Training frequency is a hot topic, and it’s a subtle one. If we look at studies on untrained individuals, the literature is quite clear that there are minimal benefits to training a muscle more than even just once a week. However, in trained individuals, the literature is more divided with multiple studies showing no benefits but also multiple studies showing significant benefits to training more often. A new study by Correa et al. helps us identify when a higher training frequency is likely to be beneficial.
The study design was simple. 2 Groups of trained men performed the exact same training program and consumed the same diets (habitual diets; no significant difference in any macronutrient intake between groups), with one exception: one group trained twice daily. Concretely, the twice-daily group did 4 sets of each exercise in the AM and PM, whereas the control group did all 8 sets for each exercise in one workout. The table below summarizes the training programs of the two-a-days (2S) and the control (1S) groups.
After 8 weeks on the programs, muscle growth was similar between groups but the twice-daily group achieved significantly greater strength gains on the squat, along with a trend for overall greater strength development. As you can see in the table below, the twice-daily group gained about double the strength on their 1RM squat in absolute (kg) as well as relative (%) terms. They also gained over 50% more strength on their 1RM bench press, and about 25% more reps on the strength-endurance tests for both exercises, but these differences did not reach statistical significance.
It’s worth noting that the study was underpowered to detect small differences between groups. Their a priori analysis indicated they needed a huge effect size of 0.75 to have 80% power, which essentially means an 80% chance of detecting an effect in the data if there is one. This could explain why the much smaller effect size differences of muscle growth, which only ranged from 0.1 to 0.4, didn’t reveal any trend.
Alternatively, it’s possible that the greater strength gains were purely due to greater neural adaptations, not increased muscle size. A higher training frequency may improve motor learning by virtue of greater repetition. Previous research has also found that twice-daily training of a muscle may increase muscle activity. To quote the researchers: ” Häkkinen and Kallinen investigated neuromuscular adaptations in 10 female athletes during an intensive strength-training period for 3 weeks and during a separate second 3-week training period, when the same total training load was distributed into 2 daily sessions. The authors concluded that isometric strength […] muscle cross-sectional area of the quadriceps […] and muscle activation increased during the twice-daily training period, but not in the first training period with 1 daily session.
Likewise, Häkkinen and Pakarinen reported that male strength athletes performing 2 weeks of twice-daily training experienced a significant increase in isometric knee-extension strength compared with the same daily training load performed in once-daily sessions.
Moreover, Hartman et al. compared the physiological responses in 10 nationally competitive male weightlifters during 3 weeks of once- versus twice-daily RT sessions with similar training volumes. There were no significant group differences for any of the dependent variables, but the twice-daily training group had a greater percentage change in isometric knee-extension strength (+5.1% vs +3.2%), clean and jerk performance (+1.9% vs +0.3%), and neuromuscular activation (+20.3% vs +9.1%).”
Over the long run, increased muscle activity should help you build muscle.
Any of these benefits should result in a higher training volume in the gym, and we can thus say that any benefits of a higher training frequency are much more likely when the greater training frequency results in a greater training volume. Indeed, in this study the total load lifted (sets x reps x weights) ended up significantly higher for the squat but not the bench press, explaining why the squat benefitted more than the bench. In this sense, your training frequency is in large part simply an extension of your rest interval: you can rest minutes or days between sets, but it’s the same principle. There’s now strong evidence that longer rest intervals enhance strength development and muscle growth when they lead to doing more reps but not when they don’t (e.g. the control group performs an extra set to equate the total reps). I predict future meta-analytic work will show the same for training frequency, although there might also be a minimum training frequency required to keep muscle protein synthesis elevated across the whole week. For muscle growth in particular, you need to increase your training volume to increase the amount of tension on your muscles, which is the primary stimulus for muscle growth.
This study provides another piece of the puzzle of when higher training frequencies are likely to be beneficial. To summarize, these are the indications for when you may benefit from training a muscle or exercise more often.
- You’re well-trained.
- You’re training with a moderate to high training volume.
- The increase in training frequency increases your total training tonnage (sets x reps x weight).
You most likely don’t need to actually train a muscle or exercise twice a day, unless you’re a high-level competitor really trying to 100% maximize your gains. This study’s design was overall similar to that of the (in)famous Norwegian Frequency Project, in which Norway’s national powerlifting team gained more strength and muscle size when splitting their 3 weekly full-body workouts into weekly 6 full-body workouts. In fact, unless you’re already training every day, it’s likely better not to train twice a day but instead to spread your volume more across the week. That way, you have more recovery in between sets and you’re thus more likely to increase muscle activity, total training tonnage and thereby your gains.
Twice-daily sessions result in a greater muscle strength and a similar muscle hypertrophy compared to once-daily session in resistance-trained men. Correa et al. J Sports Med Phys Fitness. 2021 Feb 26. doi: 10.23736/S0022-4707.21.12118-8. Online ahead of print.
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