The interference effect is the phenomenon by which adaptation to concurrent strength training and endurance training is diminished compared to separately training only strength or endurance. This is important for sports like rowing, which requires both great power and great endurance. Rowers must train both strength and endurance, so the challenge of the interference effect in rowing is how to maximize adaptation to, and minimize conflict between, the different forms of training that must necessarily occur concurrently. In this article, we’ll dig into the research on the interference effect in rowing, and discuss practical takeaways for rowers and coaches seeking better training and better performance.
I want to be up front that this gets into a level of detailed training program design that may not be an important factor for you, your rowing program, or the rowers you coach. Master the basics first. Do basic strength training, improve aerobic and anaerobic fitness via multiple means, develop great technique on the water, and of course, make rowing a positive part of your life or the lives of the athletes you coach, and this will yield the greatest results in rowing and beyond. If you have the basics down, if the athletes you coach are sufficiently advanced, and if you have the ability to structure your training program and organize your sessions, research on the interference effect offers us takeaways that might yield small performance improvements that add up in the big picture.
Interference Effect: Why It Happens
Coffey and Hawley (2017) have provided this nice, simplified illustration of the interference effect at right. The pill-shaped icon is the mitochondria, representing aerobic adaptations, and the pink tubular shape is the muscle, representing muscular adaptations. At the most basic explanation, single-mode training sends one stimulus to one receptor area, stimulating one set of physiological responses and concurrent training sends multiple stimuli to multiple receptor areas, stimulating multiple responses which may conflict with each other. The principle of specificity of adaptation suggests that the volume, intensity, frequency, and specific mode of training should be as close to the goal outcome as possible for the best results.
However, we know it to be more complex than that in actuality. If it were not, rowing training programs would be nothing more than max effort 2,000m pieces to train for 2,000m performance. Even if you followed this absurdist training approach, you would still find grey area even within specific modes, volumes, intensities, and frequencies of training. Research suggests that a max effort 2,000m rowing performance is around 80% aerobic and around 20% anaerobic, so there is a mixed stimulus even in the most specific training situation. Very few activities are 100% aerobic or 100% anaerobic, and even if they are, it’s rare that athletes only train in one modality. Most sports involve concurrent training to some extent, which means managing the divergent stimuli from aerobic and anaerobic training.
Read More: Is 2,000-meter Rowing Aerobic or Anaerobic?
The exact mechanism of the interference effect is not fully known. Researchers have identified many molecular responses in skeletal muscle and physiological processes, and it’s most likely that multiple factors interact to generate an interference effect, rather than a single, isolated mechanism. Endurance training typically consists of prolonged training sessions of either continuous or repeated bouts of submaximal muscular contractions. The physiological response to this style of training typically includes increase in mitochondrial mass, capillary density, and the amount of glucose muscles can uptake; metabolic adaptations to prefer fat-based fuel over carbohydrate-based fuel; and shift in muscle fiber type from fast-twitch to slow-twitch. Training for increased strength and power typically consists of more maximal muscular contractions performed for a short duration with external resistance. This type of training typically results in muscular hypertrophy, improved neuromuscular coordination, and little change to fuel utilization or fiber type.
While this may be neat and simple for the few sports that are purely aerobic or purely anaerobic, rowers must perform concurrent training, both within a single session and within a multi-week training program. In a concurrent training situation, the muscle is exposed to divergent stimuli, called upon to perform in different intensities, durations, and modes of contraction. 2,000m rowing is classified as a power-endurance sport because the 5.5-7 minutes of output is just too long to be considered a power sport, but too high-intensity to be considered an endurance sport. This alters the adaptive response, and athletes typically experience diminished gains in endurance, hypertrophy, strength, and power, compared to single-mode training. Below is a slightly more complicated graphic from the National Strength and Conditioning Association’s Developing Endurance (Reuter, 2012), from the “Resistance Training for Endurance Sports” chapter specifically. (Besides a tantalizing mention in the back cover’s list of endurance sports, “rowing” does not even appear in the index, and I suggest you save your $21.95.)
There are additional confounding variables in the interference effect puzzle, because athletic training does not happen in a lab or test tube. There are emotional and psychological differences in how individuals respond to training, as well as molecular ones. For example, residual fatigue from a previous session, or chronic fatigue from repeated efforts over an entire training block, might create “an interference effect” where a molecular interference effect may not exist. Much of the rowing research covered in the next section does not look specifically at gene signaling, hormone response, kinase targets, or other molecular pathways, but compares modes of training, or individual training sessions, and their effects on performance. We are largely left to hypothesize on the how and the why, and to make the best decisions for training structure based on this information.
