a sensor wh ich t u rn ed off m u scle acti vity (i nh ibited α -m n) whe n m u scle fo rce r o se b e yond saf e lev e l s ? A ff er en t acti v it y fr o m bo t h sp i n dles and Go l gi tendo ns b a lance in su ch a way th at n e it h e r m u scle force nor m u scle l e n g th sh ou l d be consi d ered as c o ntrolle d quant ity, rathe r the i r rat i o (t he sti ffnes s or chan ge i n f o rc e pe r c h a nge i n l e n g t h ) a ppea r s t o be fi xed b y t h e s t r et c h re fle x O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 0 Muscles: Effectors of the Motor System ? Th e s e n s orimot or co rtex is at th e top of the chain of command in the sensorimotor area of th e cereb ral cortex . There is a s p ec ial ized area in the cer eb ral cortex devot e d to movement of the limbs (1691, the case of a knight with a fractu red skull and paraly sis of the left sid e of the bo d y ) – T he fraction of the cer e b r al co r t ex co ntr o lling each par t of the body is by n o m e ans pr opor tional to the size of that par t – I f the cerebral cortex is r e m oved, the ani m al continues to display all the lo co m o tion r e flexes, but cannot lear n new skills ? Basal ganglia ar e a s e t of s p ec ial ized nerve ce lls i n the brain s tem . ? Cereb ellu m is a major focus of incoming sensor y information . Th e information reaching th e c e r e bellum h a s to d o with length , fo rce, velo cit y of m u s c les and position of join ts. O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 1 On Models and Ot her Demons What do you think of the following quotes? ? “ I f a ki nem a t i c o b j ect i v e f unc t i o n ca n be f o u n d t h at l e a d s t o o p t i m al tra j ect orie s t h a t accurate ly re produce t h e pa tterns of obse rved be ha vi or, i t im p lies t h at t h e brai n i g n o res no n-k i n e m a tic fact ors in selecting an d re pr o duc i n g t h at be ha vi o r” ? “If a dynam i c objecti v e funct i on ca n be found t h at lea d s t o optim al tra j ect ories th at accurately rep r od u c e t h e p a ttern s of o b serv ed b e h a v ior, it im p lies t h at th e br ai n co ns i d e r s dy nam i c fact o r s i n sel e c t i n g and rep r od uci n g t h at be hav i o r ” O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 2 Optimization Principles in Motor Control Fundam e ntal question in biom echanics ? T he hum an l i m b s a r e i n v o l v ed i n a pr o d i g i o u s va ri e t y o f t a s k s. M ovem e nt s tend t o b e gr acefu l an d usuall y inv o l v e m a ny lim b seg m ents ? D ifferent tasks typ ically requ i r e – d ifferent sequen c ing of m u scle a c tiva tion and lim b m o tion – different inform ation from sensors ? H ow a r e t h e s e m ovem e nt s o r gan i zed ? F u nd am ent a l q u est i on i n bi om ech ani c s : Which m u scles are used and i n what patte rn? [Bernste in, The Co- O rdination and R e gulation of Mo vement . Per ga m on Press, 1967] O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 3 Optimization Principles in Motor Control ? O n e wid e ly used m a th em atical too l is op tim ization t h eory Objecti v e: to discove r princ i ples t h at gui d e goal -direc ted mot o r be ha vi or ? F ou r co m p o n e n ts to an op tim ization prob lem : 1. An objective fun c tion that quan t ifies what is to be regard ed as optimum (also called performance fun c tion or cost fun c tion) 2. A d y n a m i c s y s t e m that is to be c ontrolled 3. A set of contro ls that ar e available for modulation 4. An algorithm capable of find ing an an aly t ical or numerical solution (tools of variational calcu l us) ? G ive n a m o del of m u sculo-s k e l etal dynam i cs, optim ization t h eory re -m aps Ber n stei n ’ s pr ob lem of ch oo si n g am o ng an i n fin i ty of po ssi b le p a ttern s of m u scl e act i v at i o n i n t o an eq ui val e nt p r o b l e m o f c h oo si n g a m ong a n i n fi ni t y o f pe rf o r m a nce c r iteria O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 4 Optimization Principles in Motor Control ? O pt i m i zat i o n - b a sed m odel s ha ve been d e vel o ped t o a d d r e ss t h e “e xces s deg r ee s of free dom ” p r ob l e m ? R ecall Bernstei n que sti o n: How does the m o t o r syst em select the b e h a v i or it uses fro m th e infi n ite nu m b er of p o ssi b i