emg & force

Relationship between surface EMG and muscle force

Presented by: Zinat Ashnagar

Can the surface EMG (sEMG) be utilized to quantify the force developed

by a muscle at a given time?

2EMG-Force Relationship

Force production in a muscle is regulated by two main mechanisms:

Recruitment of additional MUs the increase of firing rate of the already active

MUs. (Merletti, ELECTROMYOGRAPHY Physiology, Engineering, and Noninvasive Applications,2004)

The amplitude of the surface EMG signal depends on both the number of active MUs and their firing rates.

EMG-Force Relationship 3

Since both EMG and force increase as a consequence of the same mechanisms, it is expected that muscle force can be estimated

from surface EMG analysis.(Merletti, ELECTROMYOGRAPHY Physiology, Engineering, and Noninvasive

Applications,2004)


The possibility of estimating muscle force from the EMG signal is attractive as it allows the assessment

of the contributions of single muscles to the total force exerted by a muscle group.

This is the main reason why EMG is and probably always will be the method of choice for force

estimation in kinesiological studies.(Merletti, ELECTROMYOGRAPHY Physiology, Engineering, and Noninvasive

Applications,2004)


sEMG & Force 6

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This figure (adopted & redrawn from 10, p. 110) shows the dependency of the EMG/force ratio from angle position (A,B), which can be eliminated by normalization of the MVC of force.


C. Disselhorst-Klug et al. Surface electromyography and muscle force: Limits in SEMG–force relationship and new approaches for applications. Clinical Biomechanics 24 (2009) 225–235

The force output of a single MU is regulated by its firing rate.

The increase in force saturates at around 30–40 pulses per second, which is below the maximum MU firing rate. (Enoka and Fuglevand,2001)

An increase in firing rate above the rate at which MU force saturates is reflected in the EMG signal and this will compromise accurate force estimation.

D. Staudenmann et al. Methodological aspects of SEMG recordings for force estimation – A tutorial and review. Journal of Electromyography and Kinesiology 20 (2010) 375–387


The force increase with firing rate has been predicted by modeling to be less than proportional

(Fuglevand et al., 1993).

This also holds for the increase in EMG amplitude, due to increasing phase cancellation with an increasing firing rate (Keenan et al., 2005).


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De Luca CJ. THE USE OF SURFACE ELECTROMYOGRAPHY IN BIOMECHANICS: Delsys Incorporated; 1993. p. 21

Since MUs as contractile elements act largely in parallel, the second mechanism to control muscle force is the recruitment of additional MUs, occurs in an orderly

sequence from small to large MUs.

D. Staudenmann et al. Methodological aspects of SEMG recordings for force estimation – A tutorial and review. Journal of Electromyography and

Kinesiology 20 (2010) 375–387


Because of the size principle, the increase in force with additional MU recruitment is predicted by modeling to be more than proportional (Fuglevand et al., 1993).

While the rise in MU size with increasing force would suggest a more than proportional increase in EMG amplitude as well, this is not necessarily true as MUP amplitude also depends on the distance between the MU and the electrode

(Roeleveld et al., 1997b).


i) If the newly recruited motor unit is located close to the electrode, then the relative increase of the EMG signal will be greater than the corresponding increase of the force because the new MUAP will contribute more than an average unit of energy to the EMG signal.

ii) If the newly recruited motor unit is located far away from the electrode, then the force will increase, but the amplitude of the EMG signal will not.



Differences between muscles appear to exist in the range of force over which new MUs are

recruited, with some muscles having all MUs recruited at 50% of MVC and others recruiting new MUs up to 100% MVC (Lawrence and De Luca,

1983; Woods and Bigland-Ritchie, 1983).


It is important to note here that studies on the relationship between muscle force and EMG amplitude with very few exceptions in human

experiments (Heckathorne and Childress, 1981; Inman and Ralston, 1952) and

animal experiments (Guimaraes et al., 1995; Herzog et al., 1998; Liu et al.,

1999) did not actually measure muscle force.



Instead, the net output of a series of synergistic antagonistic muscle was

measured.

In addition, effects of gravity and effects of joint stiffness are often ignored,

although these may be quite substantial (Ridderikhoff et al., 2004).


Shape of the relation between EMG and muscle force

Linear Non-Linear

sEMG & Force 19

Linear

• Bigland and Lippold, 1954;• De Jong and Freund,1967;• DeVries, 1968; • Korner et al., 1984; • Milner-Brown and Stein, 1975


Non-LinearAlkner et al., 2000De Luca, 1997 Komi and Buskirk, 1970Potvin et al.,1996 Solomonow et al., 1986bVink et al., 1987Zuniga and Simons,1969


The shape of the relationship between firing rate and force may be different from the shape of the relationship between firing rate and EMG amplitude.

As a consequence, it may be clear that the relationship between EMG and force is not necessarily (nor physiologically, nor biophysically) linear.

It is dependent on the recruitment range and hence on muscle fiber type composition.



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This figure (redrawn from 2, p. 193) shows EMG/force ratios of 3 different muscles for MVC normalized EMG and force output data.

