Strategies for muscle activation during isometric torque generation at the human elbow

Abstract

1. We studied the patterns of electromyographic (EMG) activity in elbow muscles of 14 normal human subjects. The activity of five muscles that act in flexion-extension and forearm supination-pronation was simultaneously recorded during isometric voluntary torque generation, in which torques generated in a plane orthogonal to the long axis of the forearm were voluntarily coupled with torques generated about the long axis of the forearm (i.e., supination-pronation). 2. When forearm supination torques were superimposed on a background of elbow flexion torque, biceps brachii activity increased substantially, as expected; however, brachioradialis and brachialis EMG levels decreased modestly, a less predictable outcome. The pronator teres was also active during pure flexion and flexion coupled with mild supination (even though no pronation torque was required). This was presumably to offset inappropriate torque contributions of other muscles, such as the biceps brachii. 3. When forearm supination torque was superimposed on elbow extension torque, again the biceps brachii was strongly active. The pronator teres also became mildly active during extension with added pronation torque. These changes occurred despite the fact that both the pronator and biceps muscles induce elbow flexion. 4. In these same elbow extension tasks, triceps brachii activity was also modulated with both pronation or supination loads. It was most active during either supination or pronation loads, again despite the fact that it has no mechanical role in producing forearm supination-pronation torque. 5. Recordings of EMG activity during changes in forearm supination-pronation angle demonstrated that activation of the biceps brachii followed classic length-tension predictions, in that less EMG activity was required to achieve a given supination torque when the forearm was pronated (where biceps brachii is relatively longer). On the other hand, EMG activity of the pronator teres did not decrease when the pronator was lengthened. Triceps EMG was also more active when the forearm was supinated, despite its having no direct functional role in this movement. 6. Plots relating EMG activity in biceps brachii, brachialis, and brachioradialis at three different forearm positions revealed that there was a consistent positive near-linear relationship between brachialis and brachioradialis and that biceps brachii is often most active when brachioradialis and brachialis are least active. 7. We argue that, for the human elbow joint at least, fixed muscle synergies are rather uncommon and that relationships between muscle activities are situation dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

Redundancy or heterogeneity in the electric activity of the biceps brachii muscle? Added value of PCA-processed multi-channel EMG muscle activation estimates in a parallel-fibered muscle

“…The obviously not perfect match between EMG and force ( Fig. 2) might to a part be due to the mechanical output of the remaining muscles crossing the elbow (Buchanan et al, 1989). However, when force estimates of the BB improve, this should yield a closer correspondence with the total elbow flexion force as well, given the large contribution of BB to elbow flexion force (Kawakami et al, 1994).…”

Finger synergies during multi-finger cyclic production of moment of force

“…Earlier studies have suggested the mechanical advantage hypothesis (Buchanan et al 1989;Prilutsky 2000;Zatsiorsky et al 2002;Shim et al 2004a), which states that effectors (muscles or digits) with longer lever arms contribute more to the total rotational action as compared to effectors with shorter lever arms. In our study, two fingers (I and M) produced PR moments (M PR ) while two other fingers (R and L) produced SU moments (M SU ).…”

“…According to the mechanical advantage hypothesis (Buchanan et al 1989;Prilutsky 2000;Zatsiorsky et al 2002), these fingers were expected to produce most of the moment in PR and SU, respectively. This was indeed true.…”

Use of shoulder flexors to achieve isometric elbow extension in C6 tetraplegic patients during weight shift

“…Buchanon 24 suggests that multi-articular muscles are so complex that only one optimal pattern of recruitment exists that provides maximum force yet requires minimal energy expenditure and tissue strain. Karst and Hasan 25 propose that the muscle activation patterns can be predicted using only a few simple variables, such as initial and ®nal joint angels for a given movement.…”