What causes a muscle to be unsteady? I have noticed for myself that sometimes, certain muscles may become unsteady. Here are three examples:


Sometimes it is more difficult to hold my hand still in the air. 
Another example is how my calf muscles are not able to extend smoothly--when they get to a certain point, it sort of spasms and it is difficult to just maintain a slow steady movement.
Just today I noticed my right thumb, when holding down on the touch pad left click button, tends to spasm--my left thumb doesn't feel this way. 


I also don't believe this can be contributed to lack of motor coordination since I have good coordination to play guitar with the finger picking style and therefore my fingers should be quite dexterous. 

Though I am not sure if the first example is related to the other two. I am more curious about my thumb spasming since it can affect my guitar playing.
First, adrenalin reduces hand steadiness and increases anxiety (Basowitz, Korchin, Oken, Goldstein, &amp; Gussack, 1956), so other causes of anxiety seem likely to reduce steadiness by promoting the release of adrenalin. Next, here's an odd reference for a more voluntary psychopharmacological cause of hand-arm unsteadiness: drug abuse (including alcohol; Kaur, Sandhu, &amp; Sandhu, 2007). Caffeine and nicotine both impair hand steadiness, apparently without interacting (Smith, Tong,  &amp; Leigh, 1977; Heatherley, Hayward, Seers, &amp; Rogers, 2005; Rogers et al., 2005; Richardson, Rogers, Elliman, &amp; O'Dell, 1995; Bovim, Næss, Helle, &amp; Sand, 1995; see also for a review of caffeine effects and original evidence of a lack of interactions with alcohol  Franks, Hagedorn, Hensley, Hensley,  &amp; Starmer, 1975). Daily consumption of caffeine does not appear related though, and the effect of active caffeine can be blocked medically (Arnold, Springer, Engel, &amp; Helveston, 1993). A small dose of propranolol may also reduce hand tremors regardless of caffeine use (Humayun, Rader, Pieramici, Awh, &amp; de Juan, 1997), though interested readers should consider precautions and contraindications regarding propranolol, including hypoglycaemia or diabetes, asthma, and abnormal blood pressure, among several others (Rossi, 2006).

Physical exertion (Mürbe et al., 2001; Simon &amp; Dare, 1965; Halliday &amp; Redfearn, 1956), skill in manual labor, and sex (Gray, Sustare, &amp; Thompson, 1953) appear relevant as well. Gray and colleagues found greater hand steadiness in men, but a more recent study found the opposite, except in women during premenstrual phases and women using oral contraceptives (Hudgens, Fatkin, Billingsley, &amp; Mazurczak, 1988). A loosely related sex difference emerged in a study of marksmanship, suggesting men's accuracy endures longer than women's (Johnson &amp; Merullo, 1996); incidentally, this study found no effect of caffeine on marksmanship. Also, another study found lesser effects of caffeine on motor performance in women who use caffeine regularly (Jacobson &amp; Thurman-Lacey, 1992). A particularly old study seems to have found a weakly negative (and quite possibly spurious) correlation between performance on a mental multiplication test and hand steadiness, as well as a positive experimental effect of humidity (Stecher, 1916).

An early electroencephalographic (EEG) study emphasized the distinction between low-frequency, high-amplitude tremors, and the opposite kind, suggesting that the latter is less affected by cortical activity (Lindqvist, 1941). Two relatively recent studies produced apparently conflicting evidence about the relationship between hand and finger tremors and EEG activity: one succeeded (Isokawa &amp; Komisaruk, 1983) and another failed to influence hand tremors with rhythmic flashes of light, even though this induced changes in EEG activity (Lakie &amp; Combes, 1999). Other articles on hand steadiness research have been published in psychological journals to which I don't have access; the first of these even considers personality factors such as extraversion (Treadwell, 1960; Fleishman, 1956; Lovell, 1941).

Last (for this edition of this answer), some evidence supports the effectiveness of yoga for hand steadiness (Telles, Hanumanthaiah, Nagarathna, &amp; Nagendra, 1994), finger flexibility, grip strength and endurance (Garfinkel, Schumacher, Husain, Levy, &amp; Reshetar, 1994; Madanmohan, Jatiya, Udupa, &amp; Bhavanani, 2003; Dash &amp; Telles, 2001; Madanmohan et al., 1992; Raghuraj, Nagarathna, Nagendra, &amp; Telles, 1997; Raghuraj &amp; Telles, 1997), and stability of knee extensors (Bukowski, Conway, Glentz, Kurland, &amp; Galantino, 2006). These references were cited in a recent review that deserves mention itself (Donahoe-Fillmore, Brahler, Fisher, &amp; Beasley, 2010).

References

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