The Effect of Rate of W′ Utilisation and Acute Fatigue on W′ Reconstitution

Abstract

The critical power (CP) and W′ concept has become more integrated within applied cycling performance science. Both parameters can be assessed in the laboratory and field and have become a useful tool for coaches and athletes (Leo et al. 2022; Moritani et al. 1981; Poole et al. 1988; Spragg, Leo, and Swart 2022). It is possible to mathematically model the depletion and recovery of W′ during intermittent exercise (Skiba et al., 2012, Skiba and Clarke, 2021). Exercising at a work rate higher than CP causes a reduction in W' whereas when exercising at a work rate below CP there is an exponential reconstitution of W' (W′_REC) that is dependent on the recovery duration and a reconstitution time constant (τ_W'). This is contingent on the difference between recovery power output and CP (D_CP) (Skiba et al., 2012; Skiba et al., 2013; Skiba et al., 2014).

There are generalised equations available for W’_BAL models that allow prediction of W′_REC (Bartram et al. 2018, 2022; Pugh et al. 2022; Skiba et al. 2012). However, for optimal use individualisation of W’_BAL is advised (Welburn et al. 2023) as W′_REC has been shown to correlate with athlete specific performance parameters such as V̇O_2peak, CP and LT₁. (Bartram et al. 2018, 2022; Caen et al. 2021; Pugh et al. 2022; Welburn et al. 2023). Recent work suggests that W′_REC is influenced by work rate (Caen et al. 2019) and slows with repeated exercise (Chorley et al. 2019). However, both factors are not currently accounted for in the W’_BAL model.

Durability (or fatigue resistance) is considered to be a key determinant of cycling performance (Muriel et al. 2022; Spragg et al. 2022). It appears that work rate has the greatest impact on durability (Leo et al. 2024; Spragg et al. 2024), therefore understanding the impact of work rate (i.e. the rate of W′ utilisation) and acute fatigue on W′_REC could provide an important insight into the determinants of W’_BAL which would potentially allow improvements to the accuracy of the W′_BAL model. This study has two aims: assess the effect of (i) rate of W′ utilisation, and (ii) acute fatigue on W′_REC during intermittent exhaustive exercise.

Methods

Currently, 6 participants (4 males, 2 females; age: 21 [2] y, height; 1.78 [0.09] m, body mass; 67.8 [6.3] kg, V̇O_2max; 59.9 [7.6] mL·min^-1·kg^-1, maximal aerobic power (MAP); 361 [83] W, CP; 260 [130] W, W′ 22.45 [10.4] kJ, mean [SD]) have been recruited for this laboratory-based investigation. All tests were performed on an electronically braked cycle ergometer (Excalibur Sport, Lode). Participants initially attended the laboratory on six separate occasions for the determination of V̇O_2max, maximal aerobic power (MAP) and CP/W′. Participants subsequently performed two experimental trials, in a counter-balanced order. In both trials, participants completed intervals consisting of 40 sec on and 20 sec off. Trial one was performed at a W′ utilisation rate of 60 J×s^-1 above CP [WR_60FRESH] and trial two was performed at a W′ utilisation rate of 120 J×s^-1 above CP [WR_120FRESH]. The number of repetitions were calculated to deplete ~75% of their W′. The recovery work rate between the interval efforts was at 100 W below CP (i.e. D_CP of 100 W).  After the final interval participants then completed at an opened ended time to exhaustion (TTE) test at the same work rate. After a recovery period of ~60 min (involving cycling at 50% of LT₁), participants repeated the same interval protocol at the same W′ utilisation rate [WR_60FATIGUED & WR_120FATIGUED]. Total work done above CP (W′_totalTTE) was calculated for each open ended TTE. The Wʹ_BAL model (Pugh et al. 2022) was used to calculate the depletion and reconstruction of W′ during both trials. An individual τ_W’ was calculated for each exhaustive exercise under the assumption that at the point of task failure at the end of the TTE represents a W′_BAL of 0 kJ. A two-way repeated measures ANOVA was conducted assess differences in W′_totalTTE with W′ utilisation rate (WR₆₀ vs WR₁₂₀) and conditions (Fresh vs Fatigued) as the primary variables. Where significant main effects or an interaction was observed, Bonferroni-corrected post hoc t-tests were used to locate differences. Statistical significance was set at P ≤ 0.05. Data are presented as mean ± standard deviation [SD] unless otherwise stated.

Results

Table 1 shows W′_totalTTE during the final open-ended TTE interval for the intermittent exercise trials. There was a main effect for condition (P = 0.008), but no main effect for W’ utilisation rate (P = 0.397) or interaction (P = 0.084). W′_totalTTE was lower (P = 0.007) in WR_60FATIGUED compared to WR_60FRESH. There was no difference (P = 0.099) in W′_totalTTE between WR_120FRESH and WR_120FATIGUED.

Table 1:  W′_totalTTE during the final open-ended TTE interval for the intermittent exercise trials [n=6]. * = significant difference (P < 0.05) between Fresh and Fatigue.

WR₆₀

WR₁₂₀

W′_totalTTE Fresh

[kJ]

W′_totalTTE Fatigued [kJ]

W′_totalTTE Fresh [kJ]

W′_totalTTE Fatigued

[kJ]

11.1 ± 4.1

5.7 ± 2.5 *

8.4 ± 3.8

6.2 ± 1.8

Discussion

Preliminary data from this study suggests that acute fatigue reduces W′_totalTTE during intermittent exhaustive exercise. Moreover, there was a lower W′_totalTTE in the fatigued trials at WR₆₀ than WR₁₂₀. Although an individualised τ_W' was calculated for the fresh trials, it was not possible for the fatigued trials. This is because W′_totalTTE was reduced, suggesting that W′ does not reconstitute completely up to 1 hour after exhaustive intermittent exercise. This resulted physiological implausible τ_W' values when solving the W′_BAL model for 0 kJ at task failure in the fatigued trials (e.g. fresh τ_W': 425 sec vs fatigued τ_W': 4512 sec).

Implications for applied practitioners

These observations have important implications for future refinement of the W′_BAL model, particularly during fatiguing exercise where so-called durability becomes critical. Moreover, these data suggest the potential requirement to include a 3^rd parameter into the W′_BAL model (i.e., W′_totalTTE) to account for the deterioration in W′.