The relationship between surface EMG (sEMG) and muscle force is complex, depending on factors like motor unit recruitment and firing rates, muscle fiber type composition, fatigue, and dynamic conditions. In general, sEMG amplitude and muscle force increase proportionally with recruitment and higher firing rates. However, the exact shape of the sEMG-force relationship can vary between linear and non-linear depending on the individual muscle and contraction conditions. Dynamic contractions and muscle fatigue can further impact this relationship. While sEMG can provide an estimate of relative muscle force, many factors must be considered for accurate quantification.

1. Relationship between surface EMG
and muscle force
Presented by: Zinat Ashnagar

2. Can the surface EMG (sEMG) be
utilized to quantify the force developed
by a muscle at a given time?
EMG-Force Relationship
2

3. 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

4. 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)
EMG-Force Relationship
4

5.  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)
EMG-Force Relationship
5

6. sEMG & Force
6

7. 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.
7

8. 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
EMG-Force Relationship
8

9. 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
EMG-Force Relationship
9

10.  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).
EMG-Force Relationship
10

11. De Luca CJ. THE USE OF SURFACE ELECTROMYOGRAPHY IN BIOMECHANICS: Delsys
Incorporated; 1993. p. 21
11

12. 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
EMG-Force Relationship
12

13.  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).
EMG-Force Relationship
13

14. 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.
De Luca CJ. THE USE OF SURFACE ELECTROMYOGRAPHY IN BIOMECHANICS: Delsys
Incorporated; 1993. p. 20
EMG-Force Relationship
14

15. De Luca CJ. THE USE OF SURFACE ELECTROMYOGRAPHY IN BIOMECHANICS:
Delsys Incorporated; 1993. p. 21
EMG-Force Relationship
15

16. 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).
EMG-Force Relationship
16

17. 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.
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
EMG-Force Relationship
17

18. 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).
EMG-Force Relationship
18

19. Shape of the relation between
EMG and muscle force
 Linear
 Non-Linear
sEMG & Force
19

20. Linear
•
•
•
•
•
Bigland and Lippold, 1954;
De Jong and Freund,1967;
DeVries, 1968;
Korner et al., 1984;
Milner-Brown and Stein, 1975
EMG-Force Relationship
20

21. Non-Linear
Alkner et al., 2000
De Luca, 1997
Komi and Buskirk, 1970
Potvin et al.,1996
Solomonow et al., 1986b
Vink et al., 1987
Zuniga and Simons,1969
EMG-Force Relationship
21

22.  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.
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
EMG-Force Relationship
22

23. This figure (redrawn from 2, p. 193) shows EMG/force ratios of 3 different
muscles for MVC normalized EMG and force output data.
23

24. 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

25. 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 slowtwitch 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.
25

26. 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)
EMG-Force Relationship
26

27. 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)
EMG-Force Relationship
27

28. Other Factors
The 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).
EMG-Force Relationship
28

29. 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)
EMG-Force Relationship
29

30. Other Factors
EMG-Force Relationship
30

31. 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
EMG-Force Relationship
31

32. 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
32

33. 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.
De Luca CJ. THE USE OF SURFACE ELECTROMYOGRAPHY IN
BIOMECHANICS: Delsys Incorporated; 1993. p. 18
EMG-Force Relationship
33

34. 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.
De Luca CJ. THE USE OF SURFACE ELECTROMYOGRAPHY IN
BIOMECHANICS: Delsys Incorporated; 1993. p. 20
EMG-Force Relationship
34

35. 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
EMG-Force Relationship
35

36. 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)
EMG-Force Relationship
36

37. Tension-length curves for isolated muscle
Source: Reprinted with permission from B. Gowitzke and M. Milner, Scientific
Bases of Human Movement, 3rd edition, © 1988, Williams and Wilkins.
37

38. Force–Velocity Relationships
Source: Reprinted from G. Soderberg, Selected Topics in Surface
Electromyography for Use in the Occupational Setting: Expert Perspective.
DHHS (NIOSH), Publication No. 91-100, Washington DC, NIOSH, 1992.
38

39. 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
39

40. 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)
EMG-Force Relationship
40

41. Thank You