Furthermore, if strength and hypertrophy gains are diminished in a concurrent training program because the athlete does not eat sufficient calories to replace expenditure during calorie-intensive endurance exercise, this is not a metabolic interference effect, but it may interfere with training efficacy. If the athlete has to wake up early or stay up late to achieve all of the weekly training sessions, and therefore does not get enough sleep to fully recover and gain from all areas of training, this is not a metabolic interference effect, but it may interfere with training efficacy. I started this piece with a caution to master the basics first for this reason.
Interference Effect in Rowing Research
Rowing is a classic power-endurance sport, so this is a rare case when we have a decent amount of literature on the interference effect in rowing specifically. While there is still much we don’t understand about the exact mechanisms, we can learn a lot from the available research and use this to inform our training practices. Note: I summarize these in the “practical takeaways” section for those short on time, or uninterested in the study specifics.
Gee et al. (2011) wanted to know the effect of a single difficult strength training workout on 2km performance at 24 hours and 48 hours later. They first tested eight highly trained male club rowers in the 2km erg, 5-stroke power test, countermovement jump, and static squat jump. Approximately five days later, the participants completed a full-body strength training workout of Olympic snatch and clean, back squat, Romanian deadlift, bench press, bench pull, and weighted situps, for 3-4 sets of 5-8 reps per exercise at 75-85% of one-rep max. The researchers tested performance metrics 24 hours later, then the participants rested for approximately five days, performed the same strength training workout, and then tested performance metrics again 48 hours later. The researchers found no significant differences in 2km performance, with a 6:38.6 average baseline, 6:40.8 24 hours post-workout, and 6:40.0 48 hours post-workout, or heart rate, or blood lactate levels. The participants had significant muscle soreness, and performance decrease in squat jump, countermovement jump, and max stroke power, in both 24 and 48 hours post-workout conditions. This finding supports scheduling endurance workouts, rather than peak power workouts, in the 24-48 hours after strength training. It also suggests that a 2km test is sufficiently aerobic to not degrade following muscle-damaging exercise. It’s important to note that the participants performed a lower volume workout for full-body strength and power, not muscular endurance or hypertrophy on isolation exercises. The researchers suggest that the lack of interference was due in part to the greater fatigue of the type-II muscle fibers in strength training, minimizing heavy type-I fatigue that would have a greater effect on more aerobic rowing performance. I’d be interested in a repeat of this study done with an experimental group doing 3-4 sets of 20-25 reps, and I think we’d see more interference in that condition.
Gee et al. performed a similar study in 2016 with three strength training sessions spaced out over a week, to determine the effect of a typical week of training on 2km rowing performance. The researchers studied 28 male club-level rowers, and had them do the same strength training workouts and measured the same performance outcomes as in the 2011 study. Once again, they found no difference in 2km performance from the baseline condition compared to 24 hours after completing the third of three strength training workouts, despite an increase in muscular soreness and a decrease in performance on the power-related tests. The authors again note the importance of the study with regard to weekly training planning, and suggest using aerobic endurance training in the 24 hours after a strength training session rather than sprint, start, or other more anaerobic power work.
Note: I reviewed this study in the September, 2021 issue of Science of Rowing.
We can also look at the interference effect more broadly, in the multi-week picture of training. Gallagher et al. (2010) wanted to know the effect of different modes of strength training on 2km performance in 18 male college club rowers with an average 2km time of 6:55. For six weeks, one group did high-load, low-rep strength training, one group did low-load, high-rep strength training, and one group did no strength training, alongside two rowing sessions per week of low intensity endurance erging or rowing. The researchers did a pre-test and a post-test 2km erg test, and found no significant difference in improvement between the groups. The percent improvement between groups was not significant, but rowers in the high-load, low-rep strength training group took an average of 15 seconds off of their 2km, rowers in the low-load, high-rep group reduced 2km time by 12 seconds, and rowers in the control group improved by 11 seconds. The researchers suggested that the percent improvement was not significant, but the overall trend for just six weeks, with a very low volume of rowing training, is worth practical consideration. Their findings demonstrate that strength training at least does not hinder 2km rowing performance, and may benefit it, particularly if continued for longer than six weeks.
In “Concurrent Endurance and Strength Training Not to Failure Optimizes Performance Gains” (2010), researchers studied 43 male experienced traditional (fixed seat) rowers to better understand the effects of different resistance training programs on endurance rowing performance. The researchers tested all 43 rowers on a progressive step test on an erg, two low-intensity endurance rowing sessions, 10-stroke peak power, and bench pull max and rep strength. They divided groups into four different training categories: four exercises to failure (4RF), four exercises not to failure (4NRF), two exercises not to failure (2NRF), and a control group (CON) of no strength training. I’ve included the researchers’ table of the program below. The study period was eight weeks long, during which all four groups of athletes continued the same pre-intervention rowing training. The researchers performed the same tests after conclusion of the intervention period.
