Two methods for the measurement of voluntary contraction torque in the biceps brachii muscle
Introduction
The measurement of EMG signals during isometric contractions performed at a prescribed fraction of the maximal voluntary contraction (MVC) level provides information of considerable interest for the non-invasive assessment of muscles [7], [19], [22], [30]. Such a measurement requires the subject to produce one or more short MVCs. Subjective as well as technical factors are relevant in the production of an MVC [2]. The motivation and posture of the subject, the way in which the limb is strapped in the isometric brace, the position of the limb, the activation of muscles other than those of interest, can all affect the amount of force measured during an MVC. In the case of a subject sitting at a table with his/her arm strapped in an elbow brace which is fixed to the table, actions such as leaning forward or backward or lifting up or pushing down the arm, generate a torque output from the transducers. During an MVC, as well as during lower level contractions, the subject may involuntarily produce these actions and introduce artifacts in the force detected by the instrumented brace at the elbow joint. We have investigated an alternative method of brace support intended to reduce these artifacts. The new method is to suspend the brace by elastic cables.
A difference in experimental values measured using the two methods could be attributed to at least three different reasons:
- 1.
the suspended condition is more natural because the subject feels less constrained compared to the fixed position;
- 2.
in the fixed condition the subject could unintentionally push or pull on the brace by using muscles not involved in the contraction task that add their contribution to the total amount of MVC (e.g. back muscles);
- 3.
in the two conditions, the biceps brachii (a biarticular muscle) may act differently on the shoulder joint which is not stabilized.
To compare these two different methods of fixing the arm during MVC contractions, we have evaluated if there were significant differences between:
- 1.
the MVC values;
- 2.
variables chosen to quantify myoelectric manifestations of muscle fatigue;
The variables chosen by many investigators to quantify EMG manifestations of muscle fatigue have been spectral features, such as the mean and the median frequency (MNF and MDF), amplitude features such as the average rectified value and the root mean square value (ARV and RMS) and muscle fiber conduction velocity (CV). Such manifestations consist in a ‘slowing’ of the signal reflected by a compression of its power spectrum, a decrease of MDF and MNF and an increase of ARV and RMS. These changes have been demonstrated to reflect muscle properties and fiber constituency [11], [13], [14], [18], [23], [29], [32], [33], recruitment strategies [3], [28], [30] and have been applied in ergonomics [9], [15], [16], back muscle analysis [4], [5], [10], [24], [28] and dentistry [12].
Section snippets
Methods
A modular brace has been designed which can be assembled with different sets of mechanical components to make it suitable for torque measurements about the elbow, wrist, knee and ankle. The brace incorporates two independent torquemeters (mod. TR11, CCT Transducers, Torino, Italy), one on each side of the brace. The proximal and distal section of the brace are connected by the torquemeters. Two methods for brace support have been investigated and tested for the elbow joint (see Fig. 1):
- 1.
the
Results
Results are reported for the nine subjects tested. In the fixed brace condition, reference MVC values ranged from 45.4 to 63.8 Nm, while in the suspended condition they ranged from 38.4 to 65.4 Nm. Fig. 3 shows the MVC values for the two conditions for all subjects. No trend could be found in the three attempts. The differences between MVC values measured using the two techniques are not statistically significant (t-test, P>0.05): such differences ranged from 0.7 to 7.2% of individual MVC
Discussion
During voluntary high level contractions the activation of trunk muscles does not seem to be a source of artifact in determining the torque produced at the elbow when the brace is fixed to the table. Properly instructed subjects produce, in the two conditions, torques that are not significantly different. Initial values of EMG variables (MNF, MDF, CV, ARV and RMS) detected from the biceps are also not significantly different in the two conditions suggesting that the muscle performances are
Conclusions
The clinical investigator should be aware of the fact that measurements of isometric torque at a joint reflect the contributions of many muscles while the EMG amplitude and the EMG fatigue indices provide information mostly about one specific muscle. The same mechanical performance may be generated with different contributions by the agonists/antagonists or synergic muscles acting on the joint. The algebraic summation of these contributions may be the same while the contributions of individual
Acknowledgements
The authors are grateful to William K. Durfee of the Department of Mechanical Engineering of the University of Minnesota for the many useful discussions and his criticism in preparing the final version of this work.
This work was supported by the European Concerted Action SENIAM, Camera di Commercio di Torino, Compagnia di San Paolo, Fondazione CRT di Torino, Fondazione S. Maugeri, Regione Piemonte and Ministry of Health.
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