In the press release announcing Williams' expanded relationship with the company, she said of Tonal, “It's a different way to train that challenges me and makes me a lot stronger.”

Still, Tonal is on to something, and has a laundry list of all-star investors including Amazon, LeBron James and Serena Williams. It's easy to see why they love it, but the real challenge will be getting the price to a place where it doesn't feel so heavy for the average person to lift.

champ Serena Williams Smart home gym Tonal announced it has teamed up with tennis champ Serena Williams as its latest brand ambassador.

World-Class Athletes Trust Tonal Professional-level training at home, endorsed by the world's strongest athletes.

The main reason why Tonal weight feels heavier than barbell free weights comes down to how the resistance is generated. The digital weight on Tonal is produced by an electromagnetic motor that takes the natural momentum you'd normally experience with free weights out of the equation.

Many online Tonal workout machine reviews suggest that Tonal is every bit as effective for gaining muscle and strength as a commercial or home gym stocked with barbells and dumbbells.

Bottom line: Tonal is better for strength training and heavy lifting focus. Mirror is ideal for motivation and a wide variety of cardio classes.

The star-studded group of investors including Serena Williams, LeBron James, Steph Curry and Mike Tyson had previously spurred existing investors like private equity firm L Catterton along with investment firm Dragoneer Investment Group to take Tonal to the next level.

Williams, one of Tonal's early investors, leads a growing list of over 30 athlete investors and brand ambassadors that includes LeBron James, Stephen Curry, Klay Thompson, Tony Gonzalez, Drew Brees, Maria Sharapova, Michelle Wie, Paul George, Sue Bird, Larry Fitzgerald, and Bobby Wagner.

In the nine months that I've been exercising exclusively on Tonal, I've completed 100 workouts, lifted 1.05 million pounds, and grown 71 percent stronger while gaining 10 pounds of lean muscle. In that same time, I've grown more flexible, and eliminated the chronic pain that's plagued my back for over a decade.

That's why the programs across Tonal's three goal options - Improve Fitness, Get Lean, and Build Muscle - are all process-based and feature engaging workouts with high-energy coaches who'll help you enjoy each step you take in your fitness journey.

This accessory work improves performance in your big lifts by training your body for stable and controlled movement as resistance increases. In Build Muscle programs, you'll lift in a moderately-low rep range (4 to 8 reps) for three to five sets to stimulate hypertrophy, or muscle growth.