In 2021 I wrote my master thesis, where I compared the effects of stretching and strength training on ROM and tried to uncover if there were any differences in the adaptations that lead to the increase in ROM, after stretching and strength training.

It’s a subject that I’m still fascinated by, and this article can in many ways be seen, as an updated and expanded version of my master thesis – but hopefully easier and more exciting to read 😉

In recent years, it’s been well established that strength training can be effective for increasing Range of Motion (ROM) – maybe even as effective as stretching.
But there are gaps in out knowledge, that makes this comparison difficult.

The are meta-analysis, that compare the chronic effects of strength training and stretching on ROM, find that the two modalities are equally effective [1, 2], but the authors also conclude that more high quality studies are needed and that the heterogeneity of the current studies make comparisons difficult.

There are also other gaps, in our knowledge, that add to the difficulty of making comparisons:

• For stretching, we don’t even have a clear dose response relationship between duration, frequency or intensity and increases in ROM [3].

 

• For strength training, it’s just as unclear – No reviews or meta-analysis has been published on the subject that has been able to give any clear guidelines, most likely due to the fact, that there are still relatively very studies looking into the effects of strength training on flexibility.

 

• We have very few studies, lasting longer than a few months – the longest interventional study on stretching I’ve been able to find, lasted 6 months [4].
  This creates a gap, as it is possible that some adaptations could take several months or years to become significant.

 

In this article, my aim is to explore is the physiological adaptations that could affect ROM – and whether there’s a difference in the mechanisms that lead to increases in ROM after doing stretching or strength training respectively.

 

My hope is, that this will help kill off some of the myths and misunderstandings regarding flexibility training, as well as help you make more informed decisions, if the goal of your training is to increase your flexibility.

 

The article is based on the current literature, and I will do my very best to keep it updated, as our knowledge about this subject expands.

 

Studies looking at acute effects have been excluded, as we can’t say whether these have any meaningful impact on long term adaptations. There are also possible adaptations, that I have chosen not to include, if I’ve found the literature to be lacking regarding the chronic effects on flexibility.
One example would be fascia – it has been theorized that fascia stiffness might limit ROM [5] and recent work by Warneke, Rabitsch [6] have shown that fascia stiffness can be reduced by stretching. But since the chronic effects on ROM are yet to be uncovered, we will not be diving deeper into this subject in this article.

 

Neural and structural adaptations

The adaptations that lead to an increase in ROM, can be broadly classified as neural or structural [7, 8] – either related to the nervous system or structural changes of relevant tissue. Similar to how strength is determined by the size of the musculature and the neural drive.

 

1.0 Neural adaptations:

1.1 Stretch tolerance:

Stretch tolerance is the ability to tolerate the stretching sensation. Stretch tolerance and the underlying adaptations, are generally referred to in the literature as sensory adaptations [8] and might be linked to pain tolerance [9, 10].

 

Increased stretch tolerance has been cited in several studies as the main reason for increased ROM after stretching [11-16]. We have one study of longer duration study by Moltubakk, Villars [4], where the test subjects performed stretching over 24 weeks; post intervention, increased ROM was observed which could be explained by increased stretch tolerance.

 

Few strength training studies examine the effect on stretch tolerance, but one study found increased stretch tolerance after strength training [17] (In the study they refer to the intervention as stretching an active muscle – the intervention basically consisted of eccentric strength training), while another study found that strength training and stretching increased stretch tolerance comparably [18].
The data we have is limited, but it seems strength training can effectively increase stretch tolerance, potentially as effectively as stretching.

 

1.2 Other neurological adaptations

Stretching can lead to specific neurological adaptations, in the form of reduced T-reflex, probably due to reduced muscle spindle sensitivity [7, 19], decreased power development in the antagonist of the lengthened muscle [19] as well as decreased H-reflex measured at rest [7].

 

In the study by Guissard et al. (2004) test subjects performed stretching targeting the plantar flexors 5 days a week for 6 weeks; post-intervention, increased ROM was found, which was partly deemed to be due to reduced H and T reflex by 36% and 14%, respectively.
A study by Hayes, Harter [20] also measured H-reflex at rest measured via reflex EMG, but observed no change in H-reflex after 6 weeks of STRETCH of the soleus with 5 weekly sessions, although ROM was significantly increased. This suggests that neural adaptations may occur after periods of stretching, but the specific mechanisms remain largely unknown.

