Changes In The Rate Of Quadriceps Force Relaxation After Prolonged Cycling

PURPOSE: A reduction in the rate of force relaxation (RFR) is indicative of peripheral fatigue. This study examined quadriceps RFR following prolonged cycling in men and women. Glace et al (2012) showed no peripheral fatigue in female cyclists and a combination of peripheral and central fatigue in male cyclists. It was hypothesized that RFR changes would parallel quadriceps force fatigue, and confirm a lack of peripheral fatigue in women. METHODS: 11 men (41 ± 10 yo, 76 ± 7 kg, 181 ± 7 cm) and 8 women (36 ± 9 yo, 58 ± 8 kg, 162 ± 10 cm) cycled for 2 h at ventilatory threshold with 5, 1-min sprints followed by a 3-km time trial. Isometric quadriceps maximal voluntary contraction (MVC) was assessed prior to and after cycling. A 4 s magnetic stimulation pulse train was delivered to the femoral nerve, and the stimulated quadriceps force was also recorded. Absolute RFR was computed during the first 100 ms of force relaxation. Relative RFR was computed during force relaxation from 75-25% maximal force. Time (pre vs. post) by Sex (men vs. women) ANOVA was used to assess changes in MVC, stimulated force, and RFR. RFR was also compared between subjects with significant stimulated force fatigue (>20% decline) and those with no force fatigue (<20% decline). RESULTS: MVC declined by 22 ± 16% in women (P = 0.021) and 22 ± 17% in men (P = 0.001). There was no decline in stimulated force in women (+2 ± 21%, P = 0.942), and a marked decline in men (14 ± 19%, P = 0.003; Time x Sex P = 0.035). Absolute RFR declined in men (-13 ± 32%, P = 0.030) but not in women (+12 ± 25%, P = 0.562, Time x Sex P = 0.050). Absolute RFR declined by 30 ± 17% in 8 athletes with force fatigue and increased by 18 ± 21% 11 athletes with no force fatigue (Time x Fatigue Group P < 0.001). Decline in absolute RFR correlated with stimulated force decline (r = 0.87, P < 0.001), but not with MVC decline (r = -0.07, P = 0.782). Relative RFR did not decline (P = 0.927) with no difference between men and women (P = 0.750). Changes in relative RFR did not differ between athletes with and without force fatigue (P = 0.324). CONCLUSIONS: The absolute RFR data confirm the absence of peripheral fatigue in women. Changes in absolute RFR paralleled stimulated force fatigue. Lack of change in relative RFR indicates that RFR changes are not independent of force fatigue. RFR does not provide further insight on fatigue than that provided by force fatigue.