The calf muscles in the dominant leg were measured before the trial, at the end of weeks one, three, five, and six. Simpson et al. found that at day five, “the stretched [medial gastrocnemius] fascicles were longer than baseline and longer than the stretched [lateral gastrocnemius].” It took longer for the lateral gastrocnemius to lengthen. Muscle thickness increased by 5.6 percent, pennation angle decreased by 7.3 percent at week one, 8.76 percent at week three, and 9.53 percent at week six.
They also found that different parts of the calf muscles responded differently to the stretching. While there is no change in the length of the Achilles tendon, stretch training increased both gastrocnemius thickness and unequal fascicle lengths between the medial and lateral gastrocnemii.
“Irrespective, over the duration of training the experimental (stretched) leg changed, the unstretched leg and the control group did not,” she told Massage & Fitness Magazine in an email interview.
While this study shows promising evidence that chronic stretching with the right amount of load could elicit muscle hypertrophy, Dr. Jakobi told us that this is not a “quick fix” to getting swole.
“Skeletal muscle is extremely malleable/plastic. This potential for adaptations in muscle fascicles, albeit preliminary, does indicate that clinicians across all fields should recognize that stretching has many different levels of adaptation,” Jakobi described. “It requires both time within it stretch (note these stretches were held for three minutes) as well as a duration for adaptation. This was a very long stretch training experiment relative to most. Thus, these facts highlight that principles (frequency, intensity, time, type) once reserved for strength and endurance training to induce skeletal and cardiorespiratory adaptations like hold a place in stretch training.”
It may be possible that this study on calf muscles could be applied to other muscles, like the rotator cuffs, hamstrings, and pectoralis muscles, but there some considerations so that clinicians and trainers do not jump to conclusions.
“One must consider how these muscles (notably rotator cuff and pecs) are muscle groups that have many different contributions to a movement. The shoulder for example requires both rotation as well as stabilization,” Dr. Jakobi added. “To find a position to induce change in one group, while not limiting another would be a great challenge. In addition to gaining appropriate positioning, stretch training involves no active contraction. This would be extremely difficult in these muscle groups. There is greater potential for the hamstring. Additional to the fascicle orientation controlling synergists would be easier but consideration must be given to muscles that surround and stabilize the pelvis or assist with hamstring activity (gluteus medius, for example).
“We are continuing to exploring this conundrum relative to absolute strength. As for athletic performance, again promising but one must recognize that the measures you suggested are dynamic and involve many muscle groups.”
“The literature is equivocal,” Dr. Jakobi said. “But what is exciting is how many more scientists are working to determine the underlying mechanism and benefit of stretch training. Controversy will remain until a number of studies are done with similar training protocols to repeat and validate current observation. The largest factor is where does the athlete, coach, and clinician want the level of adaptation to occur (strength, flexibility etc.,) AND when is that adaptation needed. I would suggest the stretch training protocol done during training and off-season is very different than the pre-performance and competition stretching routine. Thus, stretching just like sprint and endurance training might have unique adaptations based upon FITT factors identified above as well as type (dynamic, static, PNF).”
So how could future research on this topic be better, such as methodology?
“Technology continues to rapidly advance and this will change the type of investigations that are undertaken across the field of neural, muscle and tendon research; inclusive of but beyond stretching. Methodology is one element, but experimental design should also be considered. The key element is ensuring that stretch training does not induce activation of the muscle.
“Irrespective of the FITT elements, the muscle must remain inactive to be considered a stretch. I think this element, whereby we recorded EMG during every training session to ensure that the muscle was not contracting, should be used by scientists as well as clinicians.”
We'd like to thank Dr. Jennifer Jakobi for her time and effort in this interview and her colleagues at UBC Okanagan for their amazing work.
1. Simpson CL, Kim BD, Bourcet MR, Jones GR, Jakobi JM. Stretch training induces unequal adaptation in muscle fascicles and thickness in medial and lateral gastrocnemii. Scand J Med Sci Sports. 2017 Jan 30. doi: 10.1111/sms.12822.
2. Behm DG, Blazevich AJ, Kay AD, McHugh M.Acute effects of muscle stretching on physical performance, range of motion, and injury incidence in healthy active individuals: a systematic review. Appl Physiol Nutr Metab. 2016 Jan;41(1):1-11. doi: 10.1139/apnm-2015-0235. Epub 2015 Dec 8.
3. Kay AD, Blazevich AJ. Effect of acute static stretch on maximal muscle performance: a systematic review. Med Sci Sports Exerc. 2012 Jan;44(1):154-64. doi: 10.1249/MSS.0b013e318225cb27.