 

Likewise, it is unclear what effect neurological adaptations due to strength training have on ROM. However, several studies have shown that heavy ST can lead to an increased H-reflex and V-wave during maximal muscle contraction [21-24].

It is possible that increased H-reflex after ST, could affect ROM, but further studies are needed to draw any strong conclusion.
From the data we have so far, I think it’s unlikely to be a significant factor regarding flexibility.

 

1.3 Summary

• The main neural adaptation for increasing ROM is increases in stretch tolerance. Both stretching and strength training seems to be effective at increasing stretch tolerance

 

• There are other possible neural adaptations that might contribute to increases in ROM after stretching, but the evidence is either lacking or conflicting.

 

• Strength training can induce other neural adaptations, but the effects of these on ROM have yet to be examined

 

Structural adaptations

2.0 Pennation angle

Watch this, for a great explanation of what pennation angle is and what the implications are.

The pennation angle is the angle between the longitudinal axis of the entire muscle and its fibers (see pictures below)

Most skeletal muscles are pennate [25] which provides greater strength but less range of motion, due to the line of pull (Again, see picture above) [26]. in theory, an increase in pennation angle would negatively affect the ability of the muscle-tendon-unit to elongate, which would hinder ROM – with the reverse being true for a decrease in pennation angle.

 

A bigger muscle cross sectional area is generally correlated to an increased pennation angle [27-29]. Several studies have shown that heavy ST can increase the pennation angle of muscles [30-34] – which should be no surprise, since ST is also for increasing the muscle cross sectional areal.

 

For stretching, the effects are less clear. One study found increased pennation angle after 6 months of STRETCH [4], While another study increase in fascicle length and decrease in pennation angle was observed after 6 weeks of loaded stretching [35], but most other studies have found no effect of STRETCH on pennation angle [36-39].

 

A handful of newer studies have shown that stretching can lead to increases in muscle cross sectional area, to the same extent as strength training (see the section about muscle CSA). However, none of these studies measured pennation angle. But as there is a correlation between increases in muscle cross sectional area and increases in pennation angle, there is a good chance that with high enough intensity and time under stretch, stretching could also lead to significant increases in pennation angle.

Unfortunately, we have no direct studies assessing whether changes in pennation angle directly affects ROM – but as stated above, in theory, an increase in pennation angle could negatively impact ROM.

However, the chronic changes in pennation angle are measured in a non-stretched position. It gets a bit more complex, because pennation angle actually changes as the muscle is stretched.
2 studies have used ultrasound to determine which structures contributed to submaximal passive elongation of the gastrocnemius (calf muscle). Both found that the pennation angles decreased as the subjects were moved into a stretched position, but the changes in pennation angle only contributed to the elongation of the muscle-tendon-unit by >10% of the maximal elongation [40, 41].

 

A limiting factor here, is the fact that the data is just from the calf muscles – it is possible that changes in pennation angle can contribute more significantly to the lengthening of the muscle-tendon-unit in other muscle groups.

 

2.1 Summary

• Strength training can increase pennation angle. Stretching might be able to increase pennation angle, if done with high enough intensity and time under stretch.

 

• In theory:
  Increased pennation angle = Less potential for MTU elongation = Negtive effect on ROM
  Decreased pennation angle = More potential for MTU elongation = Positive effect on ROM

 

• Since pennation angle decreases as the muscle is stretched, changes to resting pennation angle might not affect ROM at all. We don’t have longitudinal data on whether it’s possible to induce chronic changes to pennation angle under stretch. We still don’t know, whether changes in resting pennation angle, has any effect on pennation angle in a stretched position.

 

3.0 Tendinous tissue:

3.1 Tendon stiffness and strength training

Several studies have found increase tendon stiffness after heavy strength training [42-45]; a review by Folland and Williams [33] concluded that there is strong evidence for heavy strength training leading to an increase in tendon stiffness. Studies have found increased tendon stiffness as well as hypertrophy of the patella tendon after 9-12 weeks of strength training, performed 3 times weekly with 70-80% of 1 RM [42, 43].

