Okay, so last time we covered the effects of acute stretching on strength and performance and we learnt that strength and power might be impaired with stretch durations 60s/>. Now, let’s look at the longer term effects. Quite a provocative title: Stretch training is a waste of time, I know. But perhaps it could be; let’s look at the evidence commensurate with the intended outcomes and see what emerges.
What is Stretch Training?
What do we main by stretch training, also called flexibility training and what’s the goal? Typically stretching, no matter how long the application, is used to increase range of motion (ROM) and influence muscle-tendon properties. In rehab, this may be a focus post injury or post-surgery for example where the insult and period of reduced activity has resulted in a loss of ROM and a stiffening of the muscle-tendon unit. The training part just refers to a programme of stretching performed regularly and over a period of time.
What does Stretch Training Do?
If we stretch multiple times per week and for multiple weeks (training), then we can create a sustained increase in ROM. Not only that, research illustrates the potential benefits of stretching on muscle strength and hypertrophy. That’s right, stretching might also confer changes to muscle properties and cross-sectional area.
Stretching for Strength
A great example to illustrate the stretch-induced changes to muscle is a very recent paper by Warneke et al. (2023). They sought to investigate the effects of long‑lasting static stretching vs. hypertrophy resistance training on muscle strength, thickness and flexibility of the plantar flexors.
Participants performed either the stretch training, resistance training or nothing (control) for 6 weeks, and the results showed there were very little differences between the 2 interventions. That is to say muscle strength, thickness and flexibility increased by similar amounts in response to both interventions (10-13% changes pre- to post-intervention across all parameters). This is pretty profound, and might cause us to question the need for resistance training at all…but let’s look at what people actually did.
As already mentioned, the training protocols were each 6-weeks in duration, however, the stretching group committed to an hour per day of a prolonged stretch, achieved by an orthosis – that’s 7 hours per week of sitting around with a boot on their foot forcing the ankle into dorsiflexion. Whereas the resistance training group performed 5 sets of 10-12 reps of plantar flexions on a leg press 3 days per week; each session lasted approximately 15 minutes. Tot this up across the programmes and you see an approximate 10-fold difference in time commitment between groups: 42 hours for the stretch group, 4.5h for the resistance training group.
On balance, 42 hours of time training to receive only ~10% benefit in any of these outcomes seems a steep a commitment, especially when comparable changes can be achieved in a 10th of the time. Remember, the resistance training group also experienced positive changes in flexibility!
“Passive low intensity and self-performed stretching is unlikely to have an effect on muscle hypertrophy.”Nunes et al. (2020)
If we take a deeper and broader look at the literature what does it tell us? Can we replace resistance training for stretch training, or indeed sit back and assume stretch training will always result in positive muscular changes? A recent systematic review by Nunes et al. (2020) concludes not. Out of the 10 studies included in the systematic review, only 3 showed a positive effect on muscle hypertrophy assessed across a number of indices (see below). These studies also used an external device to overload the stretch or applied stretch alongside a resistance training programme.
Nunes et al (2020). Clin Physiol Funct Imaging. 00:1–9.
The findings of this systematic review taken together with Warneke et al. (2023) suggests that dose and intensity of stretch is really important to elicit change in muscular properties. Passive low intensity and self-performed stretching isn’t likely to have an effect on muscle hypertrophy, and just a note here, extending the training period does not compensate for this lower intensity either.
Stretching on Muscle Response Times
Let’s say you’re still committed to your stretching, you enjoy it and that’s fine, and maybe with an adequate dose (high volume & intensity) you can positively impact strength and cross-sectional area. BUT, what about timing of muscle activation? We may need to pay attention to this if we have specific sports performance demanding rapid muscle activation, or indeed have injury avoidance concerns.
Consider that mechanical joint loading during strenuous activities has the potential to exceed the tensile capacities of connecting tissue unless counteracted effectively and in a timely manner by the surrounding musculature (Minshull et al. 2007). What does that mean? It means the forces applied to joints and tissue might exceed its tensile capacity (become injured) if we don’t contract the surrounding muscles to protect/shield them.
