For fitness buffs who dream of training with a professional athlete, apps could be the next best thing.
The apps, dubbed personal trainers in your pocket, are accessible from anywhere on a smartphone or tablet and feature some of the world's elite athletes.
Professionals in sports ranging from tennis and running to gymnastics and professional football go through their workouts in the apps, demonstrating and explaining the movements.
Serena Williams, ranked No. 1 by the World Tennis Association, leads a series of workouts on the Nike Training Club app. She narrates instructions to a 15-minute workout, which focuses on core strength, and demonstrates various exercises in short videos.
"It's authentic to the core power- strengthening moves Serena needs as a pro tennis athlete," said Ms Jill Zanger, a communication manager at Nike.
Williams' workouts will be followed by workouts from fellow United States athletes like professional surfer Lakey Peterson, Olympic gold-medallist gymnast Gabby Douglas and sprinter Carmelita Jeter, the world-record holder of the 100m sprint.
Another app, by New Hampshire-based Go Pro Workouts, shows exercise routines of professionals, but in eight-week programmes. The app shows the training programmes used by athletes like Denver Broncos football linebacker Von Miller, Kansas City Chiefs' Jamaal Charles and Seattle Seahawks' Earl Thomas.
Mr Joseph Lamoureux, co-founder and chief executive of Go Pro Workouts, said: "People look up to the professional athletes, the ones who have actually done the work, made it to the highest level, and are now succeeding. They're the epitome of what every athlete should be."
Each day of the programme shows 25 exercises demonstrated by the athletes in 20- to 30-second video clips, which become progressively more challenging.
Earlier this month, fitness brand Reebok launched Reebok Fitness, an app that provides instructional videos featuring fitness experts for activities such as running, dance and yoga. The app for iPhones and Android phones is available in the US.
While the ability to access training by professional athletes through apps is convenient, Mr Kennedy Lodato of Canadian Fitness Professionals does not see apps replacing personal trainers.
He said that apps will get more people engaged in fitness activities. However, "at some point, you're going to hit a wall and not get the answer you want - those little specific things that are individual to everyone".
He said that apps do not address specific needs, such as those relating to height, weight, sex or goals that only a personal trainer can, and do not offer the same level of motivation or physical adjustments.
The Nike Training Club app, which is free, is available for Android phones. Nike said it plans to re-release the iPhone app in the next few weeks.