Activity trackers are becoming more personal – instead of fitting around the wrist, this grenade-shaped product inserts into the vagina to help strengthen pelvic floor muscles and vibrates to give workout feedback.

San Francisco start-up Minna Life designed the kGoal to help women exercise their pelvic floor muscles correctly.

"kGoal is an innovative product that will be instrumental in pelvic floor strengthening for women," said physical therapist Liz Miracle, who worked on the project.

"It's the first product that can provide tactile feedback, through vibration, of a woman's ability to perform a correct pelvic floor contraction."

Pelvic floor strengthening is recommended to provide muscular stability, help recover from pregnancy, improve bladder control and increase sensation during sex.

The user inserts the silicone device into the vagina and squeezes repeatedly to help build strength around the pelvic region.

"Since it's squishy, it conforms to a wide variety of anatomies, and you can use the vent to let air in or out and further optimise the fit," said Miracle. "We finally have a product meant to fit women of different sizes."

The soft element can measure the squeezing of pelvic floor muscles and includes a motor that provides feedback about technique through vibrations.

"It's like Fitbit for your pelvic floor muscles," a tester is quoted on the product's Kickstarter page, which launched last week.

The pressure of each squeeze, number of repetitions and average squeeze time are measured and stored by the device so the user can track progress.

The on/off button, status lights and charging point for a USB charger are located on an arm, which stays outside the body to hold the product in place whilst in use.

The product wirelessly communicates with an app that stores exercise history, and suggests workouts using algorithms developed with doctors and physical therapists.

Designs for activity-tracking devices are gradually moving on from the first wristbands designed by Nike and Jawbone. Apple CEO John Sculley's tech startup Misfit designed the Shine monitor looks more like luxury jewellery than a sports accessory and now there's even wearable technology for dogs.