Arampatzis, Karamanidis [45] found that 14 weeks of strength training performed with high (4.55±1,38%), but not low (2,88±99%,) strain of the patella tendon, led to increased tendon hypertrophy and stiffness; the authors point out that the tendon has to experience a strain above a certain limit value, to induce adaptations.

This is backed up by a meta-analysis by Lazarczuk, Maniar [46], which found that only strength training studies that involved high tendon strains of ~5% increased tendon stiffness.

 

3.2 Tendon stiffness and stretching

There are studies that find reduced tendon stiffness after PNF [47] and ballistic [48] stretching, while many other studies find no change in tendon stiffness measure under isometric contractions with a rising percentage of the maximum voluntary contraction [37, 48-52].

 

A 2024 evaluation and recalculation of the data from earlier meta-analysis [53] confirmed the conclusion of an earlier meta-analysis from 2017, which found that stretching for 5-8 weeks did not have any effects on tendon stiffness [54]. Similarly, a study by Moltubakk, Villars [4] found that 24 weeks of static stretching had no effects on tendon stiffness. Thus, stretching does not seem to have a significant effect on tendon architecture or stiffness.

 

It seems that tendons need to experience a strain of ~5% to increase stiffness [46]. It seems stretching simply doesn’t apply enough strain to cause adaptations in the tendinous tissue.
However, it is possible that adding external load to passive stretches could lead to increases in tendon stiffness.

 

The strength training studies that successfully increase tendon stiffness, usually use loads of >70% of 1RM (70% of the heaviest weight you can lift once) – in theory, it would be possible to add the same to a passive stretch, although in many cases it wouldn’t be practical.

 

3.3 Tendon Stiffness and ROM

A larger cross-sectional area of the tendon, or a stiffer tendon, leads to lower stress of the tendon at a given force, which is advantageous in terms of reducing the risk of overload injuries in the tendon tissue (Kongsgaard et al. 2007), but would increase the passive resistance to stretching, thus having a negative effect on ROM.

 

A thicker and/or stiffer tendon would also increase the passive resistance to stretching – so in theory, stiffer tendons would negatively affect ROM.

 

Unfortunately, we don’t have any studies that have been able to show, that changes in tendon stiffness affects ROM. We do have some observational data, but the results from these are mixed as well.

 

In one study [55], knee flexion ROM and patella tendon stiffness was measured for 43 individuals. The 10 subjects with the highest knee ROM, was compared to the 10 subjects with the lowest knee ROM. Tendon stiffness did not differ between groups, which could make it seem like tendon stiffness might not be an important factor for ROM.

 

However, a study by Moltubakk, Magulas [56] compared the stiffness of the achilles tendon and ankle dorsiflexion ROM of 10 professional ballet dancers to 10 non-stretching control subjects, and found that the ballet dancer had less stiff tendons and greater ROM, compared to the control group.

 

The conflicting results, could be due to the difference in the examined muscle-tendon-unit and join ROM. For the calf muscle (triceps surae), the tendon contributes relative more to the total length of the muscle-tendon-unit, whereas the patella tendon contributes relatively little to the total length on the quadriceps muscle-tendon-unit. Which could explain why lower tendon stiffness seemed to correlate with higher ROM in the study by Moltubakk, Magulas [56], but not in the study by Bojsen-Møller, Brogaard [55].

 

 

3.4 Summary

• Stretching does not seem to induce adaptations in the tendons

 

• Strength training can increase both thickness and stiffness of the tendons in the trained muscle-tendon units.

 

• The effects of tendons stiffness on ROM are unclear, but in theory a higher tendon stiffness would negatively affect ROM.
  Tendons stiffness could play a bigger role in muscle-tendon-units like the calfes, where the tendon contributes relatively more the total length of the muscle-tendon-unit.

 

 

4.0 Muscle size (CSA)

4.1 Muscle size and ROM

Muscle size is often determined by measuring the Cross-Sectional area (CSA) of the muscle. If the muscle fibers grows radially (become thicker AKA radial hypertrophy), the CSA of the whole muscle also increases.

 

Several studies have found a correlation between increased muscle mass and increased stiffness or reduced ROM [57-59]. Getting thicker muscle fibers, is similar to increasing the thickness of an elastic band – more resistance to elongation. At extreme levels, it is very possible that the muscles can get so big, that the sheer size of the muscles start to restrict movement. Elite powerlifters tend to have lower ROM than a recreationally trained population [60, 61], which could very well be due to extreme levels of hypertrophy.

