(Super-)slow vs. Traditional Weight Lifting Affects Satellite (=Muscle Progenitor) Cells, Fiber & Domain Sizes Differently

In know one could argue that you may see different results for other body parts than legs which were the only muscle group trained in the study at hand.

While the overall majority of currently available studies appears to suggest that traditional resistance training regimen with cadences of 1-2s on the concentric and eccentric parts of the exercises are superior to their (super-)slow counterparts, the debate is far from being settled.

Accordingly, the results of a recent study from the Ohio University are highly interesting. The findings Jennifer R. Herman-Montemayor, Robert S. Hikida and Robert S. Staron present in their latest paper could after all explain why "classic" resistance training resistance outperformed their (super-)slow cousins in the majority of studies.
As the authors rightly point out, previous investigations have paid little attention to the role of satellite cells during exercise-induced adaptations in human muscle (Kadi. 2005). In untrained muscle, most satellite cells are in the non-proliferative, quiescent state in. As Herman-Montemayer point out, they can however be activated by various stimuli linked to exercise:

"Several studies have shown an increase in the satellite cell population following resistance training in young and old, male and female subjects. Although additional research is needed to fully understand the various factors which may influence satellite cell response, it is clear that exercise provides a sufficient stimulus for satellite cell activation, proliferation, and possibly incorporation" (Herman.Montemayer. 2015).

The process of incorporating satellite cells into the muscle is, as I've pointed out in previous articles, highly relevant not just for the repair, but also for the growth of skeletal muscle, because the subsequent increase in the number of myonuclei and thus reduced domain sizes will allow for a reduction of the exercise induced increases in myostatin which is a key factor that limits the influx of protein into the muscle and will thus allow for increased muscle growth.
In their latest study, the scientists from the Universities of Rocky Vista and Ohio were thus interested in the training-specific effects of two standardized 5-week resistance training programs. To this ends, thirty-four, untrained females were divided into: slow speed (SS), traditional strength (TS), traditional muscular endurance (TE) groups as well as a non-training control (C) group. All groups performed the same three sets each of leg presses, squats, and knee extensions twice a week the first week and 3 days/week for the following 5 weeks. In that, ...

• the SS group performed 6-10 repetitions maximum (6-10RM) for each set with 10 s concentric (con) and 4 s eccentric (ecc) contractions for each repetition, while 
• the TS and TE group performed 6-10RM and 20-30RM, respectively, at “normal” speed (1-2 s/con and ecc contractions). 

In contrast to previously untrained men and women who had been randomized to the TE and SS who trained at the same intensity (40-60% 1RM), the subjects in the TS group trained at 80-85% 1RM.
To determine the individual effects of the workouts, the scientists performed pre- and post-training muscle biopsies that were then analyzed for fiber cross-sectional area (CSA), fiber type, SC content, myonuclear number, and MND.

Figure 1: Changes in muscle fiber size, domain size and myonuclear number (Herman.Montemayer. 2015).

As the results of the biopsies revealed, the satellite cell (SC) content of type I, IIA, IIAX, and IIX fibers significantly increased in the traditional strength training group (TS). Doing the same exercise at a significantly reduced pace, on the other hand, lead to increases in satellite cells only in the "intermediat" type IIAX and IIX fibers.

Compared to the strength endurance group, which did not see any increases in satellite cell activity, that's still a plus, though. Of greater practical relevance than the increase in satellite cell number is yet the myonuclear number which did not change in any group.
References:

• Kadi, Fawzi, et al. "The behaviour of satellite cells in response to exercise: what have we learned from human studies?." Pflügers Archiv 451.2 (2005): 319-327.
• O'Connor, Roddy S., et al. "Last Word on Point: Counterpoint: Satellite cell addition is/is not obligatory for skeletal muscle hypertrophy." Journal of Applied Physiology 103.3 (2007): 1107-1107.