Arthrokinetic reflex is defined by the way in which a joint movement and/or position can reflexively cause muscle activation or inhibition.
In any given movement an X amount of mobility is required. Some more than others, some close to nothing, and in some cases the requirements are closer to the supernatural. Where does weightlifting lay in this spectrum? In a scale between chilling in bed eating nachos and watching Netflix to getting yourself in a headlock using your own legs then weightlifting is probably somewhere in the middle. Like, being able to touch your forehead with your left foot as your itching the other foot with the left hand. What I'm trying to say is basically that the requirements are not extreme, yet, probably a tad higher than what most people deem to be average.
As a newbie to the sport of weightlifting, mobility is often seen as a challenge. Some struggle with the overhead position (especially if you're the "Chest Monday" type of person). While we're on the topic of overhead mobility, it is one of the most "unnatural" mobility requirement of the lot, as far as weightlifting is concerned. Humans are natural squatters. Our most basic and fundamental movement pattern is that of a squat. Look at any toddler and their amazing squat, we're all naturals. Overhead mobility, however, is quite something else. Yes, we do possess a good amount of mobility overhead from a young age, but since we never, like absolutely never ever, are required to do anything for our basic survival in today's society that is overhead we, and I mean everyone, will eventually lose close to all overhead mobility because we "don't need it". The same applies to the squat. We invented chairs and now we don't need to sit on the ground anymore when we're feasting, doing a number two, or socialising. And years later we struggle to flex our knees past 90 degrees... We might be the most intelligent species living on the face of the earth, yet, there is still quite a lot we still need to learn. Like, basic movement patterns. And for that mobility is required.
As mentioned above, humans are quite the adapting machine. We can adapt to almost anything, just give us enough time. I'm not talking about being able to camouflage ourself so we can fade into any background so we can hide from our spouse. That will take a lot longer, but with the current demand of this feature it will probably be available in the year 4387. Think of mobility as you do with strength training, just try and follow me for a second. For someone to get a stronger bicep they have to push the bicep to a point where the body goes: "Dude, we keep using all of this effort into contracting the bicep, like every day we have to contract it to our maximum capacity, but it never seems to get any easier. How about we add some more muscle fibres, increase the overall neurological output for contracting the bicep and then we don't have to do as much next time" Lo and behold, when you then get stronger and realise that you're able to either do more reps, a heavier weight, and can finally impress your crush, your bicep still has to work just as hard as it did before it adapted the first time around. This cycle then goes on and on, until you get 21-inch arms and actually realise that big biceps aren't life after all... What a shame. The same holds true for mobility. You stretch your muscles/tendon and your muscle spindles go all anxious and fire a signal to the brain telling it to stop with all the nonsense stretching and tells it to contract the muscle so you don't get injured. This signal is what is called the stretch reflex, or reflex arc, by the way. You know, that funky movement you leg does when your doctor tortures your patellar tendon with a hammer. That's your muscle spindles doing what they do best, restricting unnecessary stretching to a tendon. Just like a muscle eventually gives up and adds more "strength" the muscle spindles will also give in and allow themselves to be stretched further. When this happens over and over, day after day, over a long period the muscle spindles eventually just go: "F*ck it, we'll just relax a bit more and allow you to stretch a bit further this time". And from that moment on you're more mobile. This then, just like with the muscle, happens over and over again until you have the mobility that is required. Just to clarify, the muscle spindles have a much bigger role in stretches that happen within a short time. Like, when you receive a clean and have to bounce out of the bottom position to be able to stand up, as oppose to a 30 second static stretch. It works in the same way for both scenarios but it's just way more effective in the faster stretch scenario... Moving on.
The best way to acquire mobility
There are loads of different ways to get more mobile. Static stretching, dynamic stretching, loaded stretch over time, PNF stretching, MET's, massage, the list goes on. Let me just say, ALL of them work. Just like a plumber and a lawyer both earn money from their line of work, yet, one of them will earn more money than the other. One method of getting more mobile might be better than the others and should therefore be used. But that one method that is really good for one person might not be as sufficient for another person. So saying that static stretching is the best way to achieve good mobility is wrong. It might be for some people, but certainly not for everyone. I will, however, tell you what is most likely to give you the best results in regards to mobility. First; Add stretching into your routine, any type of stretching is good. As long as you to something. Second; sit in the position where you want to be more mobile in, and keep sitting. Then add some weight to that position and sit some more. This is a well known scientific principle called the SAID principle. It stands for "Specific Adaptation Imposed Demands". It basically states that if you want to be better at something, do more of that something. In other words, if you want to get a better bottom squat position, sit in a bottom squat position and you'll get better. This is true, however, in some circumstances, this might take a while. And by today's standards, when waiting longer than 1 minute at the local ice cream stand for a double cone ice cream can lead to bloody homicide, we don't want it to take a long time. So to haste the mobility gain we can add weight to the position we're trying to improve. For the squat it's easy, just load the bar with some fairly heavy weights, make it look like your taking a dump on the sideway and wait as your pray to the mobility gods.
