Only two previous studies have examined muscle fatigue after eccentric exercise-induced muscle damage (Davies & White, 1981; Balnave & Thompson, 1993). Both studies employed an electrically elicited isometric fatigue test consisting of trains of stimuli at 20 Hz, lasting 300 ms and repeated every second for 2 min. Davies & White (1981) reported that damaged muscle was weaker but no more fatiguable, whereas Balnave & Thompson (1993) reported that damaged muscle was weaker but, surprisingly, less fatiguable. This study investigated isometric strength and volitional fatigue after eccentric exercise-induced muscle damage. Seven subjects (5 males and 2 females, age 22.6 ± 4.4 years (mean ± S.D.), height 1.78 ± 0.08 m, mass 75.7 ± 11.7 kg) gave informed consent to participate in the study, which had School ethics committee approval. Strength was assessed by a maximal voluntary contraction (MVC) at 80 and 40 deg knee flexion, corresponding to an optimal and a shortened muscle length, respectively. Fatigue was assessed by a sustained 60 s MVC at each muscle length and quantified as the slope of a linear regression line (b) fitted to the force data. Plasma creatine kinase (CK) activity was also measured from a fingertip blood sample. These measures were taken before, 1 h after, and on days 1, 2, 3 and 7 following 100 repetitions of the eccentric phase of the barbell squat exercise (10 sets X 10 repetitions @ 80 % concentric 1 repetition maximum). Repeated-measures ANOVA revealed that following eccentric exercise, CK activity was significantly elevated (P < 0.05) above baseline values (mean ± S.E.M., 275 ± 105) at 1 h (505 ± 115) and day 1 (808 ± 110). Strength loss was approximately 35 % at 1 h post (P < 0.05) and had not recovered by day 7 post (P < 0.05). No significant differences (P > 0.05) in the extent of relative strength loss were observed at short and optimal muscle length. The rate of fatigue was lower for 3 days (P < 0.05) and 1 day (P < 0.05) at optimal and short muscle length, respectively (see Fig. 1).
Eccentrically exercised muscle was weaker but had an improved ability to maintain force, albeit at lower than pre-exercise levels. The post-eccentric fatigue responses were without the marked rise and rapid decline in force that is associated with the activation and rapid fatigue of type II fibres. This may imply selective damage to type II fibres.
