The influence of confounding factors on the relationship between muscle contraction level and MF and MPF values of EMG signal: a review

Figures & data

Table 1. Studies by muscle, parameter, posture in elbow joint, type of contractions and subject-related factors (age and gender).

Study	Case	Population	Age (years)	Contraction	Angle (°)	Muscle	Parameter	Electrode
[10]	a1	10M	20–34	Step	115	BB	MF	1 mm d/10 mm l
	a2	10F
	a3	10M	65–78
	a4	10F
[13]	c1	1A n/a	n/a	Ramp	90	BB, TB	MF	3 mm d/10 mm l
	c2	1A n/a						9 mm d/40 mm l
	c3	1B n/a						3 mm d/10 mm l
	c4	1B n/a						9 mm d/40 mm l
[19]	rk1	16M	23–33	Ramp	90	ED	MF, MPF	5 × 5 mm^2/20 mm
	rk2
[18]	ta	56M + 38F	5–69	Step	90	BB	MF	10 mm × 1 mm/10 mm l (4 electrodes)
[20]	e1	1A n/a	n/a	Step	90	BB, ED	MF	12 mm d/35 mm l
	e1	1A n/a				IN		9 mm d/35 mm l
	e2	1B n/a				BB, ED		12 mm d/35 mm l
[21]	b1	6M	20–29	Step	90	BB	MF	n/a/10 mm l
	b2	6M	60–69
	b3	6F	20–29
	b4	6F	60–69
[22]	p1	18 right-handed	n/a	Step	n/a	IN	MF	Non-dominant hand
	p3							Dominant hand
	p2	17 left-handed						Dominant hand
	p4							Non-dominant hand
[23]	be	3M + 4F	29.5 ± 6.8	Step	90	BB, AD	MF	n/a
[24]	r1	7M + 3F	22.2 ± 2.8	Step	115	BB	MPF	4 mm d/20 mm l
	r2							4 mm d/40 mm l
	r3							4 mm d/60 mm l
[15]	m1	13M	29.8 ± 7.1	Ramp	90	TB, AN	MF, MPF	Miniature/6 mm l
	m2	16F
[14]	o1	4M + 5F	32.5 ± 10.5; 25.8 ± 2.9	Ramp	90	AN	MF, MPF	Miniature/6 mm l
	o2
	o3
	o4			Step
	o5
	o6
[11]	l1	6M + 8F	M32.2; F28.6	Step	90	TB, AN	MF, MPF	Miniature/6 mm l
	l2	6M + 8F	M32.2; F28.7	Ramp	90
[25]	f1	13M	29.8 ± 7.1	Ramp	90	BB, TB, AN	MF, MPF	Miniature/6 mm l
	f2	16F	28.1 ± 5.8
[7]	n1	6M	28.5 ± 6.9	Ramp	90	TB, AN	MPF	Miniature/6 mm l
	n2							Miniature/30 mm l
[5]	k1	A	n/a	Step	50	BB	MPF	silver-silver chloride Red Dot^TM (Model 2259, 3M Canada Ltd, Canada)
	k2				70
	k3				90
	k4				110
	k5				130
[17]	mi	10M	30.2 ± 6.1	Step	120	BB	MPF	4-bar^a
[26]	d1	46M	23 ± 3	Step	90	BB	MPF	5 mm d/20 mm
	d2	50F	23 ± 2
[27]	g1	10M	26.6 ± 3	Step	120	BB	MPF	Array of 8 electrodes/5 mm l
	g2
	g3
[28]	h1	4M + 6F	28.5 ± 4.8	Step	135	BB	MPF	4 mm d/11 mm l
	h2							4 mm d/10 mm l^b
[16]	i1	7M	25 ± 3.1	Step	90	BB	MPF	5 mm^2/20 mm l 37 °C
	i2							5 mm^2/20 mm l 34 °C
	i3							5 mm^2/20 mm l °C
	i4							5 mm^2/20 mm l °C
[29]	j1	9F	30–40	Step	90	BB	MPF	23 mm^2/10 mm l
	j2							23 mm^2/20 mm l
	j3							23 mm^2/30 mm l
	j4			Ramp	90	BB	MPF	23 mm^2/10 mm l
	j5							23 mm^2/20 mm l
	j6							23 mm^2/30 mm l

Note: MF = median frequency; MPF = mean power frequency; Angle = angle between arm and forearm; M = males; F = females; d = diameter of the active surface of electrodes; l = inter-electrode distance; n/a = information not available; A = subject A; B = subject B; BB = biceps brachii; TB = triceps brachii; IN = interseousnoss; ED = extensor digitorum; AN = anconeus; AD = abductor digiti minimi.^aA 4-bar electrode system consisting of four silver wire electrodes 10 mm long. ^b4 mm d/10 mm l (11 Laplacian electrodes).

