Download Neuromuscular Adaptations During the Acquisition of Muscle

Neuromuscular Adaptations During the Acquisition
of Muscle Strength, Power and Motor Tasks
Toshio Moritani
Introduction
• Motor units varying in force generating capacity
• 100 or more variation in twitch force has been
observed (Garnett, 1979; Stephens, 1977)
• In voluntary contractions force is modulated by
– Recruitment and Firing rate
• The EMG is the sum of all the muscle fiber action
potentials that pass thru the recording zone of the
electrodes
Introduction (Cont)
• Under isometric
conditions there is a
linear relationship
between EMG and
force
• Deviations from this
relation are due to:
– Synchronization
(increases amp,
decreases freq)
– Not recording all of the
action potentials
– Failure to record EMG
from all muscles that
cause a given motion
Muscle Strength Gain: Neural Factors vs Hypertrophy
• During the first 4 weeks strength increases in the
absence of any measurable hypertophy
• Normalized EMG???
• Single limb training results in increased strength
in the untrained limb (Cross Education)
• Skill Acquisition may be be due to:
– Reductions in antagonistic activity
– Increased synergistic activity
– Reciprocal inhibition
Cross-Sectional Area
• A strong relationship exists between crosssectional area and strength
• Strength is determined by:
– Quantity of muscle (cross-sectional area)
– Quality of muscle (fiber type proportions)
– The extent to which the muscle mass can be activated
(Neural Factors)
Contributions of Neural Factors and Hypertrophy
• Neural Factors: Increased
EMG without hypertrophy
• Hypertrophy: changes in
strength without changes
in EMG
Time Course of Neural Factors and Hypertrophy
• During the first 4 weeks
changes in strength are
thought to be due to
Neural Factors
• After the first 4 weeks
Hypertrophy is thought to
be more important
– This may just be due to our
current inability to measure
hypertrophy
Power Training
• Effects of 30% of Fo (max vel) training upon force-velocity and power
• Right bicep was trained 30% Fo with maximum effort, 30 times/day, 3
x week, for 2 weeks
• Results showed specificity of training.
– Training with no load increased increased max vel with no load
– Training 100 % Fo caused greatest strength improvements
Effects of Power Training on EMG Amp & Freq
• EMG amplitude
RMS increased
• MPF decreased
• Synchronization:
increased amp &
decreased freq
Improved Coordination ?
• Based upon cross
correlation, there is
improved co-activation of
the long and short head of
the bicep following
training
Cross Correlation
increases due to
improved neural
control?
M Waves
• ?? Increased oscillation in the surface EMG which
would theoretically approach towards the area of
maximal evoked M waves (mass action potential),
indicating that all MU’s are now fully
synchronized (Bigland-Ritchie, 1981)???????
• Short-term training-induced shifts in forcevelocity relationship may be due to neural
adaptations: Increase EMG amp &
Synchronization
H Reflex and M Wave
M Wave is elicited by
stimulating the alpha
motor nerve. The M
wave tests the integrity
of neuromuscular
propagation. The
latency between
stimulation and the M
wave twitch is about 5
ms.
H Wave is elicited by
stimulating the afferent
Ia neurons from the
muscle spindle. It is
used to artificially test
the stretch reflex
response.
M Wave and Activation
Neuromuscular Adaptations during the Acquisition
of a Motor Task
• Effects of practice on motor output variability: force
variability, maximal rate of force development, contraction
time interval and accuracy
• Subjects produced contractions 20-60% MVC, tracing
oscilloscope (1500 trials) in 1 week
• Reduced variability in MPF and RMS
• Significant improvements in accuracy
• Motor unit recruitment is the primary factor in increasing
muscle force at low levels, while rate coding becomes
predominant at intermediate to high force levels
Neuromuscular Adaptations during
the Acquisition of a Motor Task
• The results showed much less
variability in force at 60 %
MVC
• At 60 % MVC, changes in
firing rate give much better
control of force than would
recruitment.
• IIa or IIb when recruited would
result in large force variations
• Significant increases in MPF
after extended practice may
indicate preferential recruitment
of high threshold MUs with
fast-twitch fibers have taken
place to meet the demands of
rapid alternating forceful motor
activities
Neuromuscular Adaptations during a Ballistic
Movement
•
•
•
•
Earliest adaptation to
rapid movement is not
activation, but rather a
Silent Period
When a maximal number
of MUs need to be
recruited tonically active
units need to be released
for optimal
synchronization (Conrad,
1983)
The maximal rate of
force development was
significantly greater in
trials with a silent period
(WHY???)
Significant decrease in H
wave amplitude 40 ms
prior to the appearance of
the silent period