What happens when we get more flexible?

When it comes to determining what adaptations lead to increased flexibility, two main theories have been proposed: The sensory theory, and the mechanical theory (Weppler and Magnusson 2010). To understand the difference, we’ll first need to discuss passive resistance to stretch.

Passive resistance to stretch: As you move a joint into the end ROM, there resistance to the stretch will increase, even if there is no muscle activation. This is because the muscles, tendons, ligaments, etc., works almost like elastic bands.
A decrease in passive resistance to stretch, would at least in theory have a positive effect on ROM – less force would be needed to reach a certain position the maximum ROM would most likely increase. For the passive resistance to decrease, some structures would have to adapt, either becoming longer, or less stiff. In accordance with this, several cross-sectional studies have found the flexible individuals have a lower passive resistance to stretch, than those less flexible (Blazevich et al. 2012; Abellaneda, Guissard, and Duchateau 2009; Moltubakk et al. 2018; Moltubakk et al. 2016; S.P. Magnusson et al. 1997).

Regular passive stretching for 6-8 weeks doesn’t seem to have any effect on the passive resistance to stretch, or lead to any structural adaptations (Freitas et al. 2017), but a few studies have found increase fascicle length after stretching, using external load to increase the intensity of the stretch (Simpson et al. 2017; Freitas and Mil-Homens 2015). Longer fascicles are practically the same as longer muscle fibers, which would lead to a decreased passive resistance to stretch.

Several studies have found that strength training can be effective in increasing fascicle length (G.E. McMahon et al. 2014a; G. McMahon et al. 2014b; Valamatos et al. 2018; Aquino et al. 2010); however strength training may also lead to stiffer tendons (Seynnes et al. 2009; Kongsgaard et al. 2007; Farup et al. 2014; Arampatzis, Karamanidis, and Albracht 2007) and increased pennation angle (Aagaard et al. 2001; Suetta et al. 2008; Reeves, Narici, and Maganaris 2004; Folland and Williams 2007; Bloomquist et al. 2013), which both contribute to increase the passive resistance to stretch. The effect of strength training on the passive resistance to stretch is therefore still relatively unclear.

Flexibility can also be increased without any change in passive resistance to stretch, or any structural adaptations occurring. In this case, the increase in flexibility can be explained by neural adaptions, the main adaptation being an increased stretch tolerance (Magnusson et al. 1996; Ben and Harvey 2010; P. Magnusson and Renström 2006; Ylinen et al. 2009; Halbertsma and Goeken 1994; S.P. Magnusson 1998; Weppler and Magnusson 2010).
Stretch tolerance is the ability to tolerate the stretching sensation – an increased stretch tolerance, would mean that stretching to a certain point, would feel less intense. It would thus be possible to stretch further, but getting to the new maximum ROM would also require more force to be applied.

References:

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