Reliability of EMG time-frequency measures of fatigue during repetitive lifting

Ebenbichler, Gerold R.1,2,; Bonato, Paolo3,; Roy, Serge H.3,; Lehr, Stefan4,; Posch, Martin4,; Kollmitzer, Josef2,; Della Croce, Ugo5

Medicine & Science in Sports & Exercise: August 2002 - Volume 34 - Issue 8 - pp 1316-1323

EBENBICHLER G. R., P. BONATO, S. H. ROY, S. LEHR, M. POSCH, J. KOLLMITZER, and U. DELLA CROCE. Reliability of EMG time-frequency measures of fatigue during repetitive lifting. Med. Sci. Sports Exerc., Vol. 34, No. 8, pp. 1316–1323, 2002.

Purpose: To test the short-term and long-term reliability of time-frequency electromyographic (EMG) measures of fatigue during repetitive dynamic lifting and compare it with the reliability of median frequency (MF) EMG measures of fatigue during static lifting.

Methods: Fourteen’ healthy male subjects (26 ± 5 years) repetitively (12 lifts/min) lifted and lowered a box (29 × 25 × 23 cm, 13 kg) for 4.5 min during 3 different tests on 2 different days. EMG data and the biomechanics of motion were recorded. Before and after dynamic lifting, static maximum lifting tests were performed. At the end of each of the two sessions, subjects performed a static lift at 80% of their maximum lifting force for 30 s.

Results: Significant fatigue-related changes were observed during the lifting exercise via EMG time-frequency analysis at the paravertebral L5, L2, T10, and vastus lateralis (VL) electrode sites. Two parameters for assessing fatigue during dynamic contractions [i.e., the Instantaneous Median Frequency (IMDF) and its time dependent change] were shown to be reproducible both in the short-term (2 h) and long-term (2 wk). The corresponding ICCs reflecting the reproducibility of values between sessions were 96.9% (L5), 98.1% (L2), 90.1% (T10), 96.4% (UT), 98.0% (GM), 89.5% (VL), and 99.0% (BF), respectively. For most EMG recording sites, the reliability of the IMDF measures was not dependent upon the postural strategy that the subject used to accomplish the lifting task or on the subject’s strength as measured via the static maximum lifting test. A comparison between the ICC values of the IMDF measures and those of the parameters utilized to assess fatigue during static sustained lifts [i.e., the Median Frequency (MDF) and its change during the test] revealed equally good reproducibility for most EMG recording sites. The respective ICC values that took into account time dependent trends for the IMDF parameter were 87.1% (L5), 62.4% (L2), 90.1% (T10), 0% (UT), 72.7% (GM), 45.4% (VL), and 100% (BF), and for the MDF parameter 94.9% (L5), 73.0% (L2), 80.9% (T10), 100% (UT), 89% (GM), 91.7% (VL), and 90.9% (BF), respectively.

Conclusions: The time-frequency approach allows one to derive EMG spectral parameters that can be used to monitor muscle fatigue during dynamic real-world tasks such as lifting.

1Rehabilitation Hospital “Weisser Hof”, Klosterneuburg, AUSTRIA,

2Department of PM&R, University Hospital of Vienna, AUSTRIA,

3NeuroMuscular Research Center, Boston University, Boston, MA,

4Department of Medical Statistics, University of Vienna, AUSTRIA, and

5Dipartimento di Scienze Biomediche, Università di Sassari, Sassari, ITALY

Submitted for publication August 2001.

Accepted for publication March 2002.

Address for correspondence: Gerold R. Ebenbichler, MD, Department of Physical Medicine & Rehabilitation, Wahringer Gurtel 18-20, 1090 Vienna, Austria; Email:

The first author would like to thank Professor Carlo J. De Luca, Ph.D. for hosting him at the NeuroMuscular Research Center, Boston University, during his research sabbatical year. The authors would like to thank Corinna Hatje for her help in the data processing.

This work was partially supported by The Whitaker Foundation, Biomedical Engineering Research Grants, under the project entitled “Assessment of muscle function during cyclical lifting through the analysis of surface electromyographic signals.”

Gerold R. Ebenbichler, M.D. was a research fellow at the NMRC supported by the Austrian Science Foundation (J-1630).

©2002The American College of Sports Medicine