When and How Are Flexible Templates Actually Useful?

From Volume 5, Issue 6 of MASS

When and How Are Flexible Templates Actually Useful

by Michael C. Zourdos, Ph.D., CSCS

Flexible programming – choosing which training session you’ll do based on how you feel that day – is a logical strategy. However, a new study adds to the surprisingly null findings on the topic. This article discusses specific situations in which a flexible template may have merit and how to implement flexibility.

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Study Reviewed: Effects of A Flexible Workout System on Performance Gains in Collegiate Athletes. Walts et al. (2021)

Key Points

Collegiate lacrosse players performed eight weeks of either flexible training or fixed order training. Athletes tested hex bar deadlift and bench press strength before and after training, along with vertical jump and agility performance.
The flexible training group could choose between four options for their daily workout, while the fixed order group had a predetermined weekly training order. Each group trained three times per week.
The findings showed very similar rates of progress in all outcome measures between groups. This article discusses the circumstances in which flexible templates may enhance strength performance, and whether or not the autonomy that flexible templates offer is a positive for lifters.

Autoregulating training session load and volume helps match the daily training stimulus to your daily readiness. However, quantitative assessments of daily readiness can sometimes be hard to implement. Not everyone has access to a device they can use to assess bar velocity, and not everyone can accurately assess repetitions in reserve (RIR)-based rating of perceived exertion (RPE) values. Therefore, it may be worth having a mechanism to choose the overall workout structure based on your subjective readiness when you enter the gym instead of having a specific day of the week pre-planned. Enter flexible training templates, which I’ve covered before (one, two). A basic example of a flexible template is having six heavy, six moderate, and six light training sessions within a month. In this case, you could perform each training session whenever you feel ready for it (i.e., if poor sleep, choose a light session) rather than in a set order (i.e., moderate, light, and heavy on M, W, F). One previous study has shown flexible training to enhance strength (2), while another showed a slight improvement in training adherence (3 – MASS Review). However, there isn’t much more data on the topic. The reviewed study from Walts et al (1) had collegiate athletes perform either flexible non-linear periodization (flexible group), or fixed-order non-linear periodization (fixed group) for two four-week training blocks (eight weeks total). Before each session, the flexible group would indicate their state of readiness (green, yellow, or red) and then choose a corresponding workout. A green response meant the subject could choose either a high-volume or high-intensity session, while researchers matched a yellow response with a low volume or low-intensity session. A readiness response of red meant the athlete skipped that day’s training session. The fixed order group performed training sessions in a predetermined order but could still select red and skip a session. Researchers equated the specific number of each training session (green or yellow) between groups. Findings showed that all outcome measures (bench press and hex bar deadlift 1RM, vertical jump, sprinting speed, and agility) improved, but without significant differences between groups (p > 0.05). Despite the lack of significant between-group differences, I don’t believe we should discard flexible templates entirely. A positive spin is that a lifter may receive the same training benefit while avoiding the typical rigidity of a fixed order program. However, I think the more salient argument is that the current study didn’t provide a framework for the flexible template to work. In other words, a flexible template is probably most beneficial when readiness to train is often low due to either consistently fatiguing training or extenuating life circumstances. Therefore, this article will aim to deliver the following information:

Break down the existing data on flexible training templates.
Discuss in what situations flexible training may be most useful.
Discuss different levels of flexibility (i.e., weekly, monthly, or all-time flexibility).
Examine the efficacy of various metrics to assess daily readiness.
Provide practical examples of how to implement this concept.

Purpose and Hypotheses

Purpose

The purpose of this study was to compare the effects of flexible and fixed-order training templates equated for session type on gains in strength, power, and agility over eight weeks in both men and women.

Hypotheses

The researchers hypothesized that flexible training would enhance performance in all outcome measures.

Subjects and Methods

Subjects

32 Division III collegiate lacrosse players (15 men and 17 women) completed the study. The researchers provided no information regarding previous resistance training experience, but all athletes completed a six-week familiarization phase before the study (more below). Further, the full manuscript indicated that some athletes were freshmen (which can also be inferred by the average age in Table 1). Overall, I suspect that subjects had some structured resistance training experience, but this experience varied between individuals, which is not uncommon for young athletes at the Division III level. Table 1 provides the available details of the subjects.

