So that’s how you can gradually increase force, but how do you go about holding something in place? If an action potential is only able to last that long though, how are you able to maintain a single position for minutes at a time?

When a muscle contracts in response to an action potential, it is called a ‘twitch contraction’. This lasts around 100 milliseconds and includes a short latent period (a roughly 10ms pause prior to the contraction) and a roughly 60ms relaxation period as tension is released.

What’s really going on when you hold something in place, is that your motor units are twitching extremely quickly in order to provide continuous tension. In fact, they can twitch before they have even finished completely relaxing. Meanwhile, asynchronous firing of different muscle fiber throughout the muscle helps to ensure there is no interruption to the signal. This may be why synchronized contractions across all motor units actually don’t yield the most force as you would expect (study, study).

The result of this rapid pulsing is what is known as a tetanic contraction: the sustained contraction. And that’s why if you look closely, you aren’t actually locked in place, but rather twitching and vibrating as your muscles fire rapidly to hold the position.

If you are using a yielding isometric contraction – attempting to hold a moderately heavy weight in place – then as your motor units are fatigued, your body will gradually begin to recruit other varieties of muscle fiber: such as fast twitch fiber. Now the weight begins to shake and wobble, as large, explosive contractions attempt to hold it in position. Eventually those fatigue too, and the weight begins to lower in an eccentric movement as you no longer generate enough force to keep everything in place.

The speed of these contractions is another factor that contributes to overall strength then. This is rate coding – the way in which the central nervous system encodes intensity using a digital system. The more rapidly your nerves fire (discharge), the greater the overall force. This can actually be trained, and resistance training has been shown to increase discharge rates after just four weeks of isometric contractions (study).

Rate coding is also used to convey information regarding intensity across the sensory neurons in the brain. The stronger a stimulus such as light, the more rapidly the neuron in the eye will fire. Rate coding is not unique to muscle then, but is simply the way the nervous system codifies intensity

Putting This Into Practice

So now you know all this, how does it impact on your training?

Well, we can easily see that the effect that something like a plyometric movement will have on the neural component of training will be completely different from the effect of a heavy eccentric.

In order to recruit the largest, fastest motor units – which will lead to the most hypertrophy and increased mind-muscle connection – we need to exert maximum force. Those large motor units have higher excitation thresholds, meaning that they need a big challenge in order to be recruited. This normally means either using explosive movements, or very heavy weights (usually >90% 1RM).

Intermittent Fasting Testosterone

This is also why overcoming isometrics – pushing or pulling against immovable objects – are great for increasing muscle fiber recruitment. You are attempting to produce maximum force, which requires the engagement of the largest motor units and their sustained tetanic contraction.

Another way you can engage those motor units though is by using something like a drop set, or a very lengthy yielding isometric exercise. We’ve seen that by fatiguing the slow twitch fibers, you are this way able to force the body to recruit the larger motor units – even when using lighter weights. But the recruitment pattern here is entirely different, as you are using a smaller percentage of your motor units at any given time in a bid to maintain a more constant amount of force. This is why as you tire, you will struggle to curl a barbell slowly, but may be able to ‘speed’ your way through it by calling on those as-yet-unused fast twitch fibers. It’s like a CNS form of pre-exhaust!

When performing yielding isometrics this all happens in an automated fashion. You simply try to maintain the position, and your body calls on everything it has to help you do that. This is a useful skill in itself – fine control over the amount of power you exert – and it’s one very much worth training with isometrics. Remember too that most yielding isometrics will end with an eccentric contraction as you slowly yell “noooooo”.

https://thibarmy.com/isometrics-underrated-training-tool/

Further, to release local growth factors you need to be able to cause a restriction in blood flow inside a muscle that is contracting long enough to result in a hypoxic state and an accumulation of lactic acid (about 20-40 seconds depending on contraction intensity).

ISOMETRICS TO INCREASE STRENGTH

Isometric exercises have some interesting properties when it comes to building strength.

1. You can recruit up to 10% more muscle fibers during a maximal isometric action than during a maximal concentric or eccentric one. Since fiber recruitment is one of the key neural factors affecting strength, frequent isometric training can program your nervous system to be more efficient at recruiting more fibers. Once that is done, you will become stronger in your regular lifting exercises even without adding muscle mass.

2. The firing rate of the recruited muscle fibers is higher during a maximal isometric action than during a maximal eccentric action and might be also higher than during maximal concentric actions. Again, over a certain length of time this means that by using maximal isometric actions you can train your neuromuscular system to produce a higher firing rate during all types of muscle actions. This is another way of increasing strength production.

3. During isometric exercises, strength is gained mostly at the trained joint angle. There is a carryover of about 15 degrees both sides (before and after the angle being trained) but the further away you are from the trained angle, the lesser the strength gains are. While this can be seen as a downside, it can also be beneficial since it allows you to target a specific portion of the range of motion. If you want to fix a sticking point or emphasize strength in a certain position, for example.

4. A certain form of isometrics can be used to desensitize your body’s protective mechanisms: functional isometrics. Which are a super short range partial lift (about 2 inches of movement) followed by a static hold for about 6-9 seconds. If you pick a strong point in the range of motion you can use 20-50% more than your full lift strength, getting your body used to handling such loads. Overtime it will desensitize your protective mechanisms, allowing you to use more of your strength potential.

