"Low: High"  

8 x 2 mins 

Spend the first minute at a lower rate and then step up to a higher rate for the second minute
1: r20-->r24
1: r22-->r26
1: r24-->r28
1: r26-->r30
1: r22-->r26
1: r24-->r28
1: r26-->r30
1: r28-->r32

1 minute rest between rounds