Study Overview

The athletes were split into two training groups for eight weeks. A flexible group performed a daily training session that matched their readiness. In contrast, the non-flexible (fixed order) group performed training sessions in a predetermined order (details in the next section). Athletes trained three times per week for eight weeks on non-consecutive days, but the first session of week one and last session of week eight served as pre- and post-training testing sessions; thus, each group had 22 training sessions. Both groups performed non-linear (or undulating, if you prefer) periodization. Pre- and post-testing measures were one-repetition maximum (1RM) bench press and hex bar deadlift, vertical jump height, sprint time (28.7m), and an agility test consisting of a 6.1m fly-in sprint followed by a turn and 6.1m sprint back. Lastly, all subjects completed a six-week familiarization program before the eight-week intervention. However, researchers did not provide any further details regarding the familiarization.

Training Protocol

The original paper doesn’t provide many details of the training program. However, the senior author (Dr. Kenneth Clark) put me in touch with the first author (Cory Walts), who graciously suffered through a phone call and various email exchanges with me to provide details. Huge thanks to these gentlemen for their assistance.

Researchers split the eight-week program into two four-week training blocks. There were 11 total training sessions in each four-week block (remember pre- and post-testing bookended weeks one and eight, respectively). There were two strength-focused sessions (i.e., traditional strength exercises) each week and one power-focused session (i.e., Olympic lift variations), except in weeks one and eight, when there were only two strength-focused sessions (plus a testing day). Each week also fluctuated volume and intensity; thus, the programming was non-linear both within and between weeks. There were two main workout categories, “Green” and “Yellow,” and each category had two subtypes (green: high volume or high intensity; yellow: low volume or low intensity) to make four different session-type possibilities. Further, the workouts were rated on a 1-4 scale (arbitrary units) for both volume and intensity. In other words, the workout with the highest volume was rated a 4, and the lowest volume workout was rated a 1. Table 2 displays all session types and their volume/intensity ratings.

In both groups, athletes answered the question (using the TeamBuildr phone app) “based on how your body feels and your current mindset, how ready are you for today’s training?” Subjects had the option of answering “green (good feel and mindset),” “yellow (fair feel or mindset),” or “red (poor feel or mindset).” In the fixed group, subjects performed training weeks in the order of yellow (low intensity), green (high volume), yellow (low volume), and green (high intensity) regardless of their response, as seen in Figure 1. The only caveat is that if an athlete in the fixed group answered “red,” then they skipped the training session and performed one less training session for that block.

Similar to the fixed group, a flexible athlete also skipped a workout if they answered “red” to the readiness question. However, on each training day, flexible group athletes had four workouts to choose from (the two greens and the two yellows), but no session type could be performed more than once on a specific day of the week during each block. For example, there were four Wednesdays in each training block, and if an athlete answered green on Wednesday of week one, then they chose a high volume or high intensity workout. If they chose high volume, then that workout could not be completed again on a future Wednesday during the first training block. Athletes followed the same procedures for each individual day during the training blocks. One of the four workouts was not chosen on a Monday during block one since there were only three Monday training days (i.e., the first Monday was pre-testing), and the same for Friday in block two since that’s when post-testing was conducted. Table 3 displays a possible example of the protocol in the flexible group for a four-week training block.

Additional Notes

It’s also worth noting that researchers did not supervise training in this study. Athletes answered the daily question on the TeamBuildr app, which then provided the workout based on their choice. While this is a limitation, it’s inherently not the researchers’ fault. Since the study used Division III NCAA collegiate athletes, NCAA rules had to be followed, which meant that training workouts had to be self-selected. The researchers couldn’t officially report training adherence or volume because that would amount to “tracking” an intercollegiate athlete’s off-season training. Specific interset rest intervals were not listed; however, all workouts lasted approximately 60 minutes.

Outside of lifting, the athletes also participated in speed and agility sessions twice per week for 60 minutes each time, and one “conditioning” session, which was not otherwise described.

Findings

The findings were simple. All outcome measures tended to increase in both groups; however, there were no group differences. Vertical jump increased by 3.9% and 6.4% in the flexible and fixed groups, respectively. Agility performance improved by 0.8% and 1.6% in the flexible and fixed groups, respectively. Tables 4 and 5 show the findings for bench press and hex bar deadlift 1RM strength along with percentage changes.

