A football transfer is never complete without the player being transferred undergoing a medicals, which is conducted by a team of medical personnel from the purchasing club, to ascertain his medical fitness level.
Many football fans are now accustomed to hearing that players underwent medicals before the confirmation of deals but only a paltry number are conversant with the intricacies involved in a medical and the medical law around the process.
Technically speaking, a football medical is a protocol usually observed by football clubs during the process of signing players. During a medical, players are put through mefical tests to ensure that they still possess the requisite fitness required to play the game. When a player has mitigating challenges that puts his readiness to compete in question,he is said to have failed the medicals and the purchasing club may decide to move out of a deal, though this depends on the severity of the challenge and the likelihood of full recovery. In the case where a player has no challenge whatsoever, the transfer goes ahead as planned with the player said to have passed his medicals.
It is however noteworthy that a player doesn’t just pass or fail a medical. It’s just not as simple as that. The medical staff conducting the examinations have a sole responsibility of giving a club details about the general health state of the player about to be signed but the final say rest with the club who analyse the medical reports while taking sporting sentiments into consideration. Hence, a player may technically fail a medical but the transfer is given a go ahead if there is a substantial sporting benefit the club stands to gain from the deal.
This also explains why a player may fail a medical  at one club and goes to another club and passes. A good instance being Charlie Austin whose proposed move to Hull in 2013 broke down after testing, only for him to sign for QPR three weeks later.
In a nutshell, there is basically no discrete rule concerning players’ medicals as FIFA allows clubs to use their discretion in handling it.
But on a general note most clubs follow the process explained below in carrying out medicals for players.
The first process a player undertakes when during medicals is to be put through his pace in a state-of-the-art performance lab.
 Most times a player is requested to carry out regular warm up exercises( using gadgets like bikes )to get ready before the actual examination begins.
Following this,  a Biodex assessment, which is designed to test the strength between muscle groups, is carried out on the player.
The process involves strapping the  player into a chair and asking him to extend his leg by kicking out before pulling it back to the starting position. This process is repeated five times,with more resistance added when pulling back on each occasion.
Biodex results usually record a sort of strength imbalance between the quads and hamstring of most players. This is however not a massive problem and most medical examiners recommend extra work to strengthen the hamstrings when this is observed.
Next up is what is known as the V02 max test, where the  player’s aerobic fitness is appraised. This process is very important as it used to feed back to the buying club what level their prospective signing is at. Are they ready to go straight into first-team action?
Or would this risk injury because they are not up to speed and need time working with the fitness staff instead?
Danny Drinkwater, Chelsea’s midfielder, on signing from Leicester towards the end of the Summer transfer window was certified not fit enough to slot directly into first team action. He had to undergo trainings to build up his fitness levels before being called into first team actions by Antonio Conte–Chelsea’s gaffer.
The above processes carried out in a performance lab,  mark the first phase of medicals and when a player goes through it successfully he is given a greenlight and can then proceed to the second phase.
In the second phase carried out by a  physio, orthopaedic and movement screens are used to carefully evaluate the player’s muscle and from the results judgements are made.
Usually the physio looks at joints individually, paying particular attention to ankles, knees and hips.
The mobility of the player is analysed and after, a report is written down which classifies players into three distinct categories viz: red, amber or green. The red ones are a real concern and are usually related to such issues like fluid on the knee. When a player is classified as such it means such a player is a high risk signing and clubs are advised not to sign such a player and even if they do it is on short term contract without huge financial commitments.
Also at times, especially when the signings are immensely necessary, clubs attach appearance based clauses in such players contract as a guarantee against heavy financial losses that a long term layoff of the player due to injury may cause.
However, if a player successfully scales this process too, his medical is complete and the transfer can proceed.