Nine Short Workouts (AM+PM) p. Week Yield Extra Strength, Size and Performance Gains Compared to Volume Matched 3-Day Split, All Differences are Non-Significant, Though

15 min in the AM, 15 in the PM = Win? For many of you that may sound laughable, but according to a recent study from the University of Copenhagen it is at least as effective as three "mammoth" workouts-sessions per week.

What kind of trainee are you? Do you hit the gym thrice a week, spend two hours there and crawl out of the gymdoors totally exhausted? Yeah... Well that means you're not the fitness model guy, who trains twice a day for 15-20 minutes only and swears that this is the only way to do it?

After all these questions you're probably asking yourself if the answers you gave in your mind were good or bad for ya? Right? Well, eventually, both forms of training can be equally effective. If we take a closer look at the non-significant study outcomes in a recent paper by scientists from the University of Copenhagen (Kilen. 2015), though body composition and strength may in fact benefit more if you train more frequently - even if the total workout volume is the same.
Workout volume? Yes, that's the number of sets and reps. So, let's say you do three 45-minute training sessions weekly, including 1 strength on Monday, 1 high intensity cardiovascular (HIIT) session on Wednesday, and 1 muscle endurance session on Thursday, then those 3x45 minutes + warm-up exercise are your total workout volume.

In case that's what you're doing on a regular basis, you're training just like those of the 21 study subjects (10 men, 11 women; 25 +/- 3 years) with some previous training experience who were randomly assigned to the "classical" training program in the previously mentioned study by Kilen, et al. (2015). If you're rather the fitness model type, you may recognize your own training in what the other subjects did, i.e. a "micro training" program with a total of nine 15-minute training sessions weekly that were divided equally between strength training, high-intensity cardiovascular training, and muscle endurance training and performed in the AM and PM from Monday to Friday (there was no PM training on Friday, though, and cardio and strength were rotated | see caption of Table 1).

Table 1: Description of the two different training regimen (Kilen. 2015) | *To minimize the potential negative effect of concurrent cardio + strength training, MI performed 2 days of strength (Mon and + Tue) and 1 day of cardio training (Wed) in odd weeks, and 1 day of strength (Mon) and 2 days of cardio training (Tue + Wed) in even weeks.

Unlike the training frequency and rest between workouts, the strength training, HIIT and muscular endurance training sessions, themselves, were identical in both groups:

• Strength training consisted of leg exercises (deadlifts, lunges, step-ups, and 1 leg squats), with 1–2 warm-up sets and 2–3 target sets of 8RM, and upper-body exercises (pullups, dips, weighted push-ups, and 1 arm rows), with 1–2 warm-up sets and 2–3 target sets of 5RM. For progression, the exercises were adjusted using extra loading (sandbags in 1-kg steps) if the subjects were able to accomplish more repetitions than prescribed. If the subjects were not able to perform the number of repetitions prescribed, they performed as many as they could in proper form and finished the set conducting only the eccentric phase of the exercise. 
• High-intensity cardiovascular training (HIIT) consisted of running for 2 and 4 minutes at an average speed of 15.1 and 14.5 km/h, respectively, which elicited ;90% maximal heart rate during exercise. Micro training performed two 4-minute run intervals in the morning with 3 minutes of rest in between and four 2-minute run intervals in the afternoon with 1 minute of rest in between. Classical training performed three 4-minute and six 2-minute run intervals in the same training session with the same rest as MI in between. The training volume was evaluated and the only significant difference was running distance during 2-minute and 4-minute intervals, where MI ran significantly further than CL in each interval. 
• Muscle endurance training consisted of three 5-minute exercise sessions involving 5 different exercises performed continuously for 30 seconds with 30-second rest periods. Micro training conducted 3 sessions; the first was “easy,” the second “hard,” and the third “very hard.” Classical training conducted 9 sessions in the same order, starting over with “easy” on the fourth and seventh sessions. The exercises were weighted lunges (with a 20-kg sandbag); push-ups; shuttle runs; abdominal exercises ([a] regular sit-ups from a supine position with knees bent at 908, fists in contact with the ears and the lumbar arch supported by a folded towel, and [b] diagonal sit-ups from a horizontal supine position, outstretched hand to opposite raised foot, alternating); and back exercises ([a] back extensions on an incline bench and [b] kettlebell swings in a standing position). 

As Kilen et al. point out, "[a]ll training sessions were supervised by scientific staff, and subject attendance" as well as "[h]eart rate [...] during high-intensity cardiovascular training and muscle endurance training for the last 5 weeks of the training intervention" (Kilen. 2015) were recorded.

Figure 1: Relative pre- vs. post changes in all measures performance markers (calculated based on Kilen. 2015).

After the 8-weeks on the respective training regiment, a comparison of the pre- vs. post-training data yielded the following results:

• Increases in shuttle run performance were observed in both group, albeit with a higher significance as far as the pre- vs. post-difference is concerned in the classical training (CL) vs. micro training (MI) group (MI: 1,373 +/- 133 m vs. 1,498 +/- 126 m, p < 0.05; CL: 1,074 6 213 m vs. 1,451 6 202 m, p , 0.001).
• Significant improvements in peak oxygen uptake (3,744 6 +/- 615 mL/min vs. 3,963 +/- 753 mL/min | p < 0.05), maximal voluntary isometric (MVC) force of the knee extensors (646 +/- 135 N vs. 659 +/- 209 N | p < 0.001), MVC of the finger flexors (408 +/- 109 N vs. 441 +/- 131 N, p < 0.05), and the maximal number of lunges performed in 2 minutes (65 +/- 3 vs. 73 +/- 2, p , 0.001), however, were seen only in the micro = high frequency training group.

The question you may be asking yourselves now is: Why does the headline say that there were no significant differences? Well, the lack of statistical significance of the improvements in the classical training group does not suffice for a statistically significant between difference to the micro training group. Statistical significant inter-group differences did not exist either before or after the study. The scientists conclusion that

"similar training adaptations can be obtained with short, frequent exercise sessions or longer, less frequent sessions where the total volume of weekly training performed is the same" (Kilen. 2015)

is thus absolutely correct. The fact that statistical significance for the aforementioned study outcomes was achieved in the micro, yet not in the classical training group does still suggest that the high(er) frequency training regimen may have an adaptive edge... albeit in terms of study outcomes not everyone will deem practically relevant.
References:

• Kilen, Anders, et al. "Adaptations to Short, Frequent Sessions of Endurance and Strength Training Are Similar to Longer, Less Frequent Exercise Sessions When the Total Volume Is the Same." The Journal of Strength & Conditioning Research 29 (2015): S46-S51.