 

 

However, in pennate muscles, increases in muscle thickness or cross sectional areal can also be the result of an increase in muscle fascicle length – also known as longitudinal hypertrophy [25]. This is reflected in numerous studies that have found an association between whole muscle aCSA and fascicle length, among others a systematic review by Ema, Akagi [62] that included data from 38 independent studies. This is relevant, because an increase in fascicle length, doesn’t seem to increase the stiffness of the muscles (Check out this article for more in depth explanation).

 

 

Although both longitudinal and radial hypertrophy contribute to increases in muscle CSA, radial hypertrophy is most likely the biggest contributor – but it does seem possible to train in ways, that maximizes longitudinal hypertrophy.

 

 

4.2 Muscle Size and Strength Training

Strength training is very effective at increasing muscle size, whether it’s measure in thickness, CSA, Volume, etc. [63].

 

4.3 Muscle Size and Stretching

In recent years, it’s also been well established that stretching can lead to meaningful increases in muscle size.

A meta-analysis from 2024 included 1318 participants across 42 studies and found that stretching significantly increase muscle size [64]. The same meta-analysis found that longer stretching duration and intervention period, as well as higher frequencies were needed to achieve increases in muscle size, as the effects of lower doses of stretching did not reach significant levels.

 

 

In some studies, the increase in muscle size, is even comparable to what we would expect from strength training, but in these studies, the protocols are often too aggressive to be practical. In one example, 27 subjects stretched the plantar flexors (the calf) on one leg for 1 hour per day, for 6 weeks. MCSA of the calf increased significantly by 15.2% after 6 weeks (p < 0.001) [65].
A similar protocol, stretching the pectoralis major continuously for 15minutes, 4 times a week, also found increases in muscle size and strength, that was comparable to the increases found after strength training [66].

 

We also have a study by Panidi, Bogdanis [67], that used a protocol that would be more realistic to use under normal training conditions. 21 female adolescent volleyball players did 12 weeks stretching for the gastrocnemius (calf muscle), with one leg served as the control. The participants continued their volleyball training throughout the trial. They performed 540 -> 900s of stretching pr. session, 5 times a week. So a total of 2700-4500 seconds or 45-75 minutes pr. week under stretch [67].

 

 

4.4 Summary:

 

• Increases in muscle thickness or muscle CSA, is associated with increased stiffness and decreased ROM

 

• In pennate muscles, longitudinal hypertrophy can contribute to increases in muscle thickness. In theory, the bigger the contribution from longitudinal hypertrophy, the smaller the negative impact on flexibility would be from increases in muscle thickness

 

• Strength training is very effective at increasing muscle thickness

 

• If done with very high time under stretch, high frequency and intensity, stretching can lead to significant increases in muscle size

 

5.0 Fascicle Length

Skeletal muscles are made up of bundles of muscle fibers called fascicles (FL), fascicles are made of bundles of myofibrils or muscle fibers [25]. Each muscle fiber consists of sarcomeres, which is the part of the muscle that can contract.

When fascicles grow in length, it’s most likely due to an increase in the amount of sarcomeres in series – a process called “sarcomerogenesis” [68]. Sarcomerogenesis has been studied in animals, but in order to measure the number of sarcomeres in series, you basically have to cut the muscle open – making it quite invasive and hard to get approved by an ethical committee.

 

 

We do have a few case studies. One of these is Boakes, Foran [69] who measured fascicle length and the number of sarcomeres in series via laser diffraction intra operationally before and 8 months after a femur lengthening operation in a 17-year old girl. They found that FL had increased because of an increased number of sarcomeres in series.

Increasing fascicle length, would in theory, increases the muscles extensibility and possibly reduce muscle stiffness, thereby increasing ROM.
This is supported by cross sectional studies that have found that athletes with high flexibility have longer fascicles compared to volleyball players [70] or a non-stretching control group [56] as well as a study by Hirata et al., which found a positive correlation between ROM and fascicle length [71].

Increased fascicle length is closely associated with a shift in the torque-angle curve towards longer muscle lengths [68, 72] – basically meaning, that increasing fascicle length, makes the muscles stronger in lengthened positions.

