KINESIOLOGICAL ELECTROMYOGRAPHY - Biomechanics and Motor Control of Human Movement - page 277

to be the summation of a number of randomly timed m.u.a.p.'s. How-
ever, during fatigue, a tremor (8 - 10 Hz) is evident in the EMG and
force records. These fluctuations are neurological in origin and are
caused by motor units firing in synchronized bursts.

The major EMG measures of fatigue are an increase in the amplitude of
the EMG and a decrease in its frequency spectrum. One of those measures
is median power frequency, f m , which is the frequency of the power spectral
density function below which half the power lies and above which the other
half of the power lies:

∞ ∞

f m

1
2

X 2 (f ) df X 2 (f ) df X 2 (f ) df = = (10.9)

0 f m 0

where X (f ) is the amplitude of the harmonic at frequency f , and X 2 (f ) is
the power at frequency f .
A representative paper that shows the majority of these measures
(Krogh-Lund and Jørgensen, 1991) reports a 10% decrease in conduction
velocity, a 45% decrease in the median power frequency, and a 250%
increase in the root mean square (rms) amplitude. The median power
frequency has also been shown to be highly correlated with the amplitude of
the low-frequency bandwidth (15 - 45 Hz) and the ratio of the high-frequency
bandwidth ( > 95 Hz) to the low-frequency bandwidth (Allison and Fujiwara,
2002). An alternate and very common statistical measure ( Oberg et al.,
1994) is mean power frequency (MPF):

0

· X 2 ( f ) df f

MPF =

Hz (10.10)

0

X 2 ( f ) df

where F is the maximum frequency analyzed.
There is virtually no difference in these two frequency measures and their
correlations with independent measures of fatigue (Kerr and Callaghan, 1999).

10.5 REFERENCES

Allison, G. T. and T. Fujiwara. “The Relationship between EMG Median Frequency and
Low Frequency Band Amplitude Changes at Different Levels of Muscle Capacity,”
Clin. Biomech. 17 :464 - 469, 2002.
Andreassen, S. and A. Rosenfalck. “Relationship of Intracellular and Extracellular
Action Potentials of Skeletal Muscle Fibers,” CRC Crit. Rev. Bioeng. 6 :267 - 306,
1981.
Basmajian, J. V. “Electrodes and Electrode Connectors,” in New Developments in Elec-
tromyography and Clinical Neurophysiology , Vol. 1, J. E. Desmedt, Ed. (Karger,
Basel, Switzerland, 1973), pp. 502 - 510.