Interpretation

The reviewed study from Walts et al (1) didn’t show flexible training to augment strength gains, but that doesn’t mean we should write off the concept. One could argue that similar strength gains but more autonomy over training decisions is a win for flexible templates, for starters. However, I think there’s much more to discuss. We should also consider the readiness indicator used, how fatiguing the program is, and the degree of flexibility allowed (i.e., weekly, monthly, or all-time). Therefore, this interpretation will provide a nuanced discussion of the above considerations.

Current and Previous Research

Although the concept of flexible templates is well-known, there are only three resistance training studies directly tackling the idea. Two of those studies – the currently reviewed study from Walts (1) and a study from Colquhoun et al (3 – MASS Review) – failed to show a benefit for flexible training versus fixed order training. The Colquhoun study compared a group of trained lifters using a fixed weekly order of hypertrophy-focused (Monday), power-focused (Wednesday), and strength-focused (Friday) sessions for nine weeks to a flexible group. The flexible group performed hypertrophy, power, and strength sessions within the same week, but lifters could choose the order. Lifters used a five-point Likert scale before each session to assess motivation and readiness to train. Further, subjects performed the last set to failure on both the hypertrophy and strength sessions each week in Colquhoun’s study. Weekly load changes were based on repetition performance, which allowed for between-group volume and intensity calculations. Colquhoun reported similar squat, bench press, and deadlift increases between groups, and no group differences for volume or percentage of 1RM used. Colquhoun did show a lower dropout rate in the flexible group (12.5%) versus the fixed group (31%), and fewer total missed sessions in the flexible (four) versus the fixed group (eight).

Walts’ argument regarding autonomy seems to have some value, based on Colquhoun’s adherence reporting. I’ll return to autonomy in a bit, but in the short term, I don’t think Colquhoun’s flexible group experienced enhanced performance because the fixed group was already set up well. Specifically, the fixed order of hypertrophy, power, and strength allocates weekly volume appropriately. Higher volume training sessions (i.e., traditionally hypertrophy-focused) tend to result in the most muscle damage. Thus, inserting a lighter (i.e., power) session in the middle of the week considers that a lifter may be fatigued 48 hours after hypertrophy-type training. The lifter is then recovered for Friday’s strength session and may even get a 48-hour priming effect from the power session to enhance Friday’s strength performance (4 – MASS Review). Indeed, previous data have shown this hypertrophy, power, strength set-up to result in greater volume on the strength day than a hypertrophy, strength, power configuration (5). Other factors such as sleep, travel, stress, and early morning training can affect readiness and warrant a flexible template, which may have accounted for the greater adherence in Colquhoun’s flexible group.

The other study to directly address this concept is from McNamara and Stearne (2), and was published over a decade ago. These researchers split 16 subjects (both men and women) with a little over one year of training experience into a fixed order and flexible groups and measured leg press and chest press strength before and after 12 weeks of training. The subjects trained only twice per week using one set of various exercises, but every set was to failure. The fixed order group rotated 20, 15, and 10RM sessions in that order, while the flexible group completed a 0-10 Likert scale to assess energy levels and then chose which session they wanted that day. The flexible group in this study had more autonomy than the flexible groups in the Walts or Colquhoun studies, in that flexibility was not restricted to within-week. Instead, lifters had to perform each session type eight times, but could do so in whatever order they chose. McNamara and Stearne reported no group differences for chest press increase, but leg press improvement in the flexible group roughly tripled (Figure 1 here) the fixed group’s progress. Table 6 provides a summary of both the Colquhoun and McNamara studies.