 

 

5.1 Fascicle length and stretching

Although an earlier meta-analysis from 2017 found that stretching had no effect on muscle architecture, including fascicle length [54], a new meta-analysis from 2023 showed that static stretching can indeed increase fascicle length [73]. They included 19 studies, which was divided into the high/low intensity stretching and high/low volume.

 

 

Low-intensity studies included those which described stretch intensity as “no pain perception”, “stretching preceding pain threshold”, “pain between 6 and 7 on an analog scale ranging from 1 to 10”, and “without suffering discomfort”. High-intensity studies included those which described pain perception as “highest or maximum tolerable”, “point of discomfort”, and “maximum tolerable after the onset of pain”.

The sub-group analysis showed that only studies using high intensity stretching induced increased fascicle length.

 

 

Two of the studies that found an increase in fascicle length, are characterized by the use of external load to increase the intensity [35, 39].
In the study by Freitas and Mil-Homens [39], 5 male test subjects performed 8 weeks of STRETCH 3 times weekly, at a ROM equal to the maximum tolerable torque before the pain threshold for 450s; if possible, the ROM used was further increased every 90 seconds. Knee extension ROM (11.2 ± 9.5%, p = 0.04) and and biceps femoris FL (13.6%, p = 0.04) were significantly increased post-intervention.

In the study by Simpson, Kim [35] 11 male test subjects performed 6 weeks of STRETCH for the plantar flexors 5 times a week consisting of 3 minutes STRETCH of the plantar flexors performed with an external load of 20% of the plantar flexors MVC, which was increased weekly by 5%. Post-intervention, ROM and FL in the muscle-tendon junction (25% p <.001) were significantly increased.

 

Adding an external load to stretching, might be an effective strategy to achieve an increase in fascicle length.

 

5.2 Fascicle length and strength training

Multiple studies have found increased fascicle length after strength training [17, 72, 74-81].

To maximize an increase in fascicle length, a recent meta-analysis have shown that training at long muscle lengths seems to be a key factor [82]. This has been found in both conventional [83, 84], Concentric only, [81] and isometric strength training [85, 86].

If the goal is to optimize gains in flexibility through strength training exercises, choosing exercises where ROM isn’t limited by other factors than the level of flexibility would be ideal. This seems to be independent of contraction mode.

But the different modes of contraction, might still affect the degree of the fascicles length increase.

 

 

5.3 Concentric, Eccentric or Isometric contractions for maximizing fascicle length growth?

Several studies have found increased FL after eccentric [17, 72, 74-79], conventional [72, 76, 80], concentric only [72, 79, 81] as well as isometric ST at long muscle lengths [85].

 

A systematic review from 2019, found that isometric ST, regardless of muscle length during exercise, did not seem to have a significant effect on FL [87].
However, only 2 of the included studies investigated changes in fascicle length, both investigated parts of the quadriceps muscle. One of the studies did find and increases in FL after isometric training at both long and short muscle lengths [88], while the other found no changes in fascicle length after isometric strength training [89].

In contrast, two newer studies have found increased FL after 8 weeks of isometric ST of the tibialis anterior at long, but not short muscle length [85, 86].

The data is too limited to draw any strong conclusion – but personally I find it likely that isometric strength training will be proven to be an effective way of increasing fascicle length, as more studies emerge.

 

 

Most [79, 90-93], but not all studies [72], find that eccentric training is more effective than concentric only training, for increasing fascicle length.
Unfortunately, I have not been able to identify any studies comparing eccentric and conventional strength training.

 

We do have one study that compares traditional and attenuated eccentric training [94].
In this study, 28 males were assigned to traditional (TRAD) or Attenuated Eccentric Training (AET). Both groups trained 2 times per week for 10 weeks, performing 3 sets of seated single leg knee extension. On session 1, the training was performed with a 6-RM weight, 10-RM for session 2. In each session, the loads used elicited concentric failure, with the investigator assisting the subject to complete the set.
In the AET group, 40% extra weight was added to the eccentric part of the lift.

Only the AET led to significant increases in fascicle length for the vastus lateralis (AET: + 14 ± 14%, TRAD: + 1 ± 10%) and medialis (AET: + 19 ± 8%, TRAD: + 5 ± 11%).

