I wanted to touch on a safety topic in this blog post. We are going to explain what an involuntary muscle contraction is, the different types and how to minimize an unintentional discharge.
First what is an involuntary muscle contraction? They are muscle contractions of the body or limbs the happen without conscious intent or decision. Another way of putting it, they are hard wired into your body and no amount of training can keep it from happening.1 Having an involuntary muscle contraction while your finger is not properly "indexed" (index finger out and above the trigger guard, likely resting on the frame under the slide.) can cause a unintentional discharge of your weapon.
First, Startle Reaction - An unexpected audio or visual stimulus that can cause an immediate, uncontrollable response by the muscles of the body. Which can cause your finger to make contact with the trigger and allowing the weapon to fire. I persons Startle Reaction starts with an eye blink, progresses to a bending of the neck and shoulders, elbows, fingers which in less than 200 milliseconds after the stimulus will cause a person to make a fist.
Second, Loss of Balance - "It is essentially impossible for one body segment to perform an action without causing other body segment to also move." When a person's balance is disrupted and rapid involuntary muscles contractions are evoked in an attempt to return the body to equilibrium. For example, someone who trips on a rock while holding a weapon will instinctively clutch it. The location of your trigger finger is important if this occurs, minimizing an unintentional discharge.1
Third, Sympathetic Contractions - Also called Mirror Movement. This is defined as an involuntary contraction that occurs in the muscles of one limb when the same muscles in the other limb are performing an intended forceful action. Studies have shown that this can be mirrored, on average, 25%. Again studies have shown that a maximum grip of an average man is 125lbs, which breaks down to roughly 14lbs of a sympathetic contraction to the index finger, enough to pull most triggers on handguns.1
So while no amount of training will prevent an involuntary muscle contraction, data "indicates that it is possible, with training, to modify the involuntary muscle contractions that are associated with reflexes."2 Continous training in these areas can minimize an unintentional discharge if you encounter a involuntary muscle contraction.
Source 1 Involuntary Muscle Contractions and the Unintentional Discharge of a Firearm Dr. Enoka Feb. 13, 2003 (Pg. 3, 6, 8, 9)
Source 2 Does the finger obey the brain Christopher Hein University of Frankfurt, Germany (Sec. 7.2.1)