Flexible templates are generally viewed positively. I mostly share that view; however, it’s worth noting that only one strength measure (McNamara and Stearne leg press) out of seven strength tests from three studies improved more with a flexible versus a fixed training order. However, the McNamara and Stearne study may have been better designed to see group differences than the other two studies. Specifically, as noted above, both the Colquhoun and Walts studies had power-focused training sessions in the middle of the training week, which may have helped mitigate fatigue. Further, while I’m not entirely sure about proximity to failure in the Walts study, subjects in Colquhoun’s study only went to failure on the last set of the hypertrophy and strength day (two sets per week for squats, two for bench, and one for deadlift). On the other hand, McNamara and Stearne’s subjects did at least 14 sets to failure each week on various exercises and only had a little over a year’s training experience, while Colquhoun’s lifters had been training for three years on average. Therefore, fatigue may have been greater in the McNamara and Stearne study, which provides a stronger justification for a flexible program. McNamara and Stearne prescribed the same volume in each group, but subjects in the flexible group may have been able to progress loads more frequently than subjects in the fixed group, which would explain the flexible group’s enhanced rate of strength gain. Although, even if volume or intensity was greater in the flexible group, I’m not entirely sure how to account for the roughly three times greater leg press strength increases in favor of the flexible training group in the McNamara and Stearne study. One possible explanation is that subjects in the flexible group chose mostly lighter sessions (20RM and 15RM) early in the program and thus, performed more of the heavier (10RM) sessions closer to post-testing than the fixed order group. However, that is purely speculative.

In general, the cornerstone of flexible templates is that lifters can match the day’s session to their readiness. This flexibility is beneficial when training or life circumstances (or both) are really demanding. In other words, if a training program isn’t that demanding (i.e., not high-volume or a lot of failure training) and life circumstances aren’t extraordinary, is a flexible template vital to optimize progress? Even though a flexible template may not always shine without demanding life circumstances, that doesn’t mean flexible templates still aren’t a good idea for some individuals. The discussed studies, including the currently reviewed one, show similar performance changes between flexible and fixed programs; thus, individuals should choose whatever they prefer. Having autonomy in a program is indeed a good thing, but we should also be cautious of too much autonomy. The specific population may also matter in terms of autonomy. For example, the reviewed study (1) used Division III collegiate athletes during voluntary off-season workouts, which I’m intimately familiar with as both a former Division III NCAA athlete (i.e., average athletic adult human) and former Division III strength coach. In this specific case, autonomy is probably positive. First and foremost, these athletes must voluntarily choose to do the workouts, and a coach wants the athletes to buy into the program since NCAA rules do not allow coaches to monitor athletes in the off-season. Some athletes will train and do exactly as instructed; however, others will take a bit more convincing, so on the whole, flexibility is a positive for the Walts study population. In the context of team sport athletes, flexible training may also be helpful in-season, as some athletes play more minutes in a game or match than others. For instance, in NCAA soccer, there are typically two games per week. If an athlete is playing 90 minutes and another is barely touching the field, these athletes should have different workloads (both on the field and in the weight room) during the week, and a flexible template is a vehicle to get them there. On the other hand, some autonomy is a positive for the individual strength athlete, but others hire a coach because they want exact programming. Therefore, some lifters come to a coach precisely to avoid having to make training decisions. In other words, I don’t think autonomy is the most salient defense of flexible templates for the strength athlete.

Flexibility Situations and Degrees of Flexibility

As previously stated, specific circumstances should be present to warrant use of a flexible template:

Extremely demanding training.
Currently exhaustive life (work/school/family) schedule.
Consistent travel with inconsistent gym access.

Although, I’d argue that extremely demanding training isn’t necessarily a reason to utilize a fully flexible training program. If performing an overreaching block or sustained high volume hypertrophy-type training, coaches and lifters should organize programming within a week to account for training fatigue. For example, if you are training a muscle group three times per week (i.e., M, W, F) and there are high RPE, moderate RPE, and low RPE days, then the default program structure should be to do moderate RPE training on Monday, low RPE on Wednesday, and high RPE on Friday. Similarly, if thinking in terms of high-, low-, and moderate-volume days or hypertrophy-, power-, and strength-type training sessions, the weekly order should be: high-volume/hypertrophy (Monday), low-volume/power (Wednesday), and moderate-volume/strength (Friday). The point being, no matter what type of programming you prefer, each week should be programmed to allocate training volume appropriately. If you already do this, such as Colquhoun’s fixed order group, the need for a flexible template is minimized. If you are constantly fatigued going into your next session, then a flexible template is not the solution. Instead, I’d recommend rearranging your training, or consider lowering your training volume or proximity to failure (i.e., training variables which elongate recovery).

