Aagaard
Muscle Strength Assessment and Testing
Measurements of explosive muscle strength
Measurement of maximal muscle power
Isokinetic dynamometry = evaluation of muscle strength in vivo
max eccentric muscle strength
max isometric muscle strength
max concentric muscle strength
positive: reproducibility & reliability
negative: expensive
isometric & dynamic contractions (CMJ, DJ)
Contractile Rate of Force Development (RFD) = ΔForce/ΔTime = slope of the curve -> as steep as possible for shorter times
peak concentric power Ppeak, speed and force at Ppeak
dynamic RFD, LLS, peak eccentric/concentric take-off forces
force plate analysis, power rig, flywheel methodology = dynamic & non-isokinetic (+ sometimes electromyography EMG recording)
Assessment of maximal muscle strength and muscle power & Resistance Training ... GAINs
eccentric muscle strength
maximal muscle power
explosive muscle strength (RFD)
overall maximal muscle strength
RT
tolerater higher training intensity
strength endurance
faster, reactive
reduce risk of injury
mechanical muscle function
rapid force capacity (explosive) RFD
maximal unloaded movement speed
maximal muscle force & muscle power -> slow & fast
Types of muscle contraction
concentric = shortening
isometric = constant length
eccentric = lengthening
40% higher than isometric (theory)
much higher than isometric in trained people
neoronal muscolar feature
high RT brings more results
low RT -> as fast as possible (explosive) -> better results with untrained people
peak performance = 300-500 msec
Physiological determinants of RFD ... positively influenced by ...
neural innervation of muscle fibres (motor neuron discharge frequency & recruitment rates)
fibre type composition (% fast muscle fibres (type II MHC isoforms))
muscle size & architecture (CSA & volume; fibre pennation angle)
tendon & aponeurosis (SEC) stiffness (high SEC stiffness -> higher RFD)
14 weeks of heavy RT = increased RFD
high RFD in quadriceps + hamstrings = superior acceleration capacity in top-level athletes
assessing MVC (maximal voluntary contraction) & RFD with handheld dynamometry (HHD)
muscle power
= contractile work / unit of time
Joule / second = Watt
= force * velocity
N * m/s = NM / s = Joule / s = Watt
force plate -> leg extension force (vertical ground reaction force = GRF) & power (jumping or CMJ)
12 weeks heavy RT
faster & higher (17%) jumps + shorter take-off time 11-20% + power 10% + lower limb stiffness 38% -> steeper curve of RFD 87%
Muscle and tendon adaptations to physical training
Effects of RT
tendon function (+ CSA, + stiffness, - strain, - injury)
neuromuscular function
muscle size and structure
fibre type composition
muscle size & architecture ... increases in ...
physiological fibre CSA
anatomical CSA (cross sectional area) and volume
motor cortex, cerebellum
spinal cord circuitry
explosive muscle strength
Huge implications for functional performance! (elite, old & clinical)
close relationship between muscle size & strength
max muscle strength = muscle size * neural function
angle of fibers increases -> increase in strength
more muscle mass = more muscle force, power, explosive strength (RFD) = more functional performance (acceleration, speed of movement, ...)
Assessment of muscle hypertrophy and atrophy
macroscopic measures
microscopic measures
changes in muscle size (training or inactivity)
CT-tomography: anatomical muscle CS
MRI (anatomical CSA; total muscle volume (multiple axial CSA)
ultrasound (muscle thickness)
biopsy sampling (single muscle fibre CSA, physiological; fibre types (ATPase histochemistry; MHC (myosin heavy chain) isoform composition; capillarization)
physiological CSA of individual muscle fibers
muscle fibre pennation angle
anatomical / macroscopic muscle CSA and volume
MRI / CT -> 5-15% (quadriceps)
steeper fibers -> larger muscle -> more muscle quality
type I = a lot of mitochondria, fatigue resistant = dark
type IIa = very fast, power; type IIx = extremely high power and RFD = light
RT -> changes in fibre type composition -> MHC isoform distribution
IIa = increase
I = remains the same
IIx disappear -> becomes IIA -> recovery brings boost in IIx, but individuals should have high IIx at the start
shift from IIx -> IIa -> soccer/volleyball (repetitive), cycling (endurance)
10 sets, 8-10 reps, 70% 1RM, 3 min rest, 3x/wk
proximal 6%, distal 4, size 7,5% increase
less (tensile) stress (N/cm^2) forces -> less risk of overload injury -> F/CSA
more stiffness 15% (-> also higher RFD), less elongation (strain) 8%
low RT = almost no changes; high RT = benefits in the middle
protein supplement -> changes in tendon CSA
Neuromuscular adaptation to physical training
Neuromuscular system
sensory afferent feedback
efferent motoneuron output
spinal cord
training, inactivity, injury, aging, etc.
motor cortex, cerebellum
-> adaptive changes in neuromuscular function ... changes evaluate by ...