Long legs? Pray harder!
Weightlifting is a sport in which anatomical structures play a massive role in success. That is not to say that you can't succeed if your anatomical proportions aren't the same as the majority of the Chinese, but more that of the Vladimir character in the movie Hotel Transylvania, 30% upper body and 70% legs. The mobility requirements of a lifter in which the limbs are fairly long in relation to the torso are just higher, unfortunately. These lifters either need impressive dorsiflexion or the ability to do what Ozzy Man Reviews calls a double spready. Youtube it, you won't regret it. By that, I mean good overall mobility.
Loads of good lifters, especially in the states, come from a background in gymnastics. And with gymnastics being a sport which has a way higher mobility requirement than weightlifting, at least on the women's side, they are often "too mobile". I say "too mobile" not because it's a bad thing necessarily, but due to the relationship between mobility and stability. Whenever you achieve greater mobility, the mobility that you've just acquired is just that, mobile. It's not stable, because it's a position you've never been in before. New mobility requires new stability. So when you have that gymnast that has ridiculous mobility/flexibility that they're only used to with gymnastic specific movements/positions, they struggle with weightlifting, although they have the mobility required. The same goes for the lifter with the long legs.
Weightlifting specific mobility
So what needs to be mobile, you ask. First of all, and there is no discussion about it, squat mobility. But only saying that you need to be mobile in the squat is a bit unfair, what does that even mean. To acquire a good squat the first object most face is the lack of dorsiflexion. Contrary to the popular belief, "ThE ToEs ShOuLd NeVeR tRaVeL PaSt ThE tOeS" To get a well-balanced squat, one in which you can hold a loaded barbell overhead simultaneously, you need to be upright. Otherwise, you'll dislocate your glenohumeral joint and you'll likely die. Dorsiflexion is immensely important for an upright position. This is one of the major reasons why the weightlifting shoe is a MUST in weightlifting. It provides "fake" dorsiflexion, among other benefits.
Along with dorsiflexion comes hip mobility, specifically hip flexion, abduction, and internal rotation. With lower degrees of dorsiflexion the higher the demands of hip mobility, and vice versa. Having a good amount of both gives great wiggle room in case there is a lack of balance.
Thirdly there is shoulder mobility. This is not to be defined as only the glenohumeral joint, but the whole shoulder. By that, in essence, I mean the glenohumeral joint along with the scapulothoracic joint, sternoclavicular, and the acromioclavicular joint. To be able to support a bar overhead where the arms are on the true vertical axis the scapulae have to upwardly rotate significantly. This is not possible without these joints working cooperatively. The sternoclavicular (where the sternum meets the clavicle) and the acromioclavicular (where the clavicle meets the scapula) will be mobilised indirectly as the scapula is forced in upward rotation, so no need to worry on those.
finally, the last major consideration should be thoracic mobility. The vast majority of people these days lack thoracic extension due to the extensive sitting on chairs looking into laptops, and poor sitting positions throughout the day. Take me for an example; As I'm writing this I'm slouched lazy position with my chin digging into my chest because I've got too many pillows behind "supporting" my neck. This is the new normal and is massively affecting our ability to move properly. Thoracic extension is of great importance in all the movements in weightlifting. Take the starting position for instance; without proper thoracic extension, you'll actually have to move the bar a greater distance to achieve full triple extension. In a "poor" spinal position the nervous system is unable to provide 100% neural output, making you "weaker". (This is called the arthrokinetic reflex if anyone should be interested).
The major mobility requirements for weightlifting are the ankle (dorsiflexion), hip (flexion, abduction, and internal rotation), shoulder (glenohumeral flexion/abduction, scapular upward rotation), and thoracic extension. Good luck with the mobility work, and have a great day.