TOKYO -- Falls are a common occurrence among the elderly in Japan, and recent research has pointed to the brain's misinterpretation of automatic reflexes as one possible cause. The discovery of such a mechanism could pave the way for methods to prevent tumbles.

Falls identified as major cause of accidental death among elderly

According to data published by the Consumer Affairs Agency, falls are one of the three main causes of accidental death for people 65 and over. In 2021, falls caused 9,509 deaths among this demographic, surpassing the other two main factors: choking on food and drowning in a bathtub. Over the past decade, there has been a 1.4-fold increase in deaths from falling, diverging from downward trends in other causes of death.

Data on emergency transport cases backs this tendency. According to a 2022 Tokyo Fire Department report, out of 81,901 cases, falls were the most common accident in daily life, with those 65 and older making up 75% of these emergency transports. That said, the number of cases starts rising from the time people reach their latter 40s.

In a survey by major medical and health care product company Omron Healthcare Co. of fathers in their 30s to 50s, 70 out of 602 respondents, or 11.6%, reported tumbles or injuries during relays, obstacle races and other events for parents at their children's school sports days. The most common reason cited was that their bodies did not move as anticipated.

Research published in U.S. journal Science Advances late last year by a team including Tatsuya Umeda, an associate professor in neuroscience at Kyoto University, suggests that a reason for this is a mismatch between body reflexes and brain instructions.

How the brain's commands and unconscious reflexes interact

Reflexes, such as the knee-jerk response when a seated person's knee is tapped lightly or the instinctive withdrawal of a hand when touching something hot, operate through "sensors" in the muscles and joints that react without the brain's mediation, facilitating swift or precise movements.

However, prior studies related to body movement had not fully established a link between motor cortex instructions and reflexes, with the detailed mechanisms remaining unknown. To address this, Umeda's team developed a new method to simultaneously record brain commands, signals from sensory organs (reflex signals) and actual muscle activity in Japanese macaques and other subjects, comprehensively analyzing the flow of signals in hand and arm movements.

First, three types of data were collected as macaques learned to pull a lever to receive juice and repeated the same movement. Next, when researchers made it harder for monkeys to send reflex signals and had them perform the same actions, the macaques struggled to grasp the lever and their arm movements were weaker.

An analysis of the data showed that in normal monkeys, the combined signal strength of the commands from the brain and the reflex signals matched the actual muscle movement. In the monkeys with impaired reflex signals, brain command signals became the primary signal and muscle movements were reduced.

Reduction in muscle mass causes inaccurate predictions

To move the body, the motor cortex of the brain issues a command, which moves the muscles via the spinal cord. When the muscles move, such as when a foot touches the ground, a reflex signal returns to the spinal cord, and these commands and signals are integrated to create a smooth movement. When this occurs, the brain predicts how much of the signal will be returned by the reflex, and then produces a weakened signal based on that prediction.

"If we're comparing them with humans, monkeys with limited reflexes can be likened to people who have lost muscle mass due to aging," Umeda explained. When reflex signals differ from the brain's expectations due to muscle weakness, it is thought to result in reduced muscle movement and falls, among other issues. This can be considered a phenomenon in which the brain has misperceptions.

Umeda recalled his own experience taking a fall, saying, "I was racing in the park with my kid and experienced a feeling of not being in control of my body, which was different from how I had imagined it would be."

Considering mechanisms like this, can falls be prevented? Umeda noted, "You need to keep your brain's predictions up to date by learning to understand your own muscle movements." Since such movements used in everyday situations alone cannot provide adequate information to the brain, Umeda recommends exercise or strength training that put more strain than usual.

Yukio Nishimura, head of the Tokyo Metropolitan Institute of Medical Science's Neural Prosthetics Project and part of Umeda's team, said, "Relying on your old senses to make a sudden move is like doing something for the first time or doing something unfamiliar without care. Practicing and repeating those movements over and over again is thought to prevent mistakes."

(Japanese original by Ryo Watanabe, Lifestyle, Science & Environment News Department)