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 cross-sectional 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 force-velocity relationship may be due to neural adaptations: Increase EMG amp & Synchronization

H Reflex and M WaveM 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