The differences between the large and small muscles may possibly reflect the differences in

the firing rates of the muscles (slow versus fast), their recruitment properties (which fibers

recruit as a function of the strength of the contraction) and other anatomical and electrical

considerations.CRISWELL E. CRAM’S INTRODUCTION TO Surface Electromyography.

second ed: Jones and Bartlett Publishers; 2011. p. 30.

sEMG & Force 24

In general, muscles that consist of predominantly one fiber type tend to have a more linear relationship

between force exerted and SEMG.

In muscles of a mixed fiber type (fast- and slow-twitch fibers), the relationship appears to be more

curvilinear, with the breaking point at approximately 50% of maximum voluntary contraction.

CRISWELL E. CRAM’S INTRODUCTION TO Surface Electromyography. second ed: Jones and Bartlett Publishers; 2011. p. 30.

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Both the force and the EMG amplitude are in most circumstances nonlinearly related to the neural

drive.

Apparently both nonlinearities in the relation between neural drive and EMG, on the one hand, and drive and force, on the other hand, balance each other,

leading to an often close-to-linear relation between EMG and force.

(Merletti, ELECTROMYOGRAPHY Physiology, Engineering, and Noninvasive Applications,2004)


The relationship (if any) between force and amplitude should be adapted to the muscle condition, including muscle length (joint angle), muscle temperature, fatigue, and so on.

In particular, under submaximal contractions, the fatigued muscle generates EMG signals with larger amplitude compared to the unfatigued condition, although maintaining a constant force.

(Merletti, ELECTROMYOGRAPHY Physiology, Engineering, and Noninvasive Applications,2004)


Other FactorsThe surface EMG amplitude depends strongly on the

electrode location. For locations in which EMG amplitude is very

sensitive to small electrode displacements it is expected that the relation between EMG and force

may be poorer than in other locations. Farina, D., R. Merletti, M. Nazzaro, and I. Caruso, “Effect of joint angle on surface EMG

variables for the muscles of the leg and thigh,” IEEE Eng Med Biol Mag 20, 62–71 (2001).


Considering an “optimal” electrode placement, the relation between force and EMG may depend on

the subcutaneous fat layer thickness, the inclination of the fibers with respect to the

detection system, the distribution of conduction velocities of the active MUs, the interelectrode

distance, the spatial filter applied for EMG recording, the presence of crosstalk, and the

degree of synchronization of the active MUs.(Merletti, ELECTROMYOGRAPHY Physiology, Engineering, and Noninvasive

Applications,2004)


Other Factors


The EMG-Force ratio can be used to determine the neuromuscular (training) status of a muscle.

Within static contractions with constantly increasing force output (ramping) well-trained muscles show a

clear right shift of the ratio, atrophic or very untrained muscles show a left shift.

Trained muscles need less EMG for a given force output than atrophic or fatigued muscles.

Reference: ABC of EMG – A Practical Introduction to Kinesiological Electromyography. page 43


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Schematic EMG/force relationship in ramp contractions. Depending on the muscle condition and training status the ratio can alter. Trained muscles need less EMG for a given force output than atrophic or fatigued muscles. Reference: ABC of EMG – A Practical Introduction to Kinesiological Electromyography. page 43

Muscle dynamics

In an anisometric contraction, various mechanical, physiological, anatomical and electrical modifications occur throughout the contraction that affect, in substantial ways, the

relationship between the signal amplitude and the force produced by the muscle.



For example, the force-length relationship of the muscle fibers varies non-linearly, and the shapes of the MUAPs which construct the EMG signal, are altered because the relative position of the electrode fixed on the surface of the skin changes with respect to the contracting muscle fibers.



If it is absolutely necessary to process an EMG signal detected during an anisometric contraction, then make every attempt to limit the analysis to a near-isometric epoch of the record and

extrapolate the interpretation of the analysis based on the results from this epoch.

If the anisometric contractions are repetitive, such as those found in gait and cycling, then choose for analysis a fixed epoch in the

period of the contraction. Make all comparisons in this epoch.De Luca CJ. THE USE OF SURFACE ELECTROMYOGRAPHY IN

BIOMECHANICS: Delsys Incorporated; 1993. p. 19


In dynamic contractions, muscle force, at a given neural drive and hence at a given EMG amplitude,

depends on muscle length and contraction velocity (Blix, 1894; Hill,1997)


Tension-length curves for isolated muscle

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Source: Reprinted with permission from B. Gowitzke and M. Milner, Scientific Bases of Human Movement, 3rd edition, © 1988, Williams and Wilkins.

Force–Velocity Relationships

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Source: Reprinted from G. Soderberg, Selected Topics in SurfaceElectromyography for Use in the Occupational Setting: Expert Perspective. DHHS (NIOSH), Publication No. 91-100, Washington DC, NIOSH, 1992.

The SEMG–force relationship is different in concentric versus eccentric contractions of a muscle.

(Komi and Buskirk, 1972; Linnamo et al., 2006)

While at the same force value EMG activity is increased in concentric contraction compared to

static isometric contraction, it is lower in eccentric contraction.

C. Disselhorst-Klug et al. Surface electromyography and muscle force: Limits in sEMG–force relationship and new approaches for applications. Clinical Biomechanics 24 (2009) 225–235

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Differences in Concentric Vs. Eccentric contractions

EMG amplitudes are generally less during negative (eccentric) work vs. positive (concentric) work (Komi, 1973; Komi et al., 1987)

– Preloaded tension in tendons (non-contractile elements) requires less contribution from muscle (contractile elements)


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emg & force

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