Unintended Firearms Discharges in Law Enforcement – Principle and Contributory Causes – Part 3 of 7

Inspector (Ret) Chris Butler

Welcome back to our discussion on the causes of unintended discharges. If you missed the first two articles, I highly suggest reading those before undertaking this session. You can find the series at my website at www.raptorprotection.com, just click the Blog link!

In Part 2 of our exploration of unintended discharges in law enforcement, we began to uncover some of the research principles behind the forces required to cause the trigger to be depressed as well as the fact that an officer’s sneaky little trigger finger can find its way inside the trigger guard entirely unnoticed by the officer.

Let’s summarize the important points so far that we have discussed:

  1. Officers are instructed and taught to keep their finger outside the trigger guard unless they have made the conscious decision to fire,
  2. Despite the training to the contrary, officers do put their finger inside the trigger guard even though they have not made the decision to shoot,
  3. The index finger may go inside the trigger guard without conscious awareness of the officer that it has done so, and,
  4. The greater the perceived threat conditions, the higher the possibility that the officers index finger may slip inside the trigger guard.

If an officer’s trigger finger has found its way inside the trigger guard, how is it possible for the officer holding the firearm to experience an unintended discharge? The answer is that the muscles responsible for moving the index finger and depressing the trigger can do so without the individual being consciously aware that this is happening. As Dr. Enoka has stated, this concept is not at all unusual to anyone in the study of human physiology because individuals are not aware of most muscle actions they perform during the course of daily living. When an officer intentionally decides to fire the weapon and as a result the trigger finger depresses the trigger, the muscle responds as a result of a direct command from the brain – the action is intended. This is what is termed a ‘voluntary contraction’[1]

However, muscles are also most often activated by other signals other than direct commands from the brain and when these contractions occur they are called ‘involuntary contractions’

In the existing body of literature examining involuntary contractions resulting in firearms discharges, there is a commonly accepted taxonomy which describes the various contributing conditions, any of which may, under the right circumstances, result in an involuntary contraction of the trigger finger occurring. The research conducted to date has concluded that the following conditions may elicit an involuntary contraction, which would be of sufficient force to bring about the discharge of the firearm:

  1. Sympathetic muscle contraction (also known in the research literature as muscle co-activation or muscle mirror movement, inter-limb coordination or contra-lateral contraction),
  2. Sudden loss of balance,
  3. Startle response,
  4. Hand confusion,
  5. Slip and capture error.

In our remaining time in this article, lets look at how sympathetic muscle contractions can lead directly to an unintended discharge.

Unintended Firearm Discharge – Involuntary Muscle Contraction Caused by Sympathetic Muscle Contraction

Incident: The officer from a large municipal agency had conducted a traffic stop for a moving violation. He was working alone. The officer approached the vehicle and began a conversation with the driver and lone occupant. During the interaction, the offender became aggressive with the officer and began threatening him and he opened his driver’s door and exited his car to confront the officer. As the driver was getting out, the officer drew his weapon. A physical fight ensued between the officer and the offender. The officer recalls grabbing the subject with his left hand and attempted to pull him forward, off balance, to take him to the ground. During this process, the officer’s weapon in his right hand discharged. Fortunately, the driver was not struck by the bullet, but the round struck his vehicle and lodged inside the passenger compartment.

Enoka (1991) was the first researcher to use the scientific method to examine the circumstances in which involuntary (unintended) discharges occur in the law enforcement realm. He proposed that unintended discharges might occur as a result of involuntary muscular contractions in the hand holding the firearm. Other research that supported the presence of ‘interlimb coordination’ does in fact suggest that in certain situations, especially those where the officers attention is completely directed towards a threatening stimulus, the human body can perform involuntary muscular contractions. One of the conditions that especially contribute to this occurring is when the same muscles in the opposing limb are performing an intended forceful action. For example, if an officer had his or her gun in one hand, and was attempting to push or pull as hard as they could with the opposite hand, there will be a degree of uncontrollable muscular contraction in the hand holding the firearm. In the physiology research, this effect is known as ‘muscle mirror movement’.

The question, therefore, is not if the use of other limbs can lead to involuntary contraction of the hand holding the firearm; rather, it is whether or not the forces generated by the involuntary contraction can be sufficient to overcome the resistance of the trigger pull and discharge the weapon. Researchers (Christou et al., 2002: Delwaide & Toulouse, 1983: Noteboom, Barnholt, & Enoka 2001: Weinberg & Hunt, 1976; Williams & Barnes, 1989) have determined the amount of sympathetic contractile force generated by the muscles of the non-involved limb (limb holding the firearm) can reach 25% of maximal exertion in laboratory settings. With the index finger contributing an average of 45% of the overall grip contraction force, the maximal exertion forces generated by the index finger in the hand holding the gun would average 14 lbs; more than sufficient force to overcome the trigger resistance and result in weapon discharge (Enoka, 2003).

An example of interlimb coordination leading to involuntary contraction of the trigger finger and unintended discharge occurred when a police officer attempted to physically remove a male from the front passenger seat of a vehicle. The officer reached in and grabbed the uncooperative subject and pulled vigorously to try to get the male out of the vehicle. The officer was holding his firearm in the other hand with his finger inside the trigger guard when he experienced an involuntary contraction and his weapon discharged striking the male in the torso.

In another case, an officer had conducted a traffic stop of a suspected impaired driver. As the officer approached the driver’s door, the driver suddenly began to open his door and attempted to get out of the car with a knife in his hand. The officer perceiving the threat developing, drew his sidearm and simultaneously kicked forcefully at the drivers door in an effort to prevent him from getting out of the car. The officer’s finger was inside the trigger guard and, simultaneous to his kicking action, experienced an involuntary contraction of his trigger finger causing his gun to discharge; the bullet struck the door of the vehicle.

In a tragic case from Brooklyn, New York City in November 2014, officer Peter Liang and his partner were conducting vertical patrols in stairwells of a public housing complex. With his 9mm Glock pistol in one hand and his flashlight in another, officer Liang attempted to use his shoulder to force open a stuck door and during this process his gun discharged. The bullet ricocheted off the wall and struck a 27-year-old man, Akai Gurley, who was ascending the stairwell approximately 14 steps below Liang. Gurley died of his injuries shortly after being struck.[2]

There have been numerous incidents of sympathetic muscular contractions resulting in unintended discharges. When investigating a case of a potential unintended discharge, the context and environment and the involved-officer’s statement will be critical to help to determine whether a case of sympathetic contraction might have been involved.

That’s all the time we have for this study. Thanks for stopping by and taking the time to join me in exploring this important issue. Please come back for Part 4 as we look at unintended discharges from loss of balance and other causes. Until then – stay vigilant and stay safe!

[1] Prochazka, A. Clarac, F. Loeb, G.E. Rothwell, J.C. & Wolpaw, J.R. “What do reflex and voluntary mean?” Modern views on the ancient debate; Experimental Brain Research, Vol 130, 417-432 (2000).

[2] Officer Peter Liang was indicted by a grand jury for the shooting death of Akai Gurley. He was charged with second-degree manslaughter, criminally negligent homicide, second-degree assault, reckless endangerment, and two counts of official misconduct. Liang was found guilty of manslaughter and official misconduct and on April 19, 2016 was sentenced to five years probation and 800 hours of community service.