Table 2. Studies by muscle, parameter, posture in knee joint, type of contractions and subject-related factors (age and gender).

Study	Cases	Population	Age (years)	Contraction	Angle (°)	Muscle	Parameter	Electrode
[30]	S	10M	21.6 ± 3.6	Step	45	VL, VM, RF	MF	Miniature/20 mm l
[31]	t1	11M + 11F	21–34	Step	90	VL, VM, RF	MF	3 pairs of 7 mm electrodes/30 mm l
	t2
[32]	u1	15M	26.5 ± 4.7	Step	120	VL, VM, RF	MF	8 mm d/20 mm l
	u2	15F	24.9 ± 3.7
[33]	v1	10M	32.7 ± 4.8	Step	120	VL	MF	Electrodes arranged in a 10 × 13 rectangular matrix (45 × 60 mm)/5 mm
	v3
	v2		33.4 ± 6.6
	v4
[4]	x1	11M	20.1 ± 0.4	Ramp	180	BF, SE	MF	4 mm d/15 mm l
	x2
	x3				135
	x4
	x5				90
	x6
[34]	y1	16F	20.21 ± 1.47	Balist	180	GA	MF	n/a/20
	y2			Ramp
[35]	z1	15F	28.3 ± 3.8	Step	180	TA	MF	Unipolar/20 mm l
	z2	15M	35.4 ± 7.5
	z3	15F	28.3 ± 3.8					Bipolar/30 mm l
	z4	15M	35.4 ± 7.5
[9]	q1	7M	22–43	Ramp	90	VL,VM, RF	MPF	8 mm d/20 mm l
	q2	7F	22–43		90
	q3	7M	22–43		90
	q4	7F	22–43		90
[36]	w1	14M	23.5 ± 3.2	Step	150	VL, VML, VMO	MPF	4 array electrodes 1 × 5 mm/10 mm l
	w2				120
	w3				90
	w4				150
	w5				120
	w6				90

Note: MF = median frequency; MPF = mean power frequency; Angle = angle in knee joint; M = males; F = females; d = diameter of the active surface of electrodes; l = inter-electrode distance; n/a = information not available; VL = vastus lateralis; VM = vastus medialis; VMO = vastus medialis obliquus; VML = vastus medialis longus; RF = rectus femoris; BF = biceps femoris; SE = semitendinosus; TA = tibialis anterior; GA = gastrocnemius.

Figure 1. Median frequency (MF) of biceps brachii by muscle force: (a) males; (b) females; (c) a combined group of males and females. Note: Values obtained from studies listed in Table 1 (letters indicate studies, numbers indicate cases within a study). In cases c1, c2, c3,c4, e1 and e2 information on gender is not available. MVC = maximal voluntary contraction.

Figure 2. Median frequency (MF) of triceps brachii by muscle force: (a) males (cases f1, m1) and females (cases f2, m2); (b) a combined group of males and females. Note: Values obtained from studies listed in Table 1 (letters indicate studies, numbers indicate cases within a study). In cases c1, c2, c3 and c4 information on gender is not available. MVC = maximal voluntary contraction.

Figure 3. Median frequency (MF) by muscle force: (a) interseousnoss (cases e1, p1, p2, p3, p4) and abductor digiti minimi (case be); (b) extensor digitorum (ED) (cases e1, e2, rk1, rk2). Note: Values obtained from studies listed in Table 1 (letters indicate studies, numbers indicate cases within a study). In cases p1, p2, p3,p4, e1 and e2 information on gender is not available. MVC = maximal voluntary contraction.

Figure 4. Median frequency (MF) of anconeus by muscle force: (a) males (cases f1, m1) and females (cases f2, m2); (b) a combined group of males and females (cases o1, o2, o3, o4, o5, o6). Note: Values obtained from studies listed in Table 1 (letters indicate studies, numbers indicate cases within a study). MVC = maximal voluntary contraction.

