Sports shops are putting runners at increased risk of injury by giving "unqualified advice" when selling trainers and insoles, according to a knee surgeon.
Amir Qureshi, a consultant knee and limb reconstruction surgeon at University Hospital Southampton, said sports retailers may unwittingly be putting customers at risk by claiming to provide detailed assessments about limb movement - known as gait analysis - via short videos.
This information is then used to advise people about the best footwear for the way they stand, walk or run and whether or not they require standard or custom-moulded foot orthotics such as insoles or support straps.
Mr Qureshi said: "An accurate gait assessment involves attaching markers to various points of the body and asking the patient to walk in a lab with special cameras tracking every movement of each part of the limbs, pelvis, hips, knees, ankles and feet.
"A key part is the use of a specialised plate on the floor which assesses the force going through the body and all of this information is collated and scrutinised by highly trained people with different skills including doctors, podiatrists and physiotherapists.
"However, many shoe shops and sports stores offer investigations increasingly labelled as gait analysis that would not be a correct description of the information gathered.
"The methods used, such as video recordings, are not validated and no report generated for the customer to take to their GP or healthcare professional such as a podiatrist or physiotherapist."
Mr Qureshi said professional gait analysis carried out at specialist centres can take up to four hours and added: "A well-intentioned orthotic can cause harm, either in the foot or ankle or the joints above including the knee, and result in the need for treatment - potentially undoing the hard work put in to get fit."
He called for retailers to label the services they provide correctly and, if they wished to market assessments as "gait analysis", they should advise customers to seek a professional opinion before adopting an orthotic.