Think about the typical non-contact mechanism of ACL injury: anterior tibial translation with tibial rotation. How quickly does this happen? Mere milliseconds, around 50ms to be precise according to Krausshaug et al. (2007) Forceful and importantly quick contraction of the hamstrings here can potentially restrain ‘excessive’ movement and the overloading of the ACL.
Last time we discussed an index called electromechanical delay (EMD). EMD is a component of neuromuscular performance that contributes to the timely, forceful and precise regulation of motor activation. This time delay exists from the onset of muscle activation to the onset of force development (see below). I mention this because in our consideration of muscle strength and hypertrophy changes in response to stretch training, we fail to consider the timing of muscle force production.
Given that almost half of the EMD under many circumstances is determined by the stretch of the series elastic component (tendon & apneurosis), any changes in the properties of viscoelastic structures might also confer changes to indices of performance determined by its stretch.
We studied this around 10 years ago, though I think it’s important to mention it now to illustrate this concept. We randomly allocated 18 males into one of 2 hamstring stretch training groups: passive (PASS) or proprioceptive neuromuscular facilitation (PNF). Participants underwent 8 weeks of stretching 3 times per week (full details of the paper here), 20s total stretch duration, stretch and overload applied by experimenters. After the 8-weeks of training both groups achieved a similar increase in passive hop flexibility (19%), with no changes in muscular strength (assessed at 25 deg knee flexion) – see below. Furthermore we saw a differential change in EMD; the muscle response times increased in both groups but by considerably more in the PASS group (10.8% increase) vs. PNF group (3.2% increase).
What does this mean? Well inferior force and importantly temporal (time-based) muscle performance characteristics at end-range joint positions might be important injury risk factors, especially given the elevated risk of serious injury to the ACL at extended joint positions, and flexibility training may negatively influence this.
Strengthening for Stretch
How about the other way round? Does stretching convey a better benefit than resistance training on ROM?
Well so far, it seems not. Afonso et al. (2021) set out to investigate this in their systematic review and meta analysis: Strength training vs. stretching for improving ROM. Data were pooled from 11 studies, comprising 452 people and it appears across a range of different metrics and joints, improvements in ROM were statistically similar following stretching and strength training (ES = −0.22; 95% CI = −0.55 to 0.12; p = 0.206).
We have to note here, as with many systematic reviews, protocols used by the individual studies heterogeneous, for example interventions lasted from 5-16 weeks, included 2-5 sessions per week, with different stretch modalities and with different hold times. Furthermore, many investigations involve interventions and assessments in the sagittal plane; generating resistance-based interventions that involve a rotational component to match stretching exercise may be a little more challenging.
Stretch Training Is A Waste Of Time …?
Overall what can we say about stretch training?
- Intensity and dose matter for muscle adaptation; high-intensity and longer duration stretches are likely required. So consider this if you want to improve strength &, or CSA
- Muscle strength & size changes are unlikely to come from self-regulated stretching – again consider this if you want to improve strength & or architecture from stretching
- Where increase ROM is required consider loaded (vs passive) stretching or resistance training (performed full ROM) as an alternative to preserve contractile capabilities at end-range
- It seems that stretch training performed in combination with resistance training may induce some additional muscle growth, though more research is needed
- Don’t just passively stretch without thinking about the intended goals & consequences
- Warneke et al (2023). Comparison of the effects of long‑lasting static stretching and hypertrophy training on maximal strength, muscle thickness and flexibility in the plantar flexors (LINK)
- Nunes et al (2020). Does stretch training induce muscle hypertrophy in humans? A review of the literature (LINK)
- Afonso et al (2021). Strength Training versus Stretching for Improving Range of Motion: A Systematic Review and Meta-Analysis (LINK).
- Minshull et al. (2007). Effects of acute fatigue on the volitional and magnetically-evoked electromechanical delay of the knee flexors in males and females (LINK).
- Minshull et al. (2013). The differential effects of PNF versus passive stretch conditioning on neuromuscular performance. (LINK)