## 4 Discussion and Interpretation  
As per Table 1, there is no effect in the genotype for the LT speed (in all the performed measures both during the 4 weeks of adaptation and during the 8 weeks of LT training), exercise speed, exercise time, exercise distance and even lactate concentrations during exercise. This has a key importance, since it means that mice of the two different genotypes (KO and WT) had equal amounts of exercise training (speed, distance, time and frequency) and blood lactate levels. Therefore, genotypes effects seen for the other measures will be, indeed, due to the genotype itself (consequence of _Sparc_ KO) rather than difference in the exercise amount. SPARC (osteonectin or BM-40) is a three-modular-domain [35; 36] calcium binding extracellular matrix-associated glycoprotein [37; 38]. The _Sparc_ gene localized to the central region of chromosome 11 in mice [39] and in the chromosomal site at 5q31-q33 in humans [40]. It is well known for its roles in extracellular matrix (ECM) organization, growth, cellular differentiation, cell-matrix communication, wound healing, cell cycle and tissue response to injury [35; 36; 41; 42; 43]. SPARC is also implicated in metabolism [44; 45], cancer [46] and inflammatory [47] homeostasis. Importantly, for the SM, a key metabolic tissue and the key organ for the exercise performance, SPARC represents an important element for its development [28] and function [7]. Indeed, SPARC is known for its importance is SM development and regeneration (satellite cells/myoblasts. myotubes and muscle fibers) [48]. Moreover, whereas during embryogenesis SPARC is highly expressed, its expression is mainly restricted to tissues undergoing changes and remodeling during adulthood [35; 49; 50; 51] which indicates its importance for exercising muscle; which does undergo remodeling as an adaptation to exercise [52; 53]. Importantly, SPARC modulates actin cytoskeleton within the SM structure which results in defective force recovery following in vitro fatigue stimulation in muscle from _Sparc_ KO mice [54]; but in normal and uninjured muscles, SPARC is not detectable [48]. This further indicates the importance of SPARC in the context of healing, repair, remodeling and development, especially that the ECM (important for the cellular remodeling, for instance) repair, disassembly and degradation is mediated by SPARC [55]. These impacts on regeneration and during embryogenesis suggest that SPARC deficiency could impact some tissue development and growth, as illustrated by the loss of bone mass (osteopenia) in _Sparc_ KO mice [56]. Another structural importance for SPARC in SM derives from its ability to interact with collagens. It interacts with collagen I and procollagen I [57], binds to fibrillar collagens [58], maintains SM stiffness (collagen accumulation regulation) [59] and specifically binds several molecules, including collagen  
Figure 1: Expression of both collagen type I alpha 1 chain (COL1A1) (**A**) and mitochondally encoded cytochrome c oxidase I (MT-CO1) (**B**) in the tibialis anterior muscle. The results indicate an effect of genotype\({}^{\text{G}}\) (WT \(>\) KO) for both proteins and an effect of exercise\({}^{\text{Ex}}\) (Ex \(>\) Sed) for COL1A1 (trend) and MT-CO1. For the interactions, we have one between genotype and exercise\({}^{\text{G}\times\text{Ex}}\) for COL1A1 (Ex \(>\) Sed in WT) and one between age and genotype\({}^{\text{A}\times\text{G}}\) for MT-CO1 (high in Y-WT). All data are mean \(\pm\) SEM. The number of mice: 11–12 mice per experimental condition. Abbreviations: A, age; Ex, exercise; G, genotype; KO, knockout; O, old; Sed, sedentary; WT, wild-type; Y, young. *: Trend (0.05 \(\leq p<\) 0.1). types I, III and IV [60]. SPARC deficiency has also been shown to reduce the expression of different types of collagen such as collagen type I in mesangial cells [61], collagen in skin [62] and fibrillar collagen accumulation in tibialis anterior muscle [59].