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Creatine, obviously monohydrate and no expensive and often impotent spinoff (Jäger. 2011) is useful for any athlete.

The number of items on the list of health and performance benefits of creatine is about as high as the number of boring articles about "the benefits of creatine" you can find all over the Internet. And even here at the SuppVersity they have been piling up in a way that has me ignore the majority of "creatine supplementation increases strength gains in XY" studies that appear on a monthly, sometimes weekly basis. Against that background I will cut todays creatine post short and get straight to the facts, Giuseppe Potrick Stefani et al. report in their latest paper in (how else could it be) the peer-reviewed Journal of the International Society of Sports Nutrition (Stefani. 2014).
In what turns out to be another rodent study Stefani et al. investigated whether creatine supplementation exerts intra and/or extracellular antioxidant effects and if it plays a synergistic role in the adaptation of antioxidant enzymes associated with resistance training. The actual aim of the study was thus

"to evaluate the effects of monohydrate creatine supplementation associated, or not, with RT on oxidative stress and antioxidant enzymatic activity in the plasma, the heart, the liver and the gastrocnemius of rats." (Stefani. 2014)

And the results were unambiguous. As you can see in Figure 1, the anti-oxidant capacity of plasma, heart and liver of all 40 male Wistar rats which had been divided into four groups, i.e.

• sedentary (SED),
• sedentary + creatine  (SED-Cr), and
• resistance training (RT) and resistance training + creatine (RT-Cr),

increased significantly in response to the provision of creatine (0.3 g/kg/day of creatine for seven days, 0.05 g/kg for the rest of the 8-week study period).

Figure 1: Oxidative stress in heart, liver and muscle after 8 weeks of intervention.Concentrations of MDA and CAT activity. Values are mean ± SD; n = 10 for all groups (Stefani. 2014).

As you can see, both treatments, creatine-only and creatine + resistance training led to significant improvements in heart, liver and muscle antioxidant status - and that, this is important, in the absense of those direct free radical scavenging abilities that turn vitamin C, vitamin E & co into highly questionably agents with potential anti-adaptational effects (learn more).

Works w/ and w/out exercise, but with the latter creatine really excels

Compared to the sedentary animals the rats in the exercise group did yet significantly increased catalase levels (=good, because it catalyzes the decomposition of hydrogen peroxide - the bad stuff - to water and oxygen - the benign stuff) in the heart and - obviously - increased strength gains.

Figure 2: Absolute and relative 1RM strength before and after the intervention (Stefani. 2014).

What is (positively) surprising, at first, is the fact that the latter, i.e. the increases in 1-RM strength in response to creatine supplementation occurred even in the absence of resistance training.

If you look closely, you will yet realize that the relative increase in strength, a much better gauge for lasting real-world strength gains, in the sedentary rodents was ZERO. So that it is very likely that they would disappear with the increased water the rats were holding, as soon as the creatine supplementation is seized.
References:

• Jäger, Ralf, et al. "Analysis of the efficacy, safety, and regulatory status of novel forms of creatine." Amino Acids 40.5 (2011): 1369-1383.
• Stefani, Giuseppe Potrick, et al. "Effects of creatine supplementation associated with resistance training on oxidative stress in different tissues of rats." Journal of the International Society of Sports Nutrition 11.1 (2014): 11.

The results of the study and hand strongly suggest, but dont finally prove that a brief leg-focused HIIT like spinning will not impair your biceps gains even if its done immediately before weight training.

You will probably have read about the problem that doing endurance before or immediately after resistance training will blunt the protein synthetic response to exercise. In fact, I have written about this topic repeatedly. The latest article was published only two weeks ago ("How Much Cardio Messes W/ Your Weight Training Results Depends on Recovery Times: Cardio + Weights in One Session vs. AM + PM Training vs. Doing Each on One Day" | read more).

Now, a recent study from the Swedish School of Sport and Health Sciences shows that the question whether "cardio" in the broadest sense will interfere with strength training gains cannot be answered with a simple yes or no.
In their study, Moberg, et al. (2014)set out to conduct a series of studies to examine the influence of endurance exercise on the acute stimulatory effect of resistance exercise on anabolic processes. To this ends, the effect of endurance exercise on a previously inactive muscle was investigated. The aim was to examine the influence of resistance exercise on mTORC1-signaling and rate of protein synthesis in the triceps brachii muscle with or without preceding intervals of high-intensity cycling.

Eight trained males performed, in a randomized fashion, two sessions of heavy resistance exercise (RE) with the triceps muscles, where one session was preceded by intervals of high-intensity cycling (E+RE), 5 x 4 min at 85% of VO2 peak. Mixed muscle protein fractional synthetic rate (FSR) was measured at rest, prior to exercise, and during a 3 hour recovery period following exercise by continuous infusion of L-[ring-13C6] phenylalanine. Muscle biopsies from the triceps brachii was collected twice at rest separated by three hours, directly after resistance exercise and following 90 and 180 min of recovery. Signalling in the mTORC1-and AMPK-pathway was assessed using western blot technique.
It is important to point out that the same amount of work with regard to load, total number of repetitions and total time under tension was performed in the two trials.

Figure 1: Fractional muscle protein synthesis response in the biceps brachii and corresponding changes in signalling proteins in the HIIT + resistance training (E+RE) and the resistance training only trial (Moberg. 2014).

Unlike in previous studies, where the "cardio" exercise was usually a steady state exercise and the muscle protein synthesis was measured in muscles that were trained during the "cardio" and resistance training session, mostly specifically the leg muscles, the muscle protein fractional synthesis rates (FSR) in the study at hand increased from 0.050 ± 0.006 %/h at rest to 0.078 ± 0.008 and 0.082 ± 0.0016 %/h following E+RE and RE, respectively, with no difference between trials.

The same goes for the signalling protein AMPK and eEF2,  as well as for the alleged protein synthesis gauge mTOR which was increased by 76% and 108% above rest directly after the E+RE and RE, respectively, and remained elevated in both trials during the entire recovery period.
References:

• Lundberg, Tommy R., et al. "Aerobic exercise does not compromise muscle hypertrophy response to short-term resistance training." Journal of Applied Physiology 114.1 (2013): 81-89.
• McCarthy, J., et al. "Combined strength and endurance training: Functional and morphological adaptations to ten weeks of training." No. NHRC-92-26. Naval Health Research Center San Diego CA, 1992.
• Moberg, Marcus, et al. "High-intensity cycling performed prior to resistance exercise does not influence mTORC1-signaling and the rate of muscle protein synthesis in the triceps brachii." 19th annual Congress of the European College of Sport Science, 2-5 July 2014, Amsterdam The Netherlands. 2014.