Transcript M 1 - Human Kinetics

Lecture 12:
Pre-Programmed Reactions
EMG
Voluntary
reaction
Perturbation
M2 M3
M1
Time
An unexpected perturbation of
a joint gives rise to a sequence
of EMG events in a stretched
muscle. The first one (M1)
comes at a short latency
(under 40 ms). Then two peaks
(M2 and M3) come at a latency
of between 50 and 100 ms. M2
and M3 are addressed as preprogrammed reactions. Later,
a voluntary correction comes.
Effect of Instruction on Muscle
Reactions to a Perturbation
EMG
Voluntary
reaction
Perturbation
M2 M3
M1
React
Let go
Time
Pre-programmed reactions
demonstrate a strong
dependence on the instruction.
If the subject is instructed to
resist perturbations, the preprogrammed reactions are large
(solid lines). If the subject is
asked to let the limb move,
the pre-programmed reactions
are much smaller (dashed lines).
Note that the M1 reaction is the
same.
Pre-Programmed Reactions
 Also called triggered reactions, M2–M3, functional
stretch reflex, transcortical reflex, long-loop reflex
 Come at an intermediate latency (40–100 ms)
 Are involuntary in nature but can be modified by
instruction
 Can be triggered by stimuli of virtually any modality
 Generate a quick, crude compensation for the
perturbation
 Are followed by voluntary corrections
A Possible Scheme
Central command
∆C
Tonic stretch reflex
mechanism
Afferents
Muscle
A subject is holding a position in a
joint against a load with a central
command to a muscle. If the subject
knows that a perturbation can
occur, he/she can prepare an
addition to the central command
that would compensate for the
predicted perturbation. The preprogrammed command (∆C) is
triggered by peripheral signals
generated by the perturbation and
attenuates the mechanical effects of
the perturbation.
Pre-Programmed Responses Can Be
Triggered by Any Sensory Signal
Hearing
Central
command
Vision
∆C
Tonic stretch reflex
mechanism
Proprioceptors
Muscle
The actual source of the
triggering signal for a
pre-programmed
reaction is not important
as long as the signal
carries sufficient
information. It can be
provided by
proprioceptors, a flash of
light, a loud tone, etc.
Effects of a Perturbation Applied
During a Fast Movement
If a perturbation occurs
during a fast voluntary
movement, EMG changes are
seen at a characteristic for the
pre-programmed reactions
latency. Generally, they
involve an increase in the
activity of a muscle that acts
against the perturbation, and
a decrease in the activity of a
muscle that is assisted by the
perturbation (bold lines).
Effects of Vibration on M1-2-3
Responses
M1
M2-M3
EMG
Perturbation
Vibration
Time
Muscle vibration has
different effects on
different components of
the responses to an
external perturbation. The
early response (M1) is
suppressed just like the Hreflex, whereas the preprogrammed response
(M2-M3) is unchanged.
Examples of Pre-Programmed
Reactions
 The “waiter’s response”
 Grasp adjustments
 Two-hand object holding
 Reactions to postural perturbations
(ankle and hip strategies)
 Corrective stumbling reaction
Effects of Intention
on Quick Muscle Reactions
Pre-programmed postural
corrections to a perturbation
created by platform movement
are context-dependent. If the
cup is loaded with play dough,
the corrections will be
different than if the cup is full
of hot tea.
Corrective Stumbling Reaction
A
B
Stim
Stim
Surface
A mechanical or electrical
stimulation of the paw
during locomotion induces
different reactions in the
swing and the stance phases.
In the swing phase (A), there
is a flexor reaction, so that
the leg steps over a fictitious
obstacle. In the stance phase
(B), there is an extensor
reaction, leading to the
shortening of the stance
phase for this limb.