Optimal training load periodization is a key factor in achieving maximal performance during the major event of the sports season (Avalos et al., 2003; Bosquet et al. 2007; Fry et al., 1992; Mujika et al., 1995; 1996a). The most usual program is two to four weeks of overload training followed by one to three weeks of training with a decreased load, known as taper (Houmard and Johns, 1994; Kenitzer, 1998; Mujika et al., 1995; 1996a; 1996b; 2002; Mujika and Padilla, 2003; Thomas and Busso, 2005; Thomas et al., 2008).
By consensus, the optimal strategy is assumed to consist of maintaining the training intensity while reducing the training volume (up to 60-90%), and maintaining or only slightly reducing the training frequency (no more than 20%) (Bosquet et al., 2007; Mujika and Padilla, 2003; Pyne et al., 2009). Concerning the pattern for reducing the training load, progressive non-linear tapers have been described as more beneficial to performance improvement compared with step tapers (Banister et al., 1999; Bosquet et al., 2007; Mujika and Padilla, 2003).
Most research has emphasized the importance of the overload training period preceding the taper, during which the increase in both training volume and intensity delays the stimulation of biological adaptations via an overcompensation process (Avalos et al., 2003; Bosquet et al., 2007; Fry et al., 1992; Thomas and Busso, 2005; Thomas et al., 2008). Moreover, overload volume and/or intensity training leads to a set of acute physiological and psychological disturbances that can limit short-term performance capacity (e.g., glycogen depletion, neuromuscular fatigue, decrements in red cell volume and hemoglobin, imbalance in anabolic and catabolic tissue activities, disturbance in the athlete's psychological status) (Mujika and Padilla, 2003). Consequently, to ultimately improve performance, the major challenge during the taper period is to maintain or further enhance the physiological adaptations while allowing the psychological and biological stresses of the overload periods to resolve (Bosquet et al., 2007; Houmard and Johns, 1994; Kenitzer, 1998; Mujika and Padilla, 2003; Thomas and Busso, 2005; Thomas et al., 2008).
Few studies have investigated the training load dynamics of the overload and taper periods before a major event (Avalos et al., 2003; Busso et al., 2002; Busso, 2003; Thomas and Busso, 2005; Thomas et al., 2008). Using mathematical models, these authors suggested interaction effects between training adaptation and fatigue dissipation over time. For instance, a greater training volume and/or intensity before the taper was shown to result in higher performance gains, but this required a greater reduction in the training load over a longer period (Avalos et al., 2003; Busso et al., 2002; Thomas and Busso, 2005; Thomas et al., 2008).
To ensure maximal performance improvement, the scheduling and durations of both the overload and taper periods need to be individually tailored to each athlete, taking into account the individual training profile and capacity to recover from the stress of the high daily training (Avalos et al., 2003; Hellard et al., 2005; Mujika et al., 1996a; 1996b). Previous research (Mujika et al., 1995; 1996b) has distinguished two major profiles of training response. The first is characterized by a temporary decline in performance level (as a consequence of training load-induced fatigue) followed by a long delay in the enhancement of performance via the overcompensation process (long delay in eliminating accumulated fatigue and in further enhancement or maintenance of physiological adaptations). The second profile is fast physiological adaptation to the training load without temporary performance decline (fast decay of fatigue concomitant with a continuous increase in physiological adaptations). Avalos et al. (2003) suggested longer recovery periods for older male sprint swimmers (late responders) than for young female middle-distance swimmers (early responders).
The adaptation of training load designs according to age, stroke specialty and swim standard is fundamental to ensure performance improvement throughout the athlete's career (Avalos et al., 2003; Stewart and Hopkins, 2000a; 2000b). Although several studies have pointed out changes in training response over time (Avalos et al., 2003; Busso et al., 1997; 2002), no study to our knowledge has yet analyzed these changes in response to the overload training period and taper (for some swimmers throughout their entire careers) (Pyne et al., 2009).
Thus, the aim of this exploratory observational study was to identify the most influential training designs during the final six weeks of training before a major swimming event, taking into account athletes' changes in training response over several seasons.
