Analyzing the neuromuscular controls of grip and application to lifting weights


 

An example of a precision grip. Credit to Stocksnap.io

An example of a precision grip. Credit to Stocksnap.io

Fluid movement and strength development depends heavily upon the brain. Pretty much everything in life does. In this article we are going to explore a few aspects of grip force and load force with respect to their neurological bases. Specifically we are going to look at and summarize a series of research articles related to the neuromuscular control bases for grip and lifting force. The information presented in this article should be taken with a grain of salt. Many of the studies I reference here were looking at lifting a relatively light object, between 100 g and 1 kg, with a precision grip, using only the index and the thumb. This is the most comprehensive information I have found relating lift, grip, and neuromuscular controls. I hope to see more direct research performed on neuromuscular controls and weight lifting in the future.

The direct correlation between grip force and load force

In a 1984 study by Westling and R.S. Johansson researchers measured the grip force applied by subjects lifting an object with varying weights and 3 different grip surfaces. As you’d imagine the grip force increased parallel to the load force and the grip force was significantly higher on slippery surfaces than it was on rough surfaces. The seemingly direct relation between grip force and load force is a reaction to the increased force necessary to keep the object from slipping, the “slip force.” There were 2 interesting this to note from this:

  1. The higher the slip force the higher the difference between the slip force and grip force.
  2. Slippery surfaces, suede & silk, required significantly more grip force than the rough surface, sandpaper. As we’ll see later on this requires much more effort from the nerve pathway that controls grip.

Essentially, when we’re applying a grip to pick something up we adjust our grip based on the objects weight and the texture of the object (load and friction).

Our previous lifts determine the efforts of our current lift

In a follow up 1988 study researchers found that the experience of a previous lift created a type of “sensorimotor memory.” Subjects lifted an object whose weight was altered by experimenters in a pseudorandom manner or left constant. Researchers were looking to compare the effects of lifting a heavy object after a light one, vice-versa, and lifting the same weight repeatedly.

When the weight was kept constant (control) the applied grip force was close between trials (no significant difference). Essentially subjects knew the correct force to apply and successfully did so with minimal variation.  Additionally E.M.G. imaging showed an drastic increase in activity during the time in which grip was being applied with a small reduction once the grip force stabilized (Fig 1A, third row).  This suggests that grip force, and the rate at which it is applied, is correlated with the output of electrical activity in the muscle.

The interesting findings of the study came when the weight was switched. When subjects previously lifted a heavier weight the grip force was greatly overshot in proportion to the difference between the two weights. This was exactly what you think it is: recall a time you prepared to lift a heavy box only to find it was empty. On the other hand when subjects previously lifted a lighter weight they gradually increased their applied grip force and load force until the object moved.

In the case of previously lifting a heavier weight the E.M.G. imaging showed a much longer period of increased activity before reducing (Fig 2A, third row). This is probably due to subjects preparing to grip the object and having to readjust the grip once they realize they have applied too much force.

Internal Models of Grip and Load Forces

The previously mentioned study suggests the existence of an internal model that acts to predict the magnitude and timing of forces necessary to control an object. As we have seen our previous experiences influence the rate we apply grip and load forces in preparation of lifting an object. This isn’t our only model for predicting the force necessary to lift an object. We can also use visual cues to scale our applied forces.

In a 2005 experiment by Chouinard et al researchers found that subjects could accommodate for unexpected weight changes if a visual cue, in this case an arbitrary color flash, indicated the magnitude of weight to be lifted. Suggesting our internal model for visual cues can take priority over the tendency to scale applied forces based on the previous lift.

Researchers were effectively able to interrupt this process by directing transcranial magnetic stimulation (TMS) over the dorsal premotor cortex. Similarly the tendency to begin a lift based on previous experience was also interrupted by applying TMS to the primary motor cortex. Suggesting the locations of these two internal models are very close but not the same.

Now that we have reason to believe that these models take place in distinct parts of the brain we have reason to believe that they must have a way to interact. In their 2010 research paper Loh et al sought to solve this problem. Researchers found that our previous experience acts as a preset approach to lifting an object and we can switch based on visual cues after 50-150 ms.

What the hell does this have to do with exercise and training?

Let’s review:

  • When we lift something heavy or slippery we use a stronger grip.
  • The force of our grip and the rate at which we apply it can be considered good indicators of neuromuscular exertion (at least with regards to the hands).
  • We base our approach to lifting an object on our most recent experience.
  • We are capable of adjusting our approach based on visual cues, however the model based on our previous experience appears to be more innate.

Many strength coaches are familiar with the “Law of Irradiation.” Essentially this law states as the tension of a muscle increases it will recruit additional tension from nearby muscles. This law makes a connection I’ve been hinting at throughout this article: grip exertion is a good indicator of neuromuscular exertion. Using this connection in combination with the information presented in this article we can derive a handful of training advantages.

Incremental increases in weight can effectively increase the total time under tension.

Incremental increases in weight can effectively increase the total time under tension.

Firstly, suppose you want to maximize the amount of volume within a given workout. In this case we wouldn’t want to be constantly performing at the edge of our nervous systems capabilities. We can do this by lowering the demands we make on grip with regards to weight and texture; using less than maximal weight (near 70%), gradually increasing the weight prior to the working set, and putting chalk on the barbell. This one is obvious but nonetheless critical. We can extend the length of our lifting sessions by reducing the strain on our grip.

Secondly, if we are trying to perform near-maximal sets we can actually set ourselves up to overshoot our grip and load force application, effectively making the heavy lift a little easier. This is done by performing a preceding set that is heavier than the working set, what is sometimes called a “proprioceptive set.” Suppose you want to perform a set of 6 deadlifts at 315 lbs, then you perform a single at 345 lbs beforehand. The caveat is that you’d have to do this before each set and this could eat up more time than is worthwhile.

Lastly, we can use large incremental changes in weight to increase the amount of total time under tension per lift. For instance, suppose you go from squatting 185 to 225 to 275. The lower preset approach to the lift will not be sufficient to lift the heavier weight and the rate of force will gradually increase to accommodate the new weight. Essentially you will apply the same total force to move the weight but the rate of force will increase over a greater period of time.


Please keep in mind that I am not a neurologist and none of the research cited here pertains directly to resistance training. I’m interested to see emerging research come out but in the mean time the research I have shown here, when applied to resistance training, agrees heavily with many of the lifting and coaching principles laid out by the likes of Pavel Tsatsouline, Charles Poliquin, Jim Wendler, etc…