Is stretching a valuable intervention? Recently, we have accumulated more evidence suggesting the lengthened position of a muscle is the most important stage of a movement for muscle growth. We are now starting to accrue evidence that stretching may induce hypertrophy as well.
In this recent study (PMID: 36685189) 55 athletically active and pain-free participants (28 male and 27 female) completed a stretching protocol previously used in two studies (both by Warnecke et al. in 2022). Each participant was given a calf stretching orthosis and used it for 1 hour a day (no breaks) for 6 weeks on one of their legs. The stretch was applied in sitting with the knee extended. The intensity of the stretch was a 7-8 out of 10 (10 = maximal point of discomfort) meaning it was an aggressive sustained stretch. At the start and conclusion of the 6-week period, ankle range of motion and strength was tested with the knee bent and fully extended. Calf muscle thickness was tested with ultrasound. Both legs were tested for each subject (one received the stretch intervention and one did not), providing a within-subject design, and addressing individual variability.
The results were similar to the previous studies by Warnecke. Stretching alone resulted in significant improvements across the board. There were, however, differences between males and females. With the knee extended, women improved strength by 8.7% and range of motion by 13.3%, while men improved by 15.5% and 21.4%, respectively. With the knee flexed, women improved strength by 6.2% and range of motion by 6.0%, while men improved by 8.3% and 15.5%, respectively. Lastly, for women, muscle thickness of the medial head improved by 4.2% and the lateral head improved by 5.2%, while men improved by 14.5% and 4.5%, respectively. The only significant change in the control leg was a 4.5% increase in knee extended strength in men.
Why were there sex differences? There are a couple of potential reasons, but we need future research to help answer the question. In general, females are more flexible than men. Equating stretch intensity and duration may not have yielded the same stimulus, like equating resistance training volume for people with different amounts of muscle mass. The stretch intensity was subjectively rated too. Perhaps the women in this group had a greater tolerance for stretch discomfort, leading to a difference in tension. The groups may have differed in baseline fitness as well. There was a floor for physical activity in the recruitment phase but details about training history were not gathered. Regardless of the difference, both men and women experienced substantial improvement in strength, power, and muscle size following a 6-week stretching intervention.
Does this mean stretching can replace resistance training for muscle development? No. Resistance training is more efficient and yields other benefits, such as improved bone mineral density, strength for multi-joint movements (e.g., squats), and cardio-pulmonary health. The stretching protocol is limited to a single muscle. We can train and hypertrophy more than a single muscle with 1 hour of resistance training. Stretching could be a useful adjunct if you are limited on time. This device allows you to develop your calves while sitting in a chair and working. It will be interesting to see if similar effects are achieved with hamstring, adductor, or quad stretches. You will always need to consider total training volume as it appears end-range aggressive stretching can tax a muscle in a similar manner as resistance training.