Badminton is a racket sport which is characterized by a temporal structure with actions of short duration and high intensity coupled with a short resting time, as recently reviewed (Cabello Manrique and Gonzalez-Badillo, 2003). The number of different shots used during a game can vary a lot, allowing numerous tactical choices (Hong and Tong, 2000). This sport gathers five disciplines, including Men's and Women's singles, doubles and mixed doubles, each of them requiring a specific preparation in terms of patience, control and physical fitness (Chen and Chen, 2008; 2011; Laffaye, 2011; Pearce, 2002). For more details, a recent review summarizes all the characteristics of this game (Phomsoupha and Laffaye, 2015).
Since this sport became Olympic in Barcelona in 1992, few studies reported temporal structure at a definite moment according to the total time, working time, resting time, effective playing time (EPT) (sum of the rally times divided by the match duration multiplied by a hundred) and shot frequency (number of shots divided by the effective playing time) (Abian-vicen et al., 2013; Cabello Manrique and Gonzalez-Badillo, 2003; Faude et al., 2007). During elite player matches, mean rally and rest duration revealed a high variability, with values respectively ranging from 4.6s to 9.0s and 9.7s to 24.1s (Abianvicen et al., 2013; Cabello and Lees, 2004; Cabello Manrique and Gonzalez-Badillo, 2003; Chen and Chen, 2008; Faude et al., 2007; Ming et al., 2008). In the literature, the EPT ranged from 27.3 [+ or -] 2.4% (Abianvicen et al., 2013) to 38.5 [+ or -] 3.8% (Chen and Chen, 2011), with a mean value of 32.1% (Phomsoupha and Laffaye, 2015).
Another way to analyze matches has been recently proposed, by using a notational analysis with video recordings focusing on different kinds of shots and on the way the point is won (direct point, unforced error and forced error) (Abian-vicen et al., 2013; Hong and Tong, 2000). For instance, a recent study on the Beijing Olympic Games analysis revealed a percentage of unforced error at about 41.0 [+ or -] 9.4% while the best shot for finishing a rally by direct point is the smash (29.1 [+ or -] 8.4%) (Abian-vicen et al., 2013). In another study with national level players (Taiwanese players), the percentage of unforced errors increases to 61.5% suggesting that this variable depends on the expertise (Chen and Chen, 2008). Liddle et al (1996) reported that 54.0% of the shots are overheads in singles, while a few years later, another study showed that the three most popular strokes were the smash, the overhead clear and the overhead drop (Ghosh et al., 2008). However, Ming et al. (2008) showed another stroke repartition with more clears, lobs and net shots. This suggests that the Badminton game has considerably evolved over the last two decades and that the notational analysis appears to be a good way to understand this change over time.
Up to now, studies on Badminton game characteristics focused on isolated match analysis. The aim of this study is to analyze the change of the Badminton in Men's singles through the notational and temporal structure of all the Olympic finals since 1992. It is hypothesized that Badminton has become more intensive with a higher number of offensive strokes. For validating this hypothesis, it is expected to observe an increase in the shot frequency, a decrease in the rest time and a change in the distribution of the strokes.
Period and matches
The matches selected are the Men's singles finals in all Olympic Games since 1992 (Table 1). The different finals (1996: Atlanta (OG-96), 2000: Sydney (OG-00), 2004: Athens (OG-04), 2008: Beijing (OG-08) and 2012: London (OG-12) were recovered from the archives of French Federation of Badminton, and the final of 1992: Barcelona (OG-92) from a private recording of a Chinese television broadcast. The mean age of the players is 25.8 [+ or -] 2.78 years. In total, 537 rallies and 5537 strokes have been analyzed. To be more accurate as possible, we choose to study only balanced high level matches. This was not the case for one of the semi-final during OG-00 (15-12, 154), OG-04 (15-9, 15-2) and OG-12 (21-12, 21-10). Consequently, we analyzed only finals from OG-92 to OG-12.
The study received approval from the University's ethics committee.
The film footage was analyzed frame-by-frame using Dartfish (Dartfish 4.5.2, Fribourg, Switzerland) at a frequency of 25 Hz. The movements of the players were filmed from a front-on and up-side perspective. Two categories of variables were recorded:
(i) the temporal variables were defined as proposed in the literature and include the rally time (time elapsed from the serve until the shuttlecock hits the ground or one of the players makes a mistake), number of shots per rally (total number of times the shuttle is hit by both players during the rally time), stroke time (rally time divided by the number of shots per rally), the shots' frequency, the resting time (the time elapsed from when the shuttlecock hits the ground until the next serve), the effective playing time (Abian-vicen et al., 2013; Cabello Manrique and Gonzalez-Badillo, 2003; Chen and Chen, 2008; Faude et al., 2007);
(ii) the notational variables include the different shots and the way the point is ended, and are defined in the following manner: (1) the smash is an aggressive overhead shot with downward trajectory, (2) the clear is an overhead shot with a flat (offensive clear) or rising trajectory (defensive clear) towards the back of the opponent's court, (3) the drop is a smooth shot from above the head with downward trajectory towards the front of the court, (4) the net shot is a precise shot from near the net which includes the net drop, the lob (offensive with a flat trajectory towards the back of the opponent's court and defensive with a rising trajectory) and the kill (aggressive shot with downward trajectory), (5) the drive is a powerful shot made at middle body height and in the middle of the court with a flat trajectory, (6) a direct point is a point which ends when the shuttlecock directly hits the ground, (7) a forced error is when the player is under excessive pressure from his opponent and makes an error after doing his shot (which goes in the net or outside the court) and (8) an unforced error is when the player makes an error in an expected situation without excessive pressure from the opponent (Abian-vicen et al., 2013; Cabello Manrique and Gonzalez-Badillo...