Feedforward control

The body controls movement in two main ways. Feedback control uses sensory information about what has actually happened to correct any deviations. In contrast, feedforward control anticipates and prepares for the outcome of a movement that has not yet occurred.

Imagine lifting a heavy box. If you know beforehand that the box is heavy, you tense your lower back, abdomen, and legs before lifting it. But if you lift forcefully because you expect it to be heavy and the box turns out to be empty, your body lurches upward. This happens because the brain's prediction prepared your movement in advance.

The reason feedback alone is not enough comes down to time. It takes time for sensory signals to reach the central nervous system and for motor commands to travel back to the muscles. During fast movements, the body would already have been thrown off balance by then, so the nervous system anticipates the disturbance caused by the movement and activates the muscles that maintain posture first. This advance postural regulation is called an anticipatory postural adjustment.

The most frequently cited experiment involves raising an arm quickly while standing in place. Raising the arm produces a reaction that pushes the trunk backward, but electromyographic measurements show that the trunk muscles activate before the arm muscles do. Hodges and Richardson (1996, *Spine*) measured the onset of anticipatory Transversus abdominis contraction during this task and reported that people with low back pain tended to show delayed contraction onset.

Similar control is observed in the breathing muscles. The diaphragm and transversus abdominis activate in advance when the trunk needs to be stabilized during limb movements. There are also reports that, during repetitive arm movements, the diaphragm performs the dual roles of breathing and postural stabilization. This is why feedforward control is often mentioned as a key concept when discussing Breathing and core stability.

To predict movement, the nervous system needs to know in advance what changes a motor command will produce in the body. A widely used explanation is the hypothesis that the nervous system contains an internal model. It predicts the outcome based on a copy of the motor command, compares that prediction with the sensory input that actually arrives, and continually corrects prediction errors.

A familiar everyday example of prediction is feeling less ticklish when you tickle yourself than when someone else tickles you. This is because the brain predicts the sensations you generate yourself.

Motor learning can also be explained from this perspective. Improvement with repeated practice is understood to reflect increasingly refined predictions by the nervous system, rather than simply increased muscle strength. This also explains why an unfamiliar movement feels awkward and unsteady at first but becomes smoother with repetition.

• Stairs: You get a sudden jolt when you think you have reached the bottom of the stairs but there is still one step left. This is because your brain has already predicted that your foot will meet the floor.
• Catching an object: When catching an incoming ball, you use its trajectory and speed to prepare your hands and trunk in advance.
• Lifting a heavy load: When you lift a bag without knowing its weight, your body may lurch if it is lighter or heavier than expected.
• When pain is present: Changes in the timing of this anticipatory activation have been reported in people with pain in various body regions. Experiments that deliberately induced pain also found changes in anticipatory trunk muscle responses, leading to the interpretation that these changes may be a response to pain. These findings are sometimes considered alongside the increase in Muscle co-contraction seen in painful conditions.

• Repeat new movements slowly. Refining predictions requires repeated experience of the same movement. Start by practicing accurately at a slow pace, then gradually increase the speed as you become familiar with the movement.
• Include balance practice. Exercises that require the body to anticipate and respond to disturbances, such as standing on one leg or Balance training, can help.
• Visually estimate the weight before lifting something heavy. Giving your body time to prepare can help you lift more steadily.
• Move without holding your breath. Breathing and trunk stability are connected, so moving while breathing comfortably is more natural.

The two approaches complement rather than replace each other. Feedforward control is fast, but predictions may differ from the actual situation. Feedback control responds somewhat more slowly but accurately corrects errors that have occurred. Most movement involves a combination of the two: prediction prepares the body first, and sensory information is then used to correct errors. During the feedback process, Proprioception continually provides the brain with information about the body's position and movement.