Figure 5. Median frequency (MF) of force level by upper limb muscle: (a) mean values of MF averaged over levels of contraction (mMF); (b) relative (dimensionless) difference between the lowest and the highest values of MF (aMF) – only increasing trends over entire range of maximal voluntary contraction were included. Note:

=males;

=females;

=ramp contractions;

=joint angle >90°. BB = biceps brachii; TB = triceps brachii; IN = interseousnoss; ED = extensor digitorum; AN = anconeus; AD = abductor digiti minimi.

Figure 6. Mean power frequency (MPF) of biceps brachii by muscle force: (a) males; (b) females; (c) a combined group of males and females. Note: Values obtained from studies listed in Table 1 (letters indicate studies, numbers indicate cases within a study). In cases k1, k2, k3, k4 and k5 information on gender is not available. MVC = maximal voluntary contraction.

Figure 7. Mean power frequency (MPF) of triceps brachii by muscle force: (a) males (cases n1, n2, m1, f1); (b) females (cases f2, m2) and a combined group of males and females (cases l1, l2). Note: Values obtained from studies listed in Table 1 (letters indicate studies, numbers indicate cases within a study). MVC = maximal voluntary contraction.

Figure 8. Mean power frequency (MPF) of anconeus by muscle force: (a) males (cases n1, n2, m1,f1) and females (cases f2, m2); (b) a combined group of males and females. Note: Values obtained from studies listed in Table 1 (letters indicate studies, numbers indicate cases within a study). MVC = maximal voluntary contraction.

Figure 9. Mean power frequency (MPF) by muscle force of upper limb muscle: (a) mean values of MPF averaged over levels of contraction (mMPF); (b) relative (dimensionless) difference between the lowest and the highest values of MPF (aMPF) – only increasing trends over entire range of maximal voluntary contraction were included. Note:

=males;

=females;

=ramp contractions;

=joint angle >90°;

=joint angle <90°. BB = biceps brachii; TB = triceps brachii; AN = anconeus.

Figure 10. Median frequency (MF) by muscle force: (a) vastus lateralis for males (cases s, u1, v1, v2, v3, v4) and females (case u2) and a combined group of males and females (cases t1, t2); (b) vastus medialis for males (cases s, u1), females (case u2) and a combined group of males and females (cases t1, t2); (c) rectus femoris for males (cases s, u1), females (case u2) and a combined group of males and females (cases t1, t2); (d) biceps femoris for males. Note: Values obtained from studies listed in Table 2 (letters indicate studies, numbers indicate cases within a study). MVC = maximal voluntary contraction.

Figure 11. Median frequency (MF) by muscle force: (a) tibialis anterior (TA) for females, TA for males (cases z2, z4) and gastrocnemius (GA) for females (cases y1, y2); (b) semitendinosus for males. Note: Values obtained from studies listed in Table 2 (letters indicate studies, numbers indicate cases within a study). MVC = maximal voluntary contraction.

Figure 12. Median frequency (MF) of muscle force by lower limb muscle: (a) mean values of MF averaged over levels of contraction (mMF); (b) relative (dimensionless) difference between the lowest and the highest values of MF (aMF) – only increasing trends over entire range of maximal voluntary contraction were included. Note:

=males;

=females;

=ramp contractions;

=joint angle >90°;

=joint angle <90°. VL = vastus lateralis; VM = vastus medialis; RF = rectus femoris; BF = biceps femoris; TA = tibialis anterior; GA = gastrocnemius.

Figure 13. Mean power frequency (MPF) by muscle force: (a) vastus lateralis for males (cases w1, w2, w3, w4, w5, w6, q1, q3, s) and females (cases q2, q4); (b) vastus medialis for males (cases w1, w2, w3, w4, w5, w6, q1, q3, s) and females (cases q2, q4); (c) rectus femoris for males (cases q1, q3) and females (cases q2, q4). Note: Values obtained from studies listed in Table 2 (letters indicate studies, numbers indicate cases within a study). MVC = maximal voluntary contraction.

Figure 14. Mean power frequency (MPF) of muscle force by lower limb muscle: (a) mean values of MPF averaged over levels of contraction (mMPF); (b) relative (dimensionless) difference between the lowest and the highest values of MPF (aMPF) – only increasing trends over entire range of maximal voluntary contraction were included. Note:

=males;

=females;

=ramp contractions;

=joint angle >90°. VL = vastus lateralis; VM = vastus medialis; VM-o = vastus medialis obliquus; VM-l = vastus medialis longus; RF = rectus femoris.

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