Flexible programming may shine when life gets busy, such as a month-long work project, studying for that elusive D+, preparing for family holidays, or extended travel – in other words, situations in which you have consistently lower sleep and higher stress, which can impact performance along with inconsistent gym access. The next step is not just implementing a flexible model, but also considering the degree of necessary flexibility. Suppose all of the above situations last approximately one month, and your fatigue levels and gym access are entirely unknown. In that case, you might use within-month or within-block flexibility such as the McNamara and Stearne study. For example, if you have six low RPE days, six high RPE days, and six moderate RPE days, then you would perform six of each session type anytime in the month in whatever order you choose. It may also be wise to scale back to four of each session type to ensure feasibility during stressful times. If someone doesn’t want to scale back, they could still aim for 24 total sessions, but use a breakdown with more “easier” days such as 4 high RPE, 8 moderate RPE, and 12 low RPE sessions.

Another option is within-week flexibility, such as in the Walts (1) and Colquhoun (3) studies. For within-week flexibility, there are usually two or three different types of training sessions during the week (whatever is best for the specific individual), and the lifter completes each session type during the week, but in whatever order they choose. However, I’m not sure that within-week flexibility offers much benefit if the week’s volume is already appropriately allocated so that fatigue from one session doesn’t bleed into another. If you’re already appropriately allocating volume within the week, within-week flexibility doesn’t provide much benefit. While daily stress could still warrant flexing in easier sessions, this could present problems with a within-week model. For example, if stress levels are high on Monday in a Monday (hypertrophy), Wednesday (power), and Friday (strength) setup, then you may choose to perform the power session. Then, on Wednesday, you complete one of the two more fatiguing sessions (hypertrophy or strength). Still, now fatigue from Wednesday may bleed into the hypertrophy or strength session (whichever is left) on Friday. Ironically, an actual within-week model of flexibility could exacerbate the same issue it’s trying to mitigate.

Instead of a true “within-week only” flexible model, I’d prefer a fixed order, but with an “all-time flexible option.” For example purposes, let’s assume we’re training a muscle group three times per week. I’d set up training so that you are already allocating volume to the best of your ability. Table 7 shows a conceptual example of this setup where the heaviest training day (Friday) is positioned the farthest from the high-volume day. Before looking at Table 7, just know that there are many other ways to configure training and many different exercises to include; this is solely intended for conceptual purposes.

Although the program in Table 7 isn’t easy training, the predetermined order is sound, so training fatigue alone probably won’t be an issue, assuming this is the appropriate magnitude of volume for a specific person. However, since life situations still pop up, you could have a few “easy days” in your back pocket to allow for all-time flexibility as needed. These easy days could be a low volume power session or a solely assistance work-focused session at a low RPE, or another option you prefer. In this design, you would perform the predetermined order, plug in one of the easy options when needed, and then continue with the next pre-planned day. Table 8 presents this option.

In Table 8, you can see a power day on the main lifts or a session focused on assistance work was flexed in when needed. Then, the lifter carried on with their next scheduled session-type after the flex day. This type of model may not be best when traveling and having limited gym access; however, I think it works well when you are not planning for disruptions. In other words, I would almost always have a flex option in a training program to account for unforeseen circumstances. In addition to just inadequate sleep or elevated acute stress, a lifter may have to train early in the morning unexpectedly or may suddenly be short on time. In both of these situations, having flex options works well. If we understand this concept conceptually, we can create a whole host of flex options that serve a specific purpose. For example, this video provides flexible examples for an afternoon lifter who has to train in the early morning. Of course, even in a flexible template, there should also be the option not to train if fatigue and motivation are just too low; in that case, I’d just push everything back one day. If you have to miss two days, then I’d probably repeat the training week. Our video on program troubleshooting provides additional flexible options when traveling or completely missing training.

