Flashcards in Week 15 Prehension Deck (58):
voluntary movement performed with little conscious effort
What are the 2 seamless temporally integrated stages of prehension?
The reach stage of prehension =
transport hand to target so digits align with target
* produced primarily by proximal mm
• guided extrinsically by target
The grasp stage of prehension =
pre-shapes digits; opens them to match target size
• produced primarily by distal mm of hand and fingers
• guided by intrinsic properties of target (size and shape)
carrying out target's intended use
Grasps vary because of location, size and shape of an object. What are the 4 power grips?
What are the 4 precision grips?
Power = cylindrical, spherical, hook and lateral prehension
Precision = pinch (aka precison), key, (3-jaw_) chuck, and pulp pinch
The hand is a complex mechanical structure of ______ bones activated by _________ extrinsic mm and __________ extrinsic mm.
How many DF does the kinematic model of hand consists of for each finger? the thumb? and the radio-ulnar joint? wrist?
each finger - 4
thumb - 4/5
plus 1 DF at the radio-ulnar joint
2 DF at the wrist = 23/24
with so many DF, how is the hand controlled?
What are the 2 hand synergies that account for 80% of movements alone or in combination?
MCP flexion and adduction of fingers = L line
finger aperture closure by flexion at PIPs of finger and thumb adduction and IR = R line
When reach = transport there are 2 phases: acceleration- deceleration... How is prehension different from pointing/aiming task?
1. the person intends to use the object to achieve some type of goal
2. reach has acceleration & deceleration phases
transport grip aperture =
preshaping and closure
Describe grasping kinematics:
1. hand preshapes during movement to target (object)
2. max grip aperture (distance between thumb and index finger tip) occurs within 60-80% of movement completion
3. scaling of max grip aperture traces correlated to object size
What factors affect grip aperture?
* faster reach-grasp movements --> maximum grip apertures
* grasp at normal speed vs grasp "as fast as possible" with dropping the object
• larger maximum apertures were observed for the faster movements. thus, faster reach -
• reach-grasp movements that start with an open grip aperture show a tendency of the hand grip to partially close before achieving its maximum aperture
visual regard =
locating the target
Why is vision very important in prehension?
• Enables corrections that occur just before grasp
• determines EN regulatory conditions in which the action will occur: distance/ location; size, orientation
• binocular vision aids grip size and force of grip
• person needs to look directly at object for grasp = point of gaze
Two primary findings of EMG prehension (reach and grasp) research are:
1. neck mms fire first (20-40ms) before (increase inertia) eyes, and then arm
2. arm has triphasic mm program (agonists-antagonists for breaking and then agonist)
When locating a target: visual regard, for reach, limb is directed to object by eyes. Further describe the reach -
Reach (at normal speed) is under closed loop control , visual info (feedback) is used constantly during reach and grasp
• preparation and initiation of movement - assess EN
•transports hand to object - central vision (peripheral vision provided movement feedback)
• grasp object - supplements tactile and proprioceptive feedback
Prehension and vision: development from phase 1 to phase 2:
• Infants: @ 1 week can reach for and intercept moving object, but hand is wide open- with no grasp formation which develops ~10-22 weeks
• 4 months: prehension controlled proximally; poor contact with object
• 5 months: prehension controlled distally; contact orients hand to object
• 6 months: squeeze emerges (fingers close around object)
• 9 months: prehension controlled by thumb and 1 finger (pincer grip) ; hand oreints before contact. Pre-shapes for object size; poor adjusting grip force
• 13 months: fingers oppose action of thumb without hand being stabilized
• 18 months: child can release the object
Major finding about prehension and child development:
If object is scaled to hand size, grasp is similar to adult pattern as early as age 6-7 years
Kinematics: point vs reach to grasp -
control of UE movements changes depending on goal of task
• pointing-segments controlled as a unit
• reach to grasp-hand controlled independently of arm, with arm carrying out transport; hand carries out grasp and manipulation
Kinematics: point vs reach to grasp - velocity profile and movement duration of reach vary, depending on task -
• reach the object: movement duration is > than pointing
• grasp an object: acceleration deceleration
Describe the PERIPHERAL Neural subsystems contributing to R and G:
Peripheral tell what's happening around where you are in space and where your joints are relative to each other.
• visual input is divided into 2 parallel paths
• higher cortical centers make movement plan
* basal ganglia plans forces to grasp
• cerebellum refines movement
• descending paths activate spinal cord neurons to mm
What are the 2 visual paths for R and G?
1. dorsal stream
2. ventral stream
describe the dorsal stream?
- from visual to parietal cortex
• provides action relevant info about all phases of reaching movement including object position, structure and orientation
describe the ventral stream?
- from visual cortex to temporal lobe
• provides conscious visual perceptual experience
In the posterior parietal cortex (PPC), BA5 =
BA5 = (posterior parietal lobe) integrates INTRAPERSONAL space ("body image") information; gets from BA2
BA7 = (posterior parietal lobe) cells respond to visual, auditory, and movement/tactile stimuli (from BA5)
BA 7 = integrates intrapersonal information (BA5) with critical environmental cues to perceive EXTRAPERSONAL space
Dorsal codes for:
reach WHERE for action
ventral codes for:
grasp, WHAT, object stream
Premotor cortex localizes position of object with respect to what?
with respect to our hand & specifies plan of action.
Primary motor cortex (PMC) codes for what?
movement and force; sends motor commands to spinal cord
PPC receives information from visual cortex and somatosensory cortex; localizes:
position of target with respect to the body
Cutaneous input is essential for control of grpi forces: S1. What happens without it?
uncoordinated grip and uncoordinated forces
With slippery objects, cutaneous receptors detect slip -->
activate motor activity to increase grip force in finger mm • patients wit sensory neuropathy may have difficulty with control of grip forces and have decreased ability to adapt grasp.