- 4NRF group improved the most in max strength and power output.
- 4NRF and 2NRF groups improved 10-stroke power and 20-min test more than 4RF.
- No significant difference in steady state rowing performance.
Practical Takeaways for Coaches and Rowers
In the 5 Keys to Strength Training for Rowing, I summarized a research review of 89 studies of power-endurance paddle sports, seeking to better understand training practices and how they attempted to avoid the interference effect. The authors came up with five major points that can be implemented into just about anyone’s training situation.
- Periodize your training
- Strength train 2-3 times per week
- Avoid training muscular hypertrophy and aerobic power close together
- Separate sessions by 8+ hours, or strength train before endurance
- Avoid training to failure
While these key are certainly oversimplified when we consider the entirety of research on the interference effect, I include them and think they are valuable because most rowers and rowing programs could implement these takeaways with minimal inconvenience. There are additional reasons for these five keys that are not specifically related to the interference effect at a physiological level.
For example, a thoughtful periodization plan provides opportunities for athletes to develop different physical skillsets, provides a mental break from continuous, monotonous training blocks, and reduces risk of overtraining from trying to make gains in aerobic endurance, anaerobic power, strength, power, and muscle mass all at the same time.
Strength training 2-3 times per week in some fashion, even if it’s the bare minimum, can improve performance and reduce risk of injury. If you’re training more often than that, you’re increasing risk of interference effect, and also risk of overtraining, and also could probably be better off spending time on other interests, or allowing athletes to do so.
Avoiding training muscular hypertrophy and aerobic power close together is the key most explicitly related to the interference effect. Multiple hard sets of more than 10 reps taps into aerobic power as set duration passes the 45 second mark, and as each repeated set incurs more fatigue and often increases duration, but we all know that sets of 10 on lifting doesn’t simulate the kind of aerobic power you need to row. It’s a lose-lose training situation; similar enough to conflict, but not similar enough to improve rowing performance. The authors of the research review recommend a training volume of approximately 3-5 sets of 4-6 specific and multi-joint exercises, focusing on the 70-85% of approximate 1RM range. This is good for minimizing interference, and it’s also good for improving rowing performance and reducing risk of injury. Use strength training to build general strength and power and reduce risk of injury, then use rowing, erging, and cross-training to develop technique, aerobic endurance, and specific anaerobic power.
Separating sessions or strength training before endurance training, and avoiding training to failure, are good for reducing physiological interference as well as risk of injury. A muscular or skeletal injury from strength training under fatigue from erging or rowing, or from pushing past the point of technical control and reaching all-out muscular failure, will derail training adaptations much more than interference from physiological signaling.
Here are few more specific takeaways condensed from the earlier research reviews:
- Schedule aerobic endurance training in the 24 hours following strength training, rather than peak power or more anaerobic rowing training. Use low volume strength training to minimize potential interference. A 2km race is sufficiently aerobic to avoid negative interference from 24-hour prior strength training (Gee et al., 2011).
- Three low-volume strength training sessions per week does not negatively affect 2km performance (Gee et al., 2016).
- Muscle soreness does not affect 2km performance (Gee et al., 2011 & 2016).
- Six weeks of no strength training, high-load/low-rep strength training, and low-load/high-rep strength training did not result in statistically significant differences in 2km performance. However, the high-load group improved the most (15 seconds), followed by the low-load group (12 seconds), then the no strength training group (11 seconds), so a trend may exist if training is continued for more than six weeks (Gallagher et al., 2010).
- While research on general strength training often shows a positive effect from performing higher volume work to muscular failure, rowing performance may benefit more from a moderate approach of lower volume, moderate intensity strength training (Izquierdo-Gabbaren et al., 2010).
My single biggest takeaway from research on the interference effect is the value of “keeping the goal the goal.” The goal of strength training for rowing is to improve rowing performance and reduce risk of injury, not to have the biggest biceps, the greatest 1RM squat or bench press, the highest vertical jump, or the lowest bodyfat percentage. Given this goal, it doesn’t matter if adaptation to strength training is totally maximized, just like it doesn’t matter to the powerlifter, Olympic weightlifter, or bodybuilder if the response to endurance training is totally maximized. Rowers can still improve strength, power, and muscle mass to benefit rowing performance with specific strength training practices for rowing that take into account the technical and physical demands of rowing, not strength training programs from other sports mashed together with rowing training. We need thoughtfully constructed rowing training programs that provide room for development of multiple athletic qualities over a training year, not so much rowing year-round that athletes cannot develop anything other than survival skills. If we can keep those two dual goals in mind, then the rest is details and implementation.