Determining Readiness

I’ve previously covered readiness indicators in great detail (one, two), so I’ll just provide some brief considerations here. The three flexible studies (McNamara and Stearne, Colquhoun, and Walts) vary in their methods of determining pre-training readiness. McNamara and Stearne (2) used a 0-10 Likert scale, Colquhoun (3) used a 0-5 scale assessing readiness and motivation, and Walts (1) used a specific question (quoted earlier) asking about mindset and readiness. Importantly, if using a readiness indicator to influence training choice, that indicator should have some capacity to predict performance. Yet, many common readiness indicators lack empirical support to predict lifting performance. The perceived recovery status scale (0-10 Likert scale) has a strong inverse correlation with muscle damage following very damaging sprinting (6). A general Likert scale might pick up large magnitudes of fatigue; however, if you have extreme soreness when you go into the gym, then you should consider allocating your volume differently, as discussed earlier. Further, a scale such as the perceived recovery status scale doesn’t assess well-being (anxiety and mood state), which may affect performance. While well-being scales may have merit in team sports (7), their ability to predict acute strength performance has not fleshed out (8). Even technological tools such as heart rate variability have also failed to show promise to relate to recovery of resistance training performance (9 – MASS Review) or enhance strength gains when used to guide flexible programming (10 – MASS Review). Perhaps the readiness indicator with the most empirical support to predict lower body lifting performance is vertical jump height. Watkins et al (11) assessed lifters’ vertical jump height and performed four squat sets to failure at 80% of 1RM. 48 hours later, Watkins retested both measures and observed that decreased vertical jump height was correlated with reduced squat reps (r = 0.65). A lifter could perform a quick vertical jump before each training session and set a cutoff (e.g., 1.5 cm); if their vertical jump drops below that target, they choose an easier training session. Of course, vertical jump height wouldn’t apply to upper body performance, but conceptually the Watkins study design is how researchers can determine if recovery of a particular metric is indicative of performance.

Since becoming interested in this topic about 12 years ago and writing this piece for Stronger by Science a few years back, I’ve started to wonder how much readiness indicators matter. In other words, how fatigued do you need to be to change training? Suppose you are training with a well-designed setup where volume is allocated appropriately, and you’re probably not ever too fatigued going into the next session. In that case, you probably don’t need to be 100% recovered to train effectively. Additionally, intra-session load can always be adjusted (up or down) to match performance using RPE or velocity, which mitigates the need to completely change the day’s session if you’re feeling just a touch fatigued. Let’s use the perceived recovery status scale as a simple example. If a lifter plans a heavy session when their perceived recovery is between 8-10 on a 10-point scale, does that mean performance will be worse if they do a heavy session on a day where they’d rate their perceived recovery status a 7? Probably not. I think a lifter generally knows if he or she feels completely trashed or good enough to perform. If feeling good enough to perform, tools such as RPE and velocity are there for intra-session adjustments. If the lifter is feeling trashed, then perhaps a day off is warranted, or the athlete can insert a light/power day. If a specific circumstance (i.e., morning training or travel) arises, using one of the specific flexibility options noted above is a good idea.

Conclusions and Thoughts

Overall, there’s merit in the idea of being able to flex in a different type of training session than was initially planned. Still, unless major circumstances are present (high fatigue, inadequate sleep, travel, etc.), I wouldn’t expect a huge benefit from flexible templates. The presently reviewed study does have a high ecological validity for team sport athletes training in the offseason. It’s often difficult to get those athletes to adhere to an off-season lifting program; given that a flexible approach did not hinder progress in this study, flexibility could be viewed as a potential approach to enhance adherence without sacrificing efficacy. However, for strength sports athletes, I’d be more likely to implement flexibility in specific circumstances, or always having a power/light day on hand in case it’s needed. Lastly, training flexibility isn’t limited to just session-type. A study (12) previously reviewed by Dr. Helms showed that allowing a lifter to choose from a pool of exercises each day may enhance strength. While a powerlifter needs to squat, bench press, and deadlift, a coach could give the athlete a choice of what assistance exercises to perform. For instance, I often program “back assistance (your choice)” to provide athletes with autonomy. Of course, not everyone may want that autonomy, so this is not a blanket statement to always offer this choice; instead, this is just to say that there are other ways (more than listed here) to implement flexibility into your training program.

Next Steps

Although flexible templates are logical and should work, I still feel this area needs proof of concept for resistance training. When early studies are conducted, the intervention is often overly demanding to examine if the idea is worth continuing. For example, an early static stretching study from Fowles et al (13) had subjects hold stretches for >100 seconds and found decreased acute muscle stiffness and force production. We now know that if your stretches are pretty short (i.e., <10 seconds), the risk of a strength decrease is negligible. The point being, I’d like to see a longitudinal study that compares flexible versus fixed order training when training or life circumstances are really demanding, with my preference being the latter. Potentially, college students who report typically being stressed and sleeping less during the last month of a semester would be good candidates for this study. In that case, we’d see if flexible training could enhance volume and intensity when lifters chose the hard days in the flexible group and if that led to improved outcomes over a month.

