I have had a few questions recently about why I suggest stretching prior to resistance training in the early phases of training when clients have limited flexibility. Numerous research studies have demonstrated that static and PNF stretching reduce the muscle's ability to produce force and power. What a lot of people don't realize is that not all studies have found impairments in force production after stretching. There are a lot of reasons for the huge variance in the findings of the studies. A lot of the early studies that found marked reductions in strength after stretching used unusually long stretching periods (up to half an hour on one muscle group) and did not employ warm-up sets, which means that the subjects went from maximal tension, long duration stretching to maximal muscle testing for strength. Clearly, this is not how personal trainers use stretching. Of course, researchers are interested in looking at the effect of one factor (stretching) on another factor (muscular performance). But to use this information to forbid pre-exercise stretching is not using the research data correctly.

All scientists working in this area would agree that stretching is not a warm-up. It's a modality of improving range of motion. The majority of studies have shown impairments in force production after stretching, and several studies have shown the mechanisms why; but recent investigations have failed to show any impact of stretching on performance when there was shorter, more realistic stretching protocols used, a time gap between the stretching and testing and the use of specific warm-up sets or drills.

Ultimately, personal trainers have to make a case by case decision as to who needs flexibility development and who does not. In the 1-3 hours per week that a trainer may work one-on-one with a client, I still advise trainers to stretch the tight muscles of their beginner clients prior to training with either static or PNF stretching. If you use short duration (total: ~60s per muscle group) stretching, then warm-ups sets, then a multiple set workout, then there should be no negative impact on performance (and if there is, the few percent change is hardly worth worrying about for a beginner). Of course, advanced trainees should reserve flexibility development to other times of the day or different days of the week.

Stretching is not a warm-up. So if it is used with beginner clients in the personal training session to improve range of motion so that the strength exercises can be performed more comfortably and through a greater range, then warm-up sets must be used afterwards to return neural drive to the muscle.

Take Home Advice:
For beginner clients who have flexibility issues or time-poor clients who won't stretch on other days by themselves: stretching of tight muscles with static and/or PNF stretches should be used prior to training. Limit the total duration to around 60 seconds per muscle group. Specific warm-up sets must be used afterwards.

For more experienced trainees who still have tight muscles: dedicated stretching sessions should be performed at other times of the day or on different days to strength training.