>>134746
That's a soleus trainer. You generally want gastrocnemius (the round ball-like head of the calf) training or simultaneous gastrocnemius + soleus training for bodybuilding or general athletics, which should be done with the knee joint extended—not flexed—because the gastrocnemius crosses both the ankle joint and the knee joint and cannot effectively work until lengthened enough.
There is some niche appeal for soleus training for squat-centric sport, but somehow I doubt the strength of the soleus easily becomes a limiting factor in the bottom of even the lowest squats, and it would seem that training of the calves to improve squatting can be done by squatting. Maybe it helps with footing and stability. Full squats cause the hamstrings to bounce off the calves, and I can tell from experience having bulkier calves helps with full squatting. But again that would be a niche appeal primarily for Olympic lifting, and the soleus is involved with extended knee calf raises. I guess the important question is Does the soleus work hard enough with an extended knee? for the few poor souls in Olympic lifting who can't find anything better to do in order to improve their lifts.