Applications and Takeaways

The reviewed study found that choosing each weekly session’s volume or intensity did not enhance strength performance compared to a fixed weekly training schedule.
The concept of flexible training has been around for a while and has merit; however, it’s probably most beneficial when training readiness is low due to extenuating life circumstances.
Ultimately, if life circumstances aren’t extenuating, then I’d prefer a fixed order weekly configuration. However, I’d always keep a light or power training session on hand to flex in just in case readiness is low due to unexpected poor sleep, early morning training, or time restraints.

References

Walts CT, Murphy SM, Stearne DJ, Rieger RH, Clark KP. Effects of a Flexible Workout System on Performance Gains in Collegiate Athletes. The Journal of Strength & Conditioning Research. 2021 Mar 25.
McNamara JM, Stearne DJ. Flexible nonlinear periodization in a beginner college weight training class. The Journal of strength & conditioning research. 2010 Jan 1;24(1):17-22.
Colquhoun RJ, Gai CM, Walters J, Brannon AR, Kilpatrick MW, D’Agostino DP, Campbell WI. Comparison of powerlifting performance in trained men using traditional and flexible daily undulating periodization. The Journal of Strength & Conditioning Research. 2017 Feb 1;31(2):283-91.
Tsoukos A, Veligekas P, Brown LE, Terzis G, Bogdanis GC. Delayed effects of a low-volume, power-type resistance exercise session on explosive performance. The Journal of Strength & Conditioning Research. 2018 Mar 1;32(3):643-50.
Zourdos MC, Jo E, Khamoui AV, Lee SR, Park BS, Ormsbee MJ, Panton LB, Contreras RJ, Kim JS. Modified daily undulating periodization model produces greater performance than a traditional configuration in powerlifters. The Journal of Strength & Conditioning Research. 2016 Mar 1;30(3):784-91.
Laurent CM, Green JM, Bishop PA, Sjökvist J, Schumacker RE, Richardson MT, Curtner-Smith M. A practical approach to monitoring recovery: development of a perceived recovery status scale. The Journal of Strength & Conditioning Research. 2011 Mar 1;25(3):620-8.
Mielgo-Ayuso J, Zourdos MC, Clemente-Suárez VJ, Calleja-González J, Shipherd AM. Can psychological well-being scales and hormone levels be used to predict acute performance of anaerobic training tasks in elite female volleyball players?. Physiology & behavior. 2017 Oct 15;180:31-8.
Haischer MH, Cooke DM, Carzoli JP, Johnson TK, Shipherd AM, Zoeller RF, Whitehurst M, Zourdos MC. Impact of Cognitive Measures and Sleep on Acute Squat Strength Performance and Perceptual Responses Among Well-Trained Men and Women.The Journal of Strength & Conditioning Research. 2021 Feb 1;35:S16-22.
Flatt AA, Globensky L, Bass E, Sapp BL, Riemann BL. Heart Rate Variability, Neuromuscular and Perceptual Recovery Following Resistance Training. Sports. 2019 Oct;7(10):225.
de Oliveira RM, Ugrinowitsch C, Kingsley JD, da Silva DG, Bittencourt D, Caruso FR, Borghi-Silva A, Libardi CA. Effect of individualized resistance training prescription with heart rate variability on individual muscle hypertrophy and strength responses. European journal of sport science. 2019 Jan 30:1-9.
Watkins CM, Barillas SR, Wong MA, Archer DC, Dobbs IJ, Lockie RG, Coburn JW, Tran TT, Brown LE. Determination of vertical jump as a measure of neuromuscular readiness and fatigue. The Journal of Strength & Conditioning Research. 2017 Dec 1;31(12):3305-10.
Rauch JT, Ugrinowitsch C, Barakat CI, Alvarez MR, Brummert DL, Aube DW, Barsuhn AS, Hayes D, Tricoli V, De Souza EO. Auto-regulated exercise selection training regimen produces small increases in lean body mass and maximal strength adaptations in strength-trained individuals. The Journal of Strength & Conditioning Research. 2020 Apr 1;34(4):1133-40.
Fowles JR, Sale DG, MacDougall JD. Reduced strength after passive stretch of the human plantarflexors. Journal of applied physiology. 2000 Sep 1;89(3):1179-88.