The Latest on Combined Training: 15% Increase in Muscle Size in 24 Weeks in Healthy Moderately Active Young Men Regardless of "Cardio First" or "Weights First" Training

If you think even non-significant differences in "lean gains" are significant, "weights first" is for you.

If you are about to embark on a combined strength + endurance training regimen in 2015, it does not matter if you do "cardio first" or "strength first"... well, almost. If you stick to the traditional exercise order, you may gain a non-significant 2% extra on your vastus lateralis (some people would wrongly say "quads") size.

You've read evidence for both, the superiority of "cardio first" and "strength first" on the SuppVersity before and it may in fact be a matter of personal preference, whether you do your cardio or strength training first.
The latest study from the University of Jyväskylä and the Edith Cowan University actually appears to confirm just that. In their 24-week study Schumann et al. had 34 physically active and healthy young men performed a combined strength and endurance exercise program with 16 subjects performing the endurance and 18 subjects performing the resistance training exercise first, 2-3 times a week for 24 weeks. Or, more precisely, ...

"[d]uring the first 12 wk, the subjects performed according to their corresponding training group two times 1E+1S or two times 1S+1E per week. The frequency was then increased during the second 12 wk so that two combined training sessions were performed in every first and fourth week and three combined training sessions in every second and third week (i.e., two times 1E+1S or two times 1S+1E or three times 1E+1S or three times 1S+1E, respectively)" (Schumann. 2014)

To reflect tapering before testing, both week 12 and week 24 were conducted by maintaining the training frequency but reducing training volume and intensity by reducing the number of sets, lowering the loads during the strength loading, and reducing both the total duration and time spent at high intensity (i.e., above the anaerobic threshold) during endurance cycling.
The intensity of the endurance training was controlled by HR (Polar S410; Polar Electro Oy, Kempele, Finland) associated with subject’s individual aerobic and anaerobic threshold determined during measurements at weeks 0 and 12, respectively. Subjects were instructed to maintain a constant pedaling frequency of approximately 70 rpm during each training session while the magnetic resistance of the ergometer was adjusted to achieve the required exercise intensity.

Concomitant Training: Cardio Before or After Weights? Cardio First Triggers 916% Increase in Growth Hormone. Plus: 7x Higher Testosterone & 3x Higher IGFBP-3 Peaks | read more.

"During weeks 1–7, steady-state cycling of low-to-moderate intensity (below and above the aerobic threshold) was performed, and during the remaining weeks, additional high intensity interval sessions (below and above the anaerobic threshold) were incorporated into the training program. The duration of endurance cycling progressively increased throughout the 12 wk of training from 30 to 50 min. During the second 12-wk period, the major endurance program structure was maintained whereas both training volume and intensity were further increased. The aerobic threshold represented intensities (% HRmax) of 65% T 5% and 67% T 6% in E+S and 68% T 8% and 67% T 6% in S+E at weeks 0 and 12, respectively. The anaerobic threshold represented intensities of 85% T 5% and 86% T 5% in E+S and 82% T 8% and 86% T 5% in S + E at weeks 0 and 12, respectively. The loads used during the strength training were determined by the number of repetitions and execution velocity and progressively increased throughout the two 12-wk periods" (Schumann. 2014)

Exercises for the lower body were bilateral dynamic leg press and bilateral (weeks 1–7 and 13–18) and unilateral (weeks 8–12 and 19–24) dynamic knee extension and flexion. Additional exercises for the upper body included dynamic seated vertical press, lat pulldown, and exercises commonly used to improve trunk stability (crunches, torso rotation, and lower back extension).

• During the first 2 wk, training was performed as a circuit using 2–4 sets of 15–20 repetitions at an intensity of 40%–60% of 1RM. 
• Thereafter, protocols aiming for muscle hypertrophy (2–5 sets of 8–10 repetitions at 80%–85% of 1RM, 1.5- to 2-min interset rest) and maximal strength (2–5 sets of 3–5 repetitions at 85%–95% of 1RM, 3- to 4-min interset rest) and, during the last 2 wk, protocols targeting explosive strength (two sets of 8–10 repetitions at 40% of 1RM with maximal velocity, 3- to 4-min interset rest) were performed. 
• During the second 12-wk period, the major strength program structure was maintained whereas both training volume and frequency were slightly increased to maximize fitness and health outcomes and to avoid a training plateau. 

The overall duration of the strength protocol within each combined training session was 30–50 min, resulting in a total duration of approximately 60–100 min for each combined training session (i.e., E+S and S+E, respectively)

Figure 1: Changes in leg press strength (left), time to exhaustion and aerobic power (right | Schumann. 2014).

Similar to the muscle size, the strength (Figure 1 | left) endurance performance (Figure 1 | right) increased non-significantly more in the "weights first" (S+E) group. In that, it is interesting to observe that the discrepancy between the "endurance first" (E+S) and "weigths first" (S+E) group increase over time. A fact that may suggest that after another four weeks the already measurable difference would have become significant.

Figure 2: Changes in vastus lateralis muscle cross sectional area (CSA, left) and lean mass (right | Schumann. 2014).

This is yet as speculative as the corresponding assumption that the inter-group difference in increase in lean muscle mass and vastus lateralis cross sectional area (CSA) would have achieved statistical significance if the study duration had been longer. On the other hand, that would probably imply that the "strength first group" would have ended up with significantly larger increases in upper body muscle mass.
References:

• Schumann, Moritz, et al. "Fitness and lean mass increases during combined training independent of loading order." Medicine and science in sports and exercise (2014).