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For example if i bend my arm the biceps contract and triceps relax but if I bend my arm and the triceps contract instead of biceps what effect would it have?

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    $\begingroup$ your arm wouldn't bend... $\endgroup$ – AliceD Sep 29 '15 at 14:47
  • $\begingroup$ I'm voting to close this question as off-topic because this question is too trivial $\endgroup$ – AliceD Sep 29 '15 at 14:47
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    $\begingroup$ When your triceps contract your arm straightens out. $\endgroup$ – MattDMo Sep 29 '15 at 15:23
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This can be termed co-contraction of antagonist muscles, and it can occur in several pathologic processes. It essentially results in a spastic state.

Yelnik et al. describe the state as:

Spastic co-contraction is defined as an "unwanted, excessive, level of antagonistic muscle activity during voluntary command on an agonist muscle, which is aggravated by tonic stretch in the co-contracting muscle" (2, 8). Spastic co-contraction in patients with spastic paresis is a descending phenomenon, most probably due to misdirection of the supraspinal drive. It may be facilitated by increased recurrent inhibition, causing loss of reciprocal inhibition during voluntary command (2, 9).

For example, this can occur with perinatal brain damage. See a study by O'Sullivan et al. titled "Abnormal development of biceps brachii phasic stretch reflex and persistence of short latency heteronymous reflexes from biceps to triceps brachii in spastic cerebral palsy."

The state of co-contraction can be produced in healthy individuals as well, and studies on joint biomechanics under these conditions have been done. See Wise et al.

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