Yonex Muscle Power 40 Green Dot Tennis Balls (60 Balls)The Yonex Muscle Power 40 Green Dot Tennis Balls (60 Balls) are stage 1 training balls designed to help young players develop fundamental skills with confidence. These balls travel approximately 25 % slower than standard ITF approved tennis balls, making rallies easier to sustain and giving learners more time to react and position themselves on court. Part of Yonexs progressive junior training system, Muscle Power 40 balls are ideal for school