Fifteen female and 17 male elite swimmers were followed for one to nine consecutive years. Their mean age, body mass and height at study inclusion was 18 [+ or -] 2 years, 59 [+ or -] 4 kg, and 1.68 [+ or -] 0.05 m for females, and 21 [+ or -] 3 years, 73 [+ or -] 5 kg and 1.84 [+ or -] 0.06 m for males. Eight females specialized in the 50-m and 100-m events, while the other seven swam middle-distance races: the 200-m and 400-m events. Six males specialized in the 50-m and 100-m events, six in middle-distance races: the 200-m and 400m, while the other five were specialists in a long-distance event: the 1500-m. The study was reviewed and approved by the local University Committee on Human Research and written informed consent was obtained from each participant. The swimmers trained according to the program prescribed by their coaches, and the characteristics of the training regimens and competition schedules were not modified by the present study. Values and changes in annual number of kilometers and in the best annual performances for all subjects over the ten seasons studied are indicated in Tables 1 and 2.
Studied periods and performance-related measures
The final six weeks of training (F6T) preceding the national championships, held in May each year, were studied from 1996 to 2004. Two distinct periods composed F6T: the overload training period (OP) covered the first three weeks and the taper period (TP) covered the last three weeks.
Three performance standards were considered: national, European and Olympic (i.e., participation in national, European, and Olympic or World Championships). Athletes' performances were measured in real competition (final events only), in the stroke and distance of each swimmer's main event. Performances were expressed as a percentage of the world record for the same stroke, distance and sex, in order to scale values for different swimming events (P). The performance change (AP) between mid- and post- F6T was calculated, i.e., the difference between the performance following OP, recorded during preparatory events, and the performance following TP, recorded during major events such as the national championships. A positive value indicated improved performance (faster performance after TP than OP).
Intensity levels for swim workouts were determined as proposed by Mujika et al. (1996a) and detailed in Avalos et al. (2003). An incremental test to exhaustion was performed at the beginning of each season (repeated and adjusted 4 times per season) to determine the relationship between blood lactate concentration and swimming speed. Each subject swam 6 x 200-m at progressively higher percentages of their personal best competition time over this distance, until exhaustion. Lactate concentration was measured in blood samples collected from the fingertip during the 1-min recovery periods separating the 200-m swims. All swimming sessions were divided into five intensity levels according to the individual results obtained during this test: swimming speeds 1) below ~ 2 mmol*[1.sup.-1]; 2) at ~ 4 mmol*[l.sup.1], the onset of blood lactate accumulation; 3) just above ~ 6 mmol*[1.sup.-1]; 4) at ~10 mmol*[l.sup.-1]; and 5) at maximal swimming. Workouts in the water were quantified in meters per week covered at each intensity level. Strength training included 6) dry-land workouts and 7) general conditioning (workouts involving activities like cycling, running, cross-country skiing, and collective sports) and was quantified in minutes of active exercise (Hellard et al., 2005; 2007). To scale the intensity values, the weekly training volume at each intensity level was expressed as a percentage of the maximal volume measured at the same intensity level throughout the F6T period for each subject (see Avalos et al., 2003; Hellard et al., 2005; 2007, for a full explanation of this method).
These intensity measures were synthesized as follows: the low-intensity training load, [w.sup.LIT.sub.t], was the mean weekly training volume expressed in percentage terms of intensity levels 1 to 3 at the [t.sup.th] week; the high-intensity training load [w.sup.HIT.sub.t], was the mean weekly training volume expressed in percentage terms at intensity levels 4 and 5 at the [t.sup.th] week; strength training [w.sup.ST.sub.t], was the mean weekly training volume expressed in percentage terms of dry-land workouts and general conditioning sessions at the [t.sup.th] week; and the total weekly training load, [w.sup.TTL.sub.t], representing the total physiological stress produced by the different workout sessions, was the mean weekly stimulus for each training intensity at the [t.sup.th] week. Figure 1 shows the weekly training load for the entire group of swimmers during F6T. Last, the total mileage swum, [w.sup.D.sub.t] was the total volume of swim training at the [t.sup.th] week, expressed in kilometers.
Overload and taper periods were grouped based on the similarities between the total weekly training loads using k-means cluster analysis.
We used mixed-model analysis (Proc Mixed of SAS version 9.1; SAS...