Passive force enhancement is a mechanical property of skeletal muscle that was discovered two decades ago (Herzog and Leonard, 2002). It is defined as the increase in steady-state passive force observed following deactivation of an actively stretched muscle compared to the corresponding passive force following stretching of the same muscle to the same length passively. Passive force enhancement has received recent attention (e.g., Herzog, 2019) because it is thought to be the primary contributor to the residual force enhancement property (Abbott and Aubert, 1952, de Campos et al., 2022, Edman et al., 1982, Joumaa et al., 2007, Joumaa et al., 2008b).

Passive force enhancement is known to increase with increasing stretch magnitude (Herzog and Leonard, 2002), is long lasting (Herzog et al., 2003), and occurs primarily at muscle lengths where passive forces occur naturally (Herzog and Leonard, 2002). Although the origins of passive force enhancement have not been studied much, its occurrence in single myofibrils and sarcomeres suggests that it is related to the filamentous protein titin (Joumaa et al., 2007, Joumaa et al., 2008b), as titin is the lone structure to provide passive force in these preparations (Bartoo et al., 1997, Leonard and Herzog, 2010). Residual force enhancement and passive force enhancement are small or absent in calcium-activated preparations in which cross-bridge cycling is inhibited either chemically or through disruption of the regulatory proteins (Joumaa et al., 2008b, Labeit et al., 2003, Leonard and Herzog, 2010). However, residual, and passive force enhancement are substantial when muscles are stretched and cross-bridge cycling is allowed, suggesting that it is not calcium activation but cross-bridge cycling that permits for residual and passive force enhancement to develop even at lengths beyond actin-myosin filament overlap (Powers et al., 2017).

The mechanisms underlying passive force enhancement remain unknown despite its apparent importance in actively lengthening muscle. The sole theory proposed to date is that passive force enhancement is caused by a stiffening of titin when a muscle is actively stretched and that this stiffening is maintained when a muscle is deactivated (Herzog, 2019). Stiffening of titin is thought to occur in two principal ways: (i) a stiffening of the full-length titin upon muscle activation caused by binding of calcium to specific titin domains (DuVall et al., 2013, Joumaa et al., 2007, Joumaa et al., 2008b, Labeit et al., 2003, Power et al., 2012, Rassier, 2012), and/or (ii) by binding of the proximal titin segments to actin, thereby eliminating these segments from extending when a muscle/sarcomere is stretched (Dutta et al., 2018, DuVall et al., 2017, Edman et al., 1982, Herzog and Leonard, 2002, Nishikawa et al., 2020, Noble, 1992, Rode et al., 2009). In the deactivated state in which passive force-enhancement is observed, the first of these mechanisms (calcium binding to titin) may be eliminated as it has been shown that calcium-titin binding ceases to exist when a muscle is deactivated (DuVall et al., 2013).

Herzog et al. (2003) showed that passive force enhancement is long lasting (minutes) in cat soleus muscles. They also found that passive force enhancement was abolished instantaneously by shortening the passive soleus to its pre-stretched length and stretching it back to its stretched length (Herzog et al., 2003). This incidental observation has not been repeated, but it suggested that passive force enhancement is abolished by a mechanical trigger associated with shortening of the muscle. If confirmed, this observation could potentially provide crucial hints as to the molecular mechanisms of the development and release of the passive force enhancement property.

The primary purpose of this study was to test if indeed the passive force enhancement can be abolished by shortening single skeletal muscle fibres to their pre-stretched length. We hypothesized, in analogy with the observation made on the cat soleus muscle, that passive force enhancement is abolished instantaneously by shortening a fibre to its pre-stretched length. However, this hypothesis was rejected by our primary experiments motivating additional experiments to probe further into the potential mechanisms causing passive force enhancement.