l ities open t o i t ? – I n m a them ati cal parlanc e , this is an i ll-posed prob lem in the sense that m a n y solutions ar e pos sible – F or example, most limb segments are moved b y a larger number of muscles than appear to b e ne ces s a r y – T o reach a cup o f coffee, the han d may move alo ng an infinity of paths ? R ephrasi n g t h e cent r al ques tion: How does the m o tor sys t em chooses val u es f o r t h e l a r g e num be r of pa ram e t e rs t h a t can be co nt r o l l e d i n o r de r t o pe rf o rm a g o al -o ri e n t e d m ovem e nt ? O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 5 Optimization Principles in Motor Control ? N eed t o m a k e ex plicit and q u an titat i v e h y p o th eses ab ou t the go al of m o tor acti o ns ? A re all m o tor beh a vior n ecessarily op tim al in so m e sense ? Mayb e! ? O ne a p peal i n g po ssi bi l i t y i s t h at t h e ne r v ou s sy st em has e v o l ve d t o select “ s olut ions” tha t a r e i ndeed “ o ptim al”: the hypot h ese s is t h at in pe rf o r m i ng a m o t o r t a s k , t h e C N S p r od uce s co o r di nat e d ac t i o ns t h at m i ni m i ze som e m easure o f pe r f o r m a nce (e ff or t , sm oot hne s s , et c .) O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 6 Optimization Principles in Motor Control ? K i n em at i c s ve r s u s dy nam i cs o b j ect i v e f unc t i on s ? – K inem atics r e fer s to th e tim e cou r se of an obj ect ( position, velo cit y , acce lera tion , e tc . ) – D y n amics r e fers to v a riab les such as forces and torques ? E ven si n g l e de gr ee of f r ee d o m can be pe r f o r m e d i n a va ri e t y o f way s : – P ath is constrain t – S peed along th e path can v a r y ( t r a jector y ) ? T wo di f f ere n t t y pes o f ob je ct i v e f unct i o n s ha ve been p r o p o s ed, t h ey re flect the tw o m a jo r c o m p eting t h eo rie s of m o tor co nt ro l: Kinematic objective function D y namic objective function O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 7 Optimization Principles in Motor Control Kinem a tic objective function, single-joint m ovem e nts ? T hey ar e c h ara c t e ri ze d by s i n g l e -pea ke d, bel l -s ha ped s p ee d p r o f i l es. It was po st ul at e d (H o g an , 19 8 4 ) t h at v o l u nt a r y m ovem e nt s a r e m a de t o be a s sm oo t h a s p o ssi bl e ? A qu an titati v e m easu r e of sm o o thn e ss i s n e ed ed, o n e su ch m easu r e i s the squa re d m a gnitude of t h e j e rk (ra te of c h a nge of accele r a tion or th i r d tim e d e ri v a ti v e of p o siti o n ) O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 8 Optimization Principles in Motor Control 2 t 1 ? d 3 θ ? J = t ∫ 0 ? ?? dt 3 ? ?? dt θ () is t h e jo i n t angle. Using v a riat ion a l calcu l u s, th e un iqu e tim e h i story t of joint positi ons that m i nim i zes t h is pe rform ance m easure m ay be de ri ve d a n al y t i cal l y θ () t = c 0 + c 1 t + c 2 t 2 + c 3 t 3 + c 4 t 4 + c 5 t 5 c i are uns p eci fi ed c o e f ficients whose val u es a r e dete rm ined by t h e co nd it ion s at t h e b e ginn i n g an d end of m o vem e n ts (bo undar y co nd it ion s ) O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 1 9 Optimization Principles in Motor Control ? W he n t h e m o v e m e nt i s as s u m e d t o be gi n at r e st i n on e po si t i o n a n d en d at rest i n a not he r, t h e “m i n i m um jer k ” o r “m axi m u m sm oot h n e ss” m ovem e nt t u rn s ou t t o ha ve t h e sm oot h, un i -m o d a l , bel l -s ha ped ve l o ci t y pr o fi l e t y pi ca l m o st expe rim e ntal obse rvati o ns ? T he m a xi m u m sm oot h n e ss hy po t h e s i s i s rea d i l y gene ral i ze d t o m u l t i - j oi nt m o tio ns. O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 0 Optimization Principles in Motor Control Kinem a tic o bjective function, m ulti-joint m ovements ? T he ob j ect i v e f unc t i o n ca n be wri t t en as f o l l o w s i n t h e C a rt e s i a n coo r di nat e f r a m e of t h e ha nd : J = ∫ t 1 ? ? ?? ? d 3 x ? ? 2 + ?? ? d 3 y ? ? 2 ? ? × dt t 0 ? ? ? dt 3 ?? ? dt 3 ?? ?? ? A ss um i ng t h e m ovem e nt st a r t an d e n d at zer o vel o ci t y f r o m (x 0 , y 0 ) to (x f , y f ) at tim e t f ( τ = t/t f ) x ( τ ) = x 0 + ( x 0 ? x f ) ( 15 τ 4 ? 6 τ 5 ? 