 

Another study by Presland, Opar [95], compared the effects AET with different eccentric bias on fascicle length. 20 males, did 6 weeks of flywheel leg-curls training for the hamstrings.

While traditional weights are lifted against gravity, flywheel training creates resistance through inertia working like a yoyo. So, the harder you pull – the harder it pulls back. See the above GIF for an example of how this might look.

 

As both groups did flywheel training, both groups were doing AET. However, the control group performed the leg curls with one leg on both the concentric and the eccentric part of the movement. The intervention group performed the concentric phase with two legs, and the eccentric with one leg – basically the training with more eccentric bias.

Only the eccentric biased group increased fascicle length after the training intervention. It is surprising that FL didn’t increase in both groups and I do think a bit of skepticism is in order – but it does appear that a bigger eccentric bias, leads to bigger increases in FL.

 

This is somewhat supported, by a study from Pollard, Opar [96]. 30 participants where divided in 3 groups, training either the Nordic Hamstring Exercise (NHE), Weighted NHE and one group doing Razor Hamstring Curl with added weight (RHC).
Basically 3 groups doing eccentric training at declining intensity.

 

Both weighted NHE and unweighted, but not RHC, lead to increased FL in the hamstring (Biceps Femoris long head) after 6 weeks of training, with weighted NHE increasing FL significantly more than unweighted NHE.

The reason why the RHC didn’t increase FL, might be in part due to the fact that there is movement over both the hip and the knee. This could limit the activation of the hamstring muscles, as other studies have found this to be the case for the squat, which also combine hip and knee movement [97].
In my opinion, the interesting take away here, is that higher intensity eccentric training, seems to more effectively increase FL.

 

5.4 Summary

• Increasing in fascicle length would most likely decrease muscle stiffness and have positive effects on ROM – but the extent of this effect is unclear

 

• Stretching can increase fascicle length, but high intensity is needed – to the point of discomfort or beyond. Alternatively, 7< on an intensity scale of 1-10, with 10 being the highest.
  Adding external load, might be a useful strategy for increasing the intensity appropriately

 

• Strength training is very effective at increasing fascicle length. Training at long muscle lengths seems to be a key factor for maximizing increases in fascicle length

 

• Strength training can increase fascicle length independent of contraction mode if the training is done at long muscle length

 

• Eccentric or Attenuated Eccentric Training seems to be the most effective methods for increasing FL. Eccentrics with higher loads seems to be particularly effective. However, caution is advised, if this is combined with training at long muscle length.

 

6.0 Muscle stiffness

In this article stiffness refers to how much a specific tissue resists lengthening. This would be the mechanical term of stiffness.
Stiffness and muscle stiffness in particular, are often used to describe a subjective feeling of tightness, that doesn’t correlate well to the actual properties of the tissue. People who have extreme levels of flexibility and low mechanical muscle stiffness might still feel stiff some days.

 

6.1 Muscle Stiffness and ROM

Lower muscle stiffness should increase the muscles capacity to lengthen. It would be logic to think that there should be a clear positive relationship between muscle stiffness and ROM.

 

There are multiple studies that have found increased ROM and decreased muscle stiffness after stretching, but none of these investigate whether there is a relationship between the increase in ROM and the decrease in muscle stiffness [98-100].

 

Unfortunately, only a few studies have investigated the relationship between ROM and muscle stiffness, and the results have varied. One study found muscle stiffness to be an important contributor to hip flexion, less so for hip extension and for ankle dorsiflexion [101]. Another study, found muscle stiffness with stretch tolerance to be a determinant for ankle dorsiflexion ROM in young, but not older people [102].

 

A third study by Miyamoto, Hirata [103] compared ankle dorsi flexion ROM and muscle stiffness of 20 men and 20 women. The women in this study had lower muscle stiffness and ROM – and the individual variability between ankle ROM for the women, only correlated with stretch tolerance. Conversely, both stretch tolerance and muscle stiffness, correlated with the individual variability between the men [103].

The results from this study, could interpreted as men and women having different factors limiting their ROM. But the fact that the there was only a correlation between ROM and muscle stiffness for the men, while they also had higher muscle stiffness, could also mean that if muscle stiffness is high, lowering it will have a big impact on ROM. However, if muscle stiffness is low, lowering it further might not have any further effect on ROM.

