How does emotional stress cause skeletal muscle rigidity? It is believed that when we are under stress our skeletal muscles tense up. However I am unable to explain why.Emotional stress is controlled by the sympathetic nervous system which has effects only on smooth muscles. However skeletal muscles are controlled by the somatic nervous system, so I was wondering what are some theories that explain tension on skeletal muscles?
Short answer
The stress response included norepinephrine release, in turn increasing blood pressure. This may cause muscle tension.  

Background
What an interesting question. To my surprise, I couldn't find convincing causal explanations, linking emotional stress and muscle tension.

In one study, study participants were exposed to emotional stress. The authors found a positive correlation between muscle (EMG) activity and blood pressure. They also found a significant positive correlations between stress and norepinephrine (NE) release. The latter plays an important
role in blood pressure homeostasis and is released during stress as part of the HPA axis response. These results seem to indicate that stress results in NE release, followed by increase in blood pressure (this is a known stress response). The increase in blood pressure could then lead to muscle tension. However, as the authors explain, the results do not explain whether these associations represent the effect of a general underlying factor (e.g., psychological arousal) or a more direct interdependence of blood pressure and muscular tension. In other words, a correlation was found, but no causal relationships can be deduced.

Reference
- Lundberg et al, International J Behav Med (1994); 1(4): 354-70