The two best forms of isometrics to increase strength are 

A. Overcoming isometrics (pushing or pulling against an immovable resistance, like the safety pins in a power rack): This would be the Testing mode on ReGenesis

B. Functional isometrics Lifting a bench press with maximal weight and holding it. This would be a very short range of motoin (2in) followed by static hold. This would be the Training mode on Regenesis.

With these methods, I recommend sets of 6-9 seconds with maximal effort. Well, for the overcoming isometrics you gradually build up force output over the first 3-5 seconds of the set then push all out for the last 3-5 seconds. Normally, 3 work sets are done for a position during overcoming isometrics. They are a max effort method. And I recommend the Westside approach to max effort: 3-4 max effort lifts in a workout, max. That is the recommended volume if you are using only one position, specifically to strengthen a sticking point. If you are using 2 or 3 positions to strengthen the whole range of motion I recommend 2 sets per position.

An added benefit of overcoming isometrics is that they do not cause any muscle damage. As such they can be done very frequently which will allow you to develop the neural factors involved in strength production at a much faster rate. The lack of damage also makes this form of training interesting for in-season athletes who need to be able to easily recover from their workouts so that they can perform optimally during their games and practices.

For functional isometrics, I would recommend 2 or 3 work sets because you will need a few gradually heavier warm-ups sets to reach your working weights. While these are not max effort lifts, they still fatigue the nervous system slightly. On functional isometrics, I only recommend using one position: a position that allows you to use more weight than what you use for the full lift.

Let me be clear: overcoming isometrics is not effective at stimulating muscle growth. Unless you are using a duration of 30-45 seconds per set, which is very demanding with this form of isometrics.

So how do we use isometrics to stimulate muscle growth?

You have three options.

1. Long-duration yielding isometrics (holding a weight or bodyweight at a certain point). Looks like Overcoming Isometrics to me? 

2. Iso-dynamic methods (including holds during the set). This is holding then moving after.  

2. Loaded stretching (doing a long duration hold in the stretched position) - Hanging from a bar but slightly flexed in shoulders. 

What training parameters should I use to stimulate growth?

Yielding isometrics: 2-3 sets of 45-75 seconds at the position where you can create the most tension in the target muscle. Don’t just hold the weight, flex the muscle as hard as you can.

----- NFPT https://www.nfpt.com/blog/isometrics-immovable-forces-toward-strength-and-growth -----

Push/Pull Balance of Isometrics

Whether pushing or pulling, the intent may differ but the result remains the same. We refer to overcoming isometrics as the act of attempting to shift the position of an immovable object. Yielding isometrics, on the other hand, refers to holding a weight in place (against the pull of gravity) and preventing it from falling to the ground. Finally, functional isometrics employs a static hold at a chosen stop during a lift.

Overcoming isometrics transfer a greater amount of energy to concentric strength and thereby demand more from the neurological pathways. Such moves lend themselves to short, intense efforts. This approach does not result in muscle damage and therefore, cultivates much more muscular strength versus size.

Yielding isometrics will spare the neurological system in favor of transferring eccentric strength to the muscle. As such, these moves can be executed for longer durations and increase can muscle size over time.

Functional isometrics involve using a very short range of motion or partial lift (about 2 inches of movement), immediately followed by a static hold in the end position for 6-9 seconds. By selecting the strongest position of the movement, 20-50% more weight than one’s 1RM for that complete movement can be handled. This enables the neuromuscular system to adapt to handling heavier weights, effectively desensitizing the protective mechanisms that prevent one from reaching his full strength potential.

Additional Science

Influence of physical activity on the regulation of bone density

Whalen RT, Carter DR, Steele CR.

J Biomech. 1988;21(10):825-37

Summary

The results indicate that stress magnitudes (or joint forces) have a greater influence on bone mass than the number of loading cycles. We demonstrate that by carefully considering the magnitudes of imposed skeletal forces and the number of loading cycles, it may be possible to design exercise programs to achieve predictable changes in bone mass.

PMID: 3225269 DOI: 10.1016/0021-9290(88)90015-2

Trabecular bone density and loading history: regulation of connective tissue biology by mechanical energy

Carter DR, Fyhrie DP, Whalen RT.

J Biomech. 1987;20(8):785-94.

Summary

The bone daily loading histories are characterized in terms of stress magnitudes or cyclic strain energy density and the number of loading cycles. Relationships between local bone apparent density and loading history are developed which assume that bone mass is adjusted in response to strength or energy considerations. 

Mechanical loading history and skeletal biology

Carter DR.

J Biomech. 1987;20(11-12):1095-109.

Summary

A comprehensive theory which relates tissue mechanical stresses to many features of skeletal morphogenesis, growth, regeneration, maintenance and degeneration is reviewed. The theory considers the repeated or intermittent mechanical forces which constitute the loading history on the chondro-osseous skeleton. The results of numerous mechanical stress analyses indicate that the local tissue stress history plays a major role in controlling connective tissue biology. The fact that the mechanical stress histories in skeletal tissues are directly related to the force of gravity suggests that the life forms that have evolved on Earth are closely tied to our gravitational field.