 

I think the last explanation is the most likely. Furthermore, I find it likely that when you start working on your flexibility, the neurological factors and stretch tolerance, will be the most important factor in the early stages.
Now, stretch tolerance is quite plastic – it increases relatively quick, while muscle stiffness seems to take longer to impact in a meaningful way. This means that after some time, muscle stiffness is likely to become a limiting factor, because it will become relatively high, compared to your stretch tolerance.

Since all the studies we have looking at the relationship between muscle stiffness and ROM are looking at untrained, this could also in part explain the unclear relationship between muscle stiffness and ROM.

 

6.2 Muscle stiffness and stretching

A meta-analysis by Takeuchi, Nakamura [104] found stretching to be effective at decreasing the stiffness of the muscle – this a backed up, by a 2024 evaluation and recalculation of the data from earlier meta-analysis [53]

 

One major limitation for the meta-analysis by Nakamura (and to some extent the Warneke article as well), is that 9 of the 10 included studies examined the triceps surae – the calf muscles. We can’t say for sure, but I do find it very unlikely that the effects on other muscles would differ much, but more studies are needed.

 

The meta-analysis by Takeuchi, Nakamura [104] found no relationship between stretching volume and decrease in muscle stiffness was found – and since only one study reported the stretching intensity, no clear relationship between stretching intensity and muscle stiffness were found either.
The recalculated Meta-analysis by Warneke, Lohmann [53] found that only long duration and supervised stretching effectively decrease muscle stiffness.
The authors did not perform any subgroup analysis for intensity, likely because of the lack of reporting – but there is reason to believe that when subjects are stretching under supervision, stretching intensity might be higher.

 

There is no consensus on what constitutes long duration stretching, so for this meta-analysis[53], the authors used a median cutoff point for each study, based on the total time under stretch within a training session (If the participants performed 4x30s of stretching, 120s was used for further calculations.
This method, makes it impossible to give specific recommendations in regard to the stretching duration needed to most effectively decrease muscle stiffness – but it does make it possible to conclude, that longer total stretching durations pr. session, seems to be needed. From the cutoff points used in the study, I would recommend aiming for >120s of stretching of a muscle group pr. session, if the goal is to decrease passive stiffness.
For anybody seriously training flexibility, this amount of time should be easily reached.

 

6.3 Muscle stiffness and strength training

We have limited evidence on the effects of resistance training on muscle stiffness, but from what we have, resistance training seems to increase muscle stiffness [105]. This is supported by the fact, that strength training is effective at increasing muscle CSA, would correlates with muscle stiffness. However, strength training can also increase fascicle length, which would in theory decrease muscle stiffness.

 

One study compared the effects of 8 weeks of concentric and eccentric training for the hamstring on muscle stiffness and found increased muscle stiffness in the concentric group, but decreased muscle stiffness after eccentric training [106]

However, another study also compared the effects of 15 weeks of concentric and eccentric training and found increased muscle stiffness in the vastus lateralis in both groups [107].

 

It seems that the results are less clear for eccentric training. Eccentric training also seems to be more effective at increasing fascicle length, which would in theory decrease muscle stiffness. More data is needed, but it wouldn’t be surprising if eccentric training and/or training at long muscle length could minimize the increases in muscle stiffness, or possible reduce it.

 

6.4 Summary:

• Stretching can decrease muscle stiffness, but longer times under stretch pr. session seems to be needed (Most likely +120s total time under stretch pr. session). No analysis between total weekly stretching volume and decreases in muscle stiffness were found – it can’t be ruled out, that lower stretching durations and higher frequencies could also decrease muscle stiffness.
  I do find it likely that longer stretching durations pr. session is needed, but that is only due to the fact, that it seems to be more effective for acutely decreasing muscle stiffness

 

• The most recent meta-analysis found that supervised stretching was needed for decreasing muscle stiffness – Might be because the intensity of the stretching when supervised, most likely is higher. Other than that, the chronic effects of stretching intensity on muscle stiffness are unclear

 

• Strength training seems to increase muscle stiffness – it is possible that eccentric training and/or training at long muscle length, can at the very least minimize the increase in stiffness – possible maintain or reduce it

 

• The effects of muscle stiffness on ROM are yet unclear. Lowering muscle stiffness might be more important for increasing ROM, if you have high muscle stiffness, whereas people with low muscle stiffness might not benefit much from lowering their muscle stiffness further

 

7.0 Muscle-Tendon-Unit (MTU) Stiffness

So far, we’ve been examining the tendon stiffness and muscle stiffness in isolation. We’ve concluded that neither strength training or stretching reduces the stiffness of the tendons and that only stretching seems to decrease the stiffness of the muscle.

