Can weight training really enhance your flexibility?
Some readers may believe that strength training and flexibility training are incompatible – that they will reduce their flexibility by lifting heavy weights. This is a myth: research carried out in the 1960s and ‘70s proved that correctly performed resistance exercise does not have a negative impact on flexibility(1).
In fact, I will go one step further and argue that certain types of resistance training can actually increase flexibility.
The first piece of research to demonstrate this came from the University of Sao Paulo in Brazil, where a team of researchers assessed two groups of elderly women on the sit-and-reach flexibility test at the beginning and end of a 10- week study period(2).
During that time, a training group of 11 women followed a supervised strength training programme three times a week. Each workout consisted of the following exercises, all using resistance machines: chest press, seated row, shoulder press, seated biceps curl, seated triceps press, leg press, calf press, ab crunch. No flexibility exercises were included in this training programme. Meanwhile, a matched control group of eight women performed no training at all during the study period.
By the end of the study period, the training group had significantly improved their sit-andreach scores, while the control group showed no change. The researchers concluded that a regular series of strength exercises without any element of flexibility training could improve the flexibility of elderly women as measured by the sit-andreach test.
This is an interesting finding – an unexpected adaptation to strength training. But the study had limitations that need to be taken into account. First, all the subjects were over 60 and, while healthy, had no track record of fitness training. It could be that for this type of adult any kind of activity would be enough to promote flexibility improvement. A young healthy adult, however, might not respond in the same way to a general strength programme.
The second limitation of the study is that the sit-and-reach test measures only spine and hip flexion and, while common, is quite a nonspecific test(1).
A subsequent study, also on elderly subjects (aged 55-83), used specific range-of-motion tests for neck, shoulder, hip, knee and ankle to assess the impact of resistance training on flexibility(3). This group of US researchers studied 43 subjects split into two groups, both following a 10-week, thrice-weekly training programme of Body Recall classes – an exercise routine, franchised across the US, which is specifically designed as a painfree, rhythmic workout to promote health and fitness.
The difference between the two groups was that one used hand and ankle weights while performing the routines and the other used body weight alone.
At the end of the study period, the researchers found that, even though the resistance group used only light weights (1-3 lbs), they significantly bettered the control group on five out of 10 measures of flexibility: neck rotation left and right, hip extension, knee flexion and ankle dorsiflexion.
The researchers concluded from their findings that, because the exercises in the classes were performed through a full joint range of motion, the increased resistance promoted extra flexibility adaptation.
On an anecdotal basis, you can see that this must be true simply by observing elite weightlifters. These athletes must have excellent range of motion in their ankle, knee, hip and shoulder joints to perform movements like the full snatch with correct technique.
Documented support for this concept in younger athletes has come from Soviet research, which showed how active stretching combined with strength exercises can reduce the gap between the passive and the active range of motion(4).
Active vs passive range of motion
The passive range of motion, as explained above, is the stretch produced by an external force or person and is usually greater than the active range of motion, which is the stretch you can generate yourself by using the strength of the opposing muscle group. Imagine, for example, stretching the hamstrings by lying on your back and lifting your leg with the quadriceps and hip flexors only, and then achieving the same feat by pulling with your arms. You are most likely to increase the stretch in the hamstrings with the latter technique.
The findings of this study were that an active stretching programme combined with strength training could reduce the passive/active deficit which, the researchers claimed, was correlated with athletic performance. And it would seem logical that the range of motion athletes can generate themselves has a closer link with performance than the range resulting from an external force.
Explaining how resistance training can increase flexibility is more difficult. In fact, adaptation to flexibility training is one of the least well understood areas of sports science research. Interestingly, the textbook by Alter referenced earlier in this article does not contain a single chapter on adaptations to flexibility training(1).
A study by a Danish research team is more illuminating(5). They used specifically-designed apparatus to measure the passive force response of the hamstring muscle to an imposed stretch and noted the following findings:
- Holding a static stretch for up to 45 seconds produces a relaxation effect on the muscle, reducing the resistance within the muscle to the stretch; this is the expected viscoelastic response to strain;
- The relaxation effect lasted only one hour;
- Long-term adaptation to a daily static stretching routine showed that the resistance within the muscle to the stretch did not decrease for a given angle, but the maximum joint range of motion did increase. This suggests that flexibility training increases the stretch tolerance – ie the ability to withstand the sensation or force of the stretch;
- Subjects with poor flexibility levels demonstrated a lower stretch tolerance and increased resistance to the stretch than subjects who had good levels of flexibility.
The relevance of these findings to the issue of developing flexibility through strength training is that they show that the response of the muscle tendon unit to static stretching is unclear. Performing a stretch will relax and lengthen a muscle in the short term; but the development of flexibility in response to regular static stretching is not the result of chronic changes in muscle tendon resistance to stretch but a tolerance to the stretch itself.
If this is the case, then it would seem that dynamic or active stretching would be just as effective as static stretching, and more relevant for athletic movements. If the muscle adaptation required for increased flexibility is to become more tolerant to being stretched, then simply taking a muscle through a full range of motion, inducing a stretch, should be sufficient to stimulate increased flexibility. The static hold/relaxation part of stretching seems less relevant.
In addition, if less flexible subjects have increased stiffness (greater muscle resistance to the stretch), as the Danish research suggests, then it may be that the benefit of adding weights to dynamic movements is that it enables greater range of motion due to the extra force applied to the muscle.
That last paragraph is my own hypothesis, supported by personal experience of increasing range of motion in athletes with tight shoulder joints, using large range dynamic movements with moderate weights. I believe that sports science may still have some way to go before we fully understand the mechanisms and most effective methods of developing flexibility.
If any readers know of further research on this subject, I would be happy to hear from them. Meanwhile, however, I can recommend with some conviction that resistance exercises and dynamic movements involving body weight, which take the joints through a full range of motion, will promote increased flexibility effectively.
- Alter, 1996. The Science of Flexibility, Human Kinetics, p151
- Journal of Strength and Conditioning Research, 2002, 16(1), 14-18
- Journal of Strength and Conditioning research, 2003, 17(2), 374-378
- 1983, Soviet Sports Review 18(1), cited in Alter 1996
- Scandinavian Journal of Medicine and Science in Sports 1998, 8, 65-77