EMG (recording intramuscular & surface)
spinal motoneuron responses (H-reflex, V-wave)
transcranial magnetic/electrical stimulation of cortical neurons & subcortical axons, motor evoked responses (MEP)
MU activation & muscle force generation
Motor Unit = MU =
its motor axone +
all the muscle fibers innervated by MN
single spinal motor neuron (MN) +
EMG signals = sum of motor unit action potentials (MUAPs) from different MUs
Neuromuscular adaptions ... related to ...
explosive muscle strength (RFD)
exitability of spinal motorneurons
Training-induced changes in neuromuscular function
contractile rate of force development (RFD) ... positively influenced by ...
muscle size & architecture
neural innervation of muscle fibers (motor neuron discharge frequency & recruitment)
fibre type composition
tendon & aponeurosis (SEC) stiffness
high-density surface EMG analysis
motor units firing
MU discharge rate
beginning = extremely high firing of MN
linear relationship between motorneuron discharge rates & RFD
heavy RT
14 weeks
explosive muscle RFD -> increased neuromuscular activity within initial 200 ms increased -> acceleration -> increased max. RFD
increased RFD along with increases in iEMG & elevated rate of EMG rise
functional consequences
faster movements
elevated muscle force & power in fast movements
enhanced acceleration
reduced risk of falls
e.g. soccer: 20 min high intensity small sided games -> 40 min heavy RT
potential adaption mechanism
increased max. discharge frequency of individual motorneurons (MU) -> 12 ballistic-type RT wks 60-80% increase
increased number of discharge doublets in the individual motorneuron (MU) firing pattern -> very short = double-firing
nervous system learns high firing rates
after training -> 1/3 neurons learnt double-firing -> extremely strong -> increase in force
Spinal & supraspinal adaptions
H-reflex -> stimulating nerve instead of Knee (patella tendon) -> amplified artificial stretch reflex -> very easy activation on MN
V-Wave response
stretch reflex -> activates spinal neurons (Hammer am Knie)
electric stimulus applied to afferent axons -> evoked efferent motorneuron response (H-reflex)
increased H-reflex amplitude = altered spinal circuit state -> more excitability of spinal motoneurons; -> presynaptic inhibition of afferents; -> postsynaptic inibition of spinal motoneurons
maximal stimulus strength -> H-reflex response disappears -> one moves in wrong direction = cancel each other (resting muscle)
H-reflex re-appears in active muscle contraction -> maximal stimulus = now named V-wave
increased V-wave amplitude: increased descending supraspinal motor drive; increased spinal motoneuron excitability; decreased pre/post synaptic inhibition of MNs
14 wks heavy RT
V-waves = brain + spinal factors
H-reflex = only spinal responses
increase in H-reflex not as much as in V-wave
elevated V-wave & H-reflex responses during max. muscle contraction
RT -> neural adaptation at supraspinal & spinal levels
enhanced descending motor drive from higher CNS centres
Adaptive neuromuscular plasticity with resistance training in neurological (multiple-sclerosis = MS) patients
MS = neuro-degenerative disease -> loss of myelinization & destruction of peripheral axons -> difficult to activate muscles -> losing mobility (lower limb) -> not able to drive active MUs at high firing frequencies -> max. muscle strength = 30-70% reduced
3 wks heavy RT = huge increase in max. strength
Parkinson patients = same results
improved neuromuscular function
rapid force capacity
maximal muscle power
maximal muscle strength
eccentric muscle strength
Exercise in Aging - Functional aspects
Changes in neuromuscular functions
Spinal motor neurons
MU
Sarcopenia
(Isometric) muscular strength
loss of muscle mass
reduced muscle CSA 40% (decline starts at early adulthood, accelerates at 50)
more non-contractile tissue (fat & connective tissue)
reduction in muscle fiber size
risk of disability, balance abnormality, falls
reduced muscle mass -> 70 yrs 40%, 80 yrs 60% less muscle fibers
25% loss of spinal MN
60 yrs 50% less MN
less motor neurons in spinal cord
less excitable MUs 60-70 yrs
60-95 yrs = 1/3 MUs than young
preserved till 50-60 yrs
decreases 1-1,5% per year from 60-65 yrs
max. muscle power = important in elderly -> affects functional capacity!
losing power much faster than strength = 3,5% per year
weightlifters max. muscle power declines similar to untrained but are at higher level = gaining 20 years biological age
loss in muscle mass & decrease in max. muscle strength & power slowed/reversed = with RT
increase in muscle CSA & MVC
12 wks 3x/wk, 3 sets x 8 reps, 70% 1RM
85-98 yrs, 3x/wk, 12 wks, 80% 1RM
type IIa fibre CSA 22%
quadriceps strength 40-45%
chair rising time 30% faster
max. walking speed 25% faster
explosive (high velocity) RT
trained 80 yrs & untrained 60 yrs = no significant difference -> 50% reduced age-related power deficit = 10 yrs younger
increase of MVC, RFD, SSC muscle power, 1-leg muscle power
rehab hip replacement surgery
CSA
RT leads to better results than electrical stimulation or rehab
RT beginning 50% 1RM, end 80% 1RM
better RFD -> improved functional performance
summary of RT in elderly
neural factors
muscular factors
strength & power properties
muscle power
rapid force capacity (RFD)
dynamic muscle strength, isometric muscle strength
maximal motor neuron firing frequency
improved force steadiness, enhanced fine motor control
EMG amplitude and rate of EMG rise
myogenic satellite cell activation
muscle fiber pennation angle, tendon stiffness
single muscle fiber size, whole muscle size