10 τ 3 ) y ( τ ) = y 0 + ( y 0 ? y f ) ( 15 τ 4 ? 6 τ 5 ? 10 τ 3 ) O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 1 Optimization Principles in Motor Control ? T he m a xim u m s m oothness theory yields in the m ulti-joint m ove m e nt several explicit predictions: 1. Tra j ect ories of the l i m b s a r e s t rai g ht li ne pat h s 2. The t a n g ent i al vel o ci t y al on g t h at pat h i s sm oo t h an d u n i - m oda l 3. The s h a p e of t h e lim b t r a j ec t o ries a r e inva ri ant unde r tra n s l ation, rotati o n , an d am p litu d e scal ing ? T hese predictions are in agreem ent with experim e ntal observations O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 2 Optimization Principles in Motor Control Lim itations of the ki nem a tic objecti v e functions ? A trou bling asp ect of t h is t h eo ry is th at it imp lies t h at at high er lev e ls i n the m o t o r syste m , the brai n does not ta ke i n t o account a n y dynam ic consi d erati o ns suc h a s e n ergy re qui re d, the l o ads on t h e lim b segm ent s or th e fo rce an d fatigu e lim itati on s of t h e n e urom u s cu lar system ? I n oth e r word s, it im p lies t h at th e brai n d e term in es th e “op ti m al” tra j ect ory i nde pende n tly of t h e physical syst em that will ge ne rate t h e m ovem e nt , i .e. , t h e l i m b ! “It seems ver y strange that the op timal trajector y of our movement is determined perfectly indep e ndent of th e d y n a mic quantities such as arm length , pay l oad , motor command, torqu e or exter n al for ce, etc.” Y. Uno a nd M . Kawato, 1989 O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 3 Optimization Principles in Motor Control Lim itations of the k inem atic objective functions (suite) ? T he t r a j ect o r i e s de ri ve d f o r t h e m i ni m u m jer k m odel a r e i n va ri an t wi t h re spect t o t h e re gi on o f t h e wo r k -s pac e an d i n depe n d e nt o f e x t e rna l fo rce s ? T he m i ni m u m je r k m odel de t e rm i n es t r a j ect or i e s i r re spec t i ve o f gra v i t y ? T o ci rcu m v e n t th is prob lem with i n t h e fram ework of op tim ization t h eo ry, a seco n d t y pe o f ob j ect i v e fu nct i o n s wa s f o rm ul at e d base d o n dy nam i c va ri abl e s (joint t o rques , m u scle forces , etc.) O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 4 Optimization Principles in Motor Control Dyna m i c objective function ? M odels using a dynam i c objective function in m ovem e nts assum e tha t the CNS solves th e th re e f o llowing com putational problem s at different levels: 1. Det e rm i n at i o n of a desi re t r a j ect o r y 2. Tran s f o r m a t i o n of v i s u al c o o r d i na t e s o f t h e desi re d t r a j ec t o ry t o bo dy c o o r di nat e 3. Gene rat i on o f m o t o r com m a nds ( f o r ces a n d t o r q ues ) t o rea l i ze t h e desi re d t r a j ec t o ry O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 5 Optimization Principles in Motor Control Dyna m i c objective function, m ulti-joint m ovem e nts ? O ne dy nam i c o b j ect i v e f unc t i on p r op o s ed i s t h e f o l l owi n g : t f n 2 J = ∫ ∑ ? ? dz i ?? dt t 0 i ? dt ? ?z i is t h e m o t o r comm and fe d t o t h e i - t h ac tua t or (m uscle ) out of n ac tuat ors ? I n ord e r t o com p u te op tim al traj ect ories p r ed icted b y t h is m in im u m to rqu e chan ge m o del , t h e dy nam i cs e qua t i o ns o f t h e m u scul o - s k el e t al sy st em m u s t fi rst be s p eci fi ed beca use J d e pen d s on t h e dy nam i cs of t h e co nt ro l l e d o b j ect O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 6 Optimization Principles in Motor Control ? P roblem : it is difficult to describe the the m usculo-skeletal system exactly because it is a com plex system . ? C onsider the f o llowing two-join t sys t em : O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 7 Optimization Principles in Motor Control x y L 1 1 θ 2 θ S 1 D D D D z 1 = ( I 1 + I 2 + 2 M 2 L 1 S 1 cos( θ 2 ) + M 2 L 1 2 ) × θ 1 + b 1 θ 1 D D D D D + ( I 2 + M 2 L 1 S 2 cos( θ 2 ) ) × θ 2 ? M 2 L 1 S 2 ( 2 θ 1 + θ 2 ) × θ 2 sin( θ 2 ) D D D D D D z 2 = ( I 2 + M 2 L 1 S 2 cos( θ 2 ) ) × θ 1 + I 2 θ 2 + b 2 θ 2 D θ 1 + M 2 L 1 S 2 () 2 sin( θ 2 ) O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 8 