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

Abstract

The purpose of this article is to gather results of studies on the relationship between median frequency (MF) and mean power frequency (MPF) and the level of muscle contraction, and to use those results to discuss the differences in the trends according to factors related to measurement technique and subject. Twenty-one studies with 63 cases for upper limb muscles and nine studies with 31 cases for lower limb muscles were analysed. Most results showed an increase in parameters with an increased level of muscle contraction, only some studies showed a decrease. The influence on parameters of the level of muscle contraction and factors such as subjects, type of contraction, muscle length and electrodes was analysed for each muscle. It was concluded that when analysing the influence of different factors on MF and MPF, because those factors interact they should be considered together, not separately.

Keywords: age and gender groups; electrodes; mean power frequency; median frequency; muscle contraction; muscle length; power spectrum.

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.