(control of hand posture is seperate from regulation of contact force)
Posture and force are not independent because hand must shaped properly so the correct set of fingers makes contact with the object. What have fMRI studies demonstrated?
Demonstrated that caudate and ant. putamen signal level of grip force prediction through: cortex, ventral thalalmus, cerebellum (efference copy/ corollary discharge/ reafference)
(motor cortical control of movement) Reaching movement: what precedes activity in M1
PMA precedes activity in M1, mm activation and finger movement
PMA = plan
M1 = do
EMG = activation
Finger = fingers GO
Anterior interparietal sulcus (AIP) or parietal cortex and premotor cortex are involved in:
visual motor transformation
• sensory coordinates related to object - motor objects
(motor systems) Descending pathways -
Reach: arm movement ransports hand to target, moves together while hadn pre-shping of fingers for grasp occurs
(• children with pyramidal lesions show problems with grasp, transport may be normal. Suggests that midbrain pathways (rubrospinal reticulospinal) may control more proximal mm involved in reaching), corticospinal required for fine control of grasp
(motor systems) Precision vs power grip:
neurons in M1 fire only during execution of precise movement, but not power grip. Indicates connections to intrinsic hand mm rather than with forearm mm.
Postural support of reaching:
• control of body's position in space for stability and orientation
• reach has shown key brain structure for learning of anticipatory postural adjustments during bimanual task is the cerebellum
• postural requirements are task dependent. Requirements in supported sitting are
Slippage, what causes? What provides better grip?
• Slippage is related to cutaneous sensation.
• evolution of normal and tangential forces with finger pad contact on glass
• --> increase grip forces
• "stick" decreases with increase in tangential forces
** finger prints improve grasp
(adaptation of grip forces) Aging -->
decrease manual dexterity with larger grip forces
What happens with loss of sensation?
larger grip forces, no matter the weight or texture of the material
Application of the prosthetic hand:
• five fingered prosthetic hand consisting of digits driven by DC motors
• force sensors resistors (FSR) placed at finger tip of and potentiometers attached at proximal and middle joints. Information from FSR can detect level of normal force exerted and also slippage between fingers and object
What type of control is prehension under? Give examples of feedforward (open loop):
• anticipate requirements of task and obstacles that might perturb arm trajectory and correct for effects of perturbation
• anticipatory control takes advantage of previous experience to predict consequences - occurs before sensory receptors are stimulated and reduces reliance of feedback
What type of control is prehension under? Give examples of feedback (closed loop):
• feedback from vision and proprioception needed at end of movement to ensure hitting target accurately
Motor program: invariant characteristics -
• path of the wrist during arm movement is unaffected by movement speed or by load (wts in hand).
• these support joint angle coordinates at end-points
• NS can directly control joints and produce straight line movement by varying onset times for joint movements with all joints stopping simultaneously
Motor programming: distance vs location -
• Both strategies probably used, depending on task and context
• Distnace theories: when making arm move to target, people visually perceive the distance (to target). activate mm to propel arm proper distance... turn off Aag mm and activate Antag mm to brake movement
• Schmidt's impulse variability model
• RELEVANCE to PT and retraining: important to practice fast movement of varying amplitudes so pt learns to program forces appropriate for quick, accurate movement.
Location program: NS program balance of tensions/ stiffness of AG and Antag mm sets. Every location in space corresponds to a stiffness relationship between opposing mm
(subjects blindfolded fingers anesthetize with cuff. Beforehand training to move fingers to specific position in space. Given perturbations, with loss of finger sensation, very little difference between perturbed and unperturbed movements.
Schmit's impulse variability model:
make a fast mvmt (high velocity) over a fixed distance. - size of error increased in proportion to amonut of force; "make a fast but accurate mvmt" large forces caused increased force variability. Resulting in decreases accuracy.
Dynamical systems: How does NS plan movement?
• NS possess a central representation of the movement that is in the form of a 'motor image'
• believed that propriocepetion was important to final movement achievement
• suggested that high number of DF for complex movements allow organization of actions in terms of synergies or groups of mm and joints acting together as a unit
(ex. point of 2 targets (1 close & large, 1 far and small - Hands move simultaneously, even if pointing is different)
Systems theory =
predicts specific neural and musculoskeletal subsystem contribute to control for each, grasp and manipulation
(musculoskeletal = ROM, flexibility, mm properties, biomechanical relationships of linked segments)
(neural = motor processes - coordination of eye, hand and trunk, and arm , sensory processes - coordination of visual, vestibular and somatosensory, internal representation for mapping sensation to action, higher level processes for adaptation and anticipatory aspects of manipulation, manipulation - voluntary and reflexive aspects_feedforward and feedback)
What do we observe in patients?
• visual processing is more complex when a person reaches across their body
• reaching a target on opposite side is slower and less accurate then movements to target on same side as arm
• so, when you examine a patient, consider where you have the target and for your intervention, you may want to begin ipsilateral reaching prior to progressing to contralateral reaching.
Age and prehension; young =
• infant can intercept moving objects; able to predict where an object will be which is anticipatory
• at 7 weeks: thumb opposition
• children open hands wider
• children > variability
• grip formation not mature til ~ 7 year
age and prehension; elderly =
• 30% decrease in speed of reach
• more time in target deceleration phase
• accuracy is about the same for drawing tasks, just slower
• increase 20-40% in time to do fine manipulations skills by age 70.
• most decrements are reversible with practice