Optimization Principles in Motor Control ? S in ce th e d y n a m ics o f th e m u lti-jo i n t system is no n l in ear, t h e prob lem of fi n d i n g t h e un i q ue t r a j ec t o ry t h at m i ni m i zes J is a non lin e ar o p tim izatio n pro b lem . ? C o n s equ e ntly, it seem s im p o ssib le t o o b t ain an alytical expressi o n of th e so l u ti on of th is prob lem , un li k e t h e case with th e m in imu m j e rk pr o bl e m ? P re di ct i o ns v s expe ri m e nt – T raje ctor y depen d s on arm pos tur e and ext e rnal fo rces – N ot alway s str a ight paths ? The m i nim u m torque c h a nge m odel s u ccee ded i n re produci ng o bser v ed t r ajector ies und er v a r i o us co nd it ions O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 2 9 Optimization Principles in Motor Control ? P hysiolog i cal advan t ag e of each m odel: W hy would the CNS want to m i nim i ze – t or qu e ch an g e ? –J e r k ? O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 3 0 Pre-Programmed Muscle Respons e During Dow n ward Jumps ? L it er at ure r e vi ew – E n gb e r g and Lu ndb er g (19 69) E M G activity dur ing walking in cat li m b s “E M G was triggered 5 to 10 m s p r io r to i m p act”, sor t of feedfor w ar d activation “a centr a lly pr ogr a m m e d event anticipating s t ance” – Melv ill -Jon es an d W a tt (1 971 ) T e sted the above conclusion on hu m a ns dur ing sudden falls . Found consistent EM G bur s t activity beg i nning 75 m s af ter d r op. Concluded that “deceler ation r esulted f r o m a ti m e d bur s t of pr e- p r ogr a m m e d m u scle activity ”. Pr oble m with this study : d r opped subjects f r o m heights up to 20cm ! Activ ity tr igger e d by ves tibular input? – Gree n w o o d a n d H o pki ns (1 9 7 6 ) Studied EM G activity dur ing voluntary and unexpected ju m p s, heights up to 120cm . Findin g s: T w o peaks of activ ity : 80 m s after release only in unexpected ju m p s + consistent ti m e befo r e landing (r elated to the voluntar y control of landing) O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 3 1 Pre-Programmed Muscle Respons e During Dow n ward Jumps – D yh r e - P ou lsen and Lau r sen (1 980 , 19 83 , 198 5) Analy z ed landing m e chanis m s and EM G activity in m o nkey s dur ing downwar d ju m p s. Onset of E M G activity s t arted occu rr ed with gr eat precision 8 0 m s befo r e landin g . Still an ar gu m e n t against p r e- p r ogr amm i ng : visual m o nitoring of distance dur ing ju m p ? L i ghts tur n ed off, s a m e act ivat ion patt e r n, locked to the t i m e of expected i m p a c t – M cKin ley and Sm ith (19 83) Per f or m e d si m ilar exper i m e nts on blindfolded and laby rinthecto m ized cats . – W att et al . ( 198 6) p l u s nu m e ro us o t h e r st ud i e s It is wid e ly ackn owledged that microgravity expo sure causes prof ound chang e s in hum an balan ce, posture con t rol a nd locom o tion . W a tt e t a l . test ed astronau t s subjected to sudden dr ops. All subjects ar e “unsteady po stflight”. Reaons for decr e m ent in per f or m a n ce? Astronauts sta ted t h e f loor co m ing up to m e et the m , and is the r e b ef o r e the y we re r ead y f o r it O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 3 2 ? ? ? Pre-Programmed Muscle Respons e During Dow n ward Jumps ? T he m i ssi ng l i nk : A p r o p o se d m odel t o acc ou nt f o r t h e a b ove o b s e r v at i o ns – T he pr evious ex periments suggest that th e “fly in g object” has an estimate of th e tim e of im pact T impact wh y? – P rior to jum p , a visual estim ate o f the heigh t is pe rform ed H 0 – W ha t i s nee d e d to go from e sti ma t e d he i ght t o e st ima t e d ti me of impa c t ? A representation of th e gr avity field in th e sensorimotor s y stem, or an intern al g-model T impact = CNS g H 0 2 ? O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 3 3 ? Pre-Programmed Muscle Respons e During Dow n ward Jumps ? W hen astronauts perform postflight jum ps, one hypotheses regarding the perform a nce decrem ent (other than m uscle atrophy) is th a t intern al repr esen ta tion of the g r av ity f i e l d is alte red : g CNS < g true ? T impact > T true ? H ence the floor is “there before [they are] ready for it”! O p t i m i za t i o n Pr i n ci p l es i n M o t o r Co n t ro l 3 4