 

There are limited studies on the effects of strength training on MTU stiffness – the review article by Thomas, Ficarra [105] included 3, but one of these studies only had the participants perform 2 sessions [108]. The other two studies found no significant effect of strength training on passive MTU stiffness [109, 110]. This is no surprise, we’ve seen in the earlier sections, that strength training can increase muscle CSA, tendon CSA and stiffness, which would also contribute to muscle-tendon-unit stiffness.

 

We’ve also concluded that stretching decreases muscle stiffness and have no effect on tendon stiffness, so it might be surprising to discover that when looking at the muscle-tendon unit as a whole and when measuring passive resistance of joint movements (which could also be seen as a measure of the combined stiffness of all the structures crossing the joint) – static stretching does not seem to reduce stiffness [53, 111]. Although, in the meta-analysis by Takeuchi et al., there was a non-significant tendency of decreased overall stiffness [111].

 

We do have observational data, that found rhythmic gymnasts to have lower passive resistance to stretch compared to a non-stretching control group, measured in knee extension [112]. Similarly, ballet dancers were found to have lower passive stiffness in the calf muscles, compared to a non-stretching control group [56].

Since this is observational data, we don’t know whether these very flexible individuals have been able to lower their passive muscle-tendon-stiffness through years of training, or whether they were simply born this way. I think it’s likely to be a combination.
We’ve established muscle stiffness can be lowered through stretching, which was found in studies that usually last for 6-12 weeks. Now, if the same studies had the participants stretching for years, I find it likely that muscle stiffness would decrease far enough, to also make muscle-tendon-stiffness decrease significantly.

 

I do have some anecdotal evidence to back this up – but I want to highlight, that it is purely anecdotal, applying some critical thinking.
From my own personal experience, there are several positions, where I used to need lots of external weight, to access certain ranges. But after close to 10 years of flexibility training, the same load is no longer needed.
Other coaches who’ve focused on flexibility for a long time report the same observations, and I’ve seen it with people I’ve trained. Keep in mind, that there are a lot of uncontrolled factors and variables – and you’d be right to apply some healthy skepticism – but combined with what we know about changes in muscle stiffness, I find it likely that long term flexibility training, can also decrease muscle-tendon-unit stiffness.

 

8.0 Main takeaways

• Both stretching and strength training can be effective for increasing ROM

 

• Stretch tolerance seems to be the main adaptation that causes an increase in ROM after both strength training and stretching.
  It is very possible that the neurological adaptations account for most of the early increases in ROM and that the structural adaptations contribute more and more to increases in ROM, the longer you continue training.

 

• Structural adaptations, in the form of increased fascicle length can occur from both strength training at long muscle lengths and from high intensity stretching.
  Eccentric strength training at long muscle lengths, might be the most effective way to increase fascicle length.
  Increased fascicle length, seems to positively increase ROM, while also increasing strength at longer muscle lengths.

 

• Stretching, but not strength training can decrease muscle stiffness. This might not be an important factor for increasing ROM, unless you have high muscle stiffness or have been training flexibility consistently for >8-12 weeks or longer.

 

• Neither stretching nor strength training seem to be able to decrease the stiffness of the muscle-tendon-unit. Flexible subjects that have been stretching for years, have been found to have lower muscle-tendon-unit stiffness than an non-stretching control group – so it is still possible that stretching can decrease muscle-tendon-stiffness, but that the adaptations take longer to occur, than the length of the studies vi currently have available.

 

• Stretching doesn’t make your joints “loose”. Stretching doesn’t seem to make neither your tendons nor your connective tissue less stiff.
  And although stretching can decrease muscle stiffness, long term and intense stretching can increase both your strength and muscle size – and more active strength, easily makes up for the lower stiffness of the muscles.
  If you generally have low connective tissue stiffness (Either because of genetics, or disorders like HSD), stretching might not be the best option – but otherwise, don’t worry about getting “loose” or instable joints.

 

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