Muscle strength and activity in men and women performing maximal effort biceps curl exercise on a new machine that automates eccentric overload and drop setting

Abstract

Purpose: Connected adaptive resistance exercise (CARE) machines are new equipment purported to adjust resistances within and between repetitions to make eccentric (ECC) overload and drop sets more feasible. Here, we examined muscle strength, endurance, electromyographic activity (EMG), and perceptions of fatigue during unilateral bicep curl exercise with a CARE machine and dumbbells. We also tested for sex differences in muscle fatigability.

Methods: Twelve men and nine women attempted 25 consecutive coupled maximal ECC-concentric (CON) repetitions (ECC_max-CON_max) on a CARE machine. Participants also completed a CON one repetition maximum (1RM) and repetitions-to-failure tests with 60 and 80% 1RM dumbbells.

Results: Maximal strength on the CARE machine was greater during the ECC than CON phase, illustrating ECC overload (men: 27.1 ± 6.8, 14.7 ± 2.0 kg; women: 16.7 ± 4.7, 7.6 ± 1.4 kg). These maximal resistances demanded large neural drive. Biceps brachii EMG amplitude relative to CON dumbbell 1RM EMG was 140.1 ± 40.2% (ECC) and 96.7 ± 25.0% (CON) for men and 165.1 ± 61.1% (ECC) and 89.4 ± 20.4% (CON) for women. The machine's drop setting algorithm permitted 25 consecutive maximal effort repetitions without stopping. By comparison, participants completed fewer repetitions-to-failure with the submaximal dumbbells (e.g., 60%1RM-men: 12.3 ± 4.4; women: 15.6 ± 4.7 repetitions). By the 25th CARE repetition, participants reported heightened biceps fatigue (~ 8 of 10) and exhibited large decreases in ECC strength (men: 63.5 ± 11.6%; women: 44.1 ± 8.0%), CON strength (men: 77.5 ± 6.5%; women: 62.5 ± 12.8%), ECC EMG (men: 38.6 ± 20.4%; women: 26.2 ± 18.3%), and CON EMG (men: 36.8 ± 20.4%; women: 23.1 ± 18.4%).

Conclusion: ECC overload and drop sets occurred automatically and feasibly with CARE technology and caused greater strength and EMG loss in men than women.

Keywords: Bicep curl; Eccentric; Muscle fatigue; Resistance exercise; Sex difference.

Fig. 1

Eccentric (ECC, black circles) and concentric (CON, white circles) phase loads (kg) (A, C) and biceps brachii electromyographic (EMG) activity (B, D) during one set of 25 maximal eccentric, maximal concentric (ECC_max–CON_max) repetitions (50 contractions) of the unilateral bicep curl on the connected adaptive resistance exercise (CARE) machine. A For men, ECC phase strength was greater than CON phase strength. ECC and CON phase strength were reduced most markedly within the first 15 repetitions after which they tended to plateau. B For men, root mean square (RMS) amplitude of biceps brachii EMG was similar in the ECC and CON phases throughout exercise. Biceps brachii EMG amplitudes followed a trend similar to strength, with reductions most marked within the first 15 repetitions followed by a plateauing behavior. C For women, ECC phase strength were greater than CON phase strength. Both ECC and CON phase strength decreased progressively over the first 12 repetitions after which they tended to plateau. D For women, RMS amplitude of biceps brachii EMG was similar in the ECC and CON phases throughout exercise. Brachioradialis EMG amplitudes (not displayed) followed a similar trend to biceps brachii in both men and women. All data are presented as mean ± SD

Fig. 2

Group means (black circles) and individual participant data (white or gray circles) for highest average eccentric (ECC) (A, B, D, E) and concentric (CON) (A, C, D, F) phase loads at the start (initial) and end (final) of one set of 25 ECC_max–CON_max repetitions of the unilateral bicep curl on the connected adaptive resistance exercise (CARE) machine. A Men exhibited greater average ECC than CON phase strength. The mean difference [95% CI] between ECC and CON phase strength at the start of exercise was 10.5 kg [7.6, 13.4 kg] (Hedges g [95% CIs] = 1.528 [0.696, 2.359]). B All men exhibited large reductions in ECC phase strength from the start to end of exercise. The mean percent loss in ECC phase strength was 64%. Not displayed is the mean raw difference between initial and final ECC phase strength which was − 17.9 kg [− 21.5, − 14.2 kg]. C All men exhibited large reductions in CON phase strength from the start to end of exercise. The mean percent loss in CON phase strength was 78%. Not displayed is the mean raw difference between initial and final CON phase strength which was − 12.9 kg [− 15.5, − 10.4 kg]. D Women exhibited greater average ECC than CON phase strength. The mean difference between ECC and CON phase strength at the start of exercise was 7.1 kg [5.4, 8.9 kg] (Hedges g = 3.669 [1.366, 5.972]). E All women exhibited large reductions in ECC phase strength from the start to end of exercise. The mean percent loss in ECC phase strength was 44%. Not displayed is the mean raw difference between initial and final ECC phase strength which was − 6.8 kg [− 8.2, − 5.4 kg]. F All women exhibited large reductions in CON phase strength from the start to end of exercise. The mean percent loss in CON phase strength was 63%. Not displayed is the mean raw difference between initial and final CON phase strength which was − 4.8 kg [− 5.8, − 3.8 kg]. See Table 1 for between-sexes effect sizes

Fig. 3

Means (black circles) and individual participant data (gray circles) for root mean square (RMS) amplitudes of biceps brachii electromyographic (EMG) activity during the eccentric (ECC) (A, C) and concentric (CON) (B, D) phases of the CON bicep curl one repetition maximum (1RM) with a dumbbell and during ECC_max–CON_max repetitions on the connected adaptive resistance exercise (CARE) machine. A In men, biceps brachii EMG RMS amplitude during the ECC phase of ECC_max–CON_max repetitions on the CARE machine was greater than during the ECC phase of the CON dumbbell 1RM (mean raw difference [95% CI] = 0.000188 V [0.000097, 0.000279 V]; Hedges g [95% CI] = 0.565 [0.184, 0.946]). B In men, biceps brachii EMG RMS amplitude during the CON phase of ECC_max–CON_max repetitions on the CARE machine was similar to during the CON phase of the CON dumbbell 1RM (mean raw difference = − 0.000032 V [− 0.000505, − 0.000147 V]; Hedges g = − 0.081 [− 0.339, 0.176]). C In women, biceps brachii EMG RMS amplitude during the ECC phase of ECC_max–CON_max repetitions on the CARE machine was greater than during the ECC phase of the CON dumbbell 1RM (mean raw difference = 0.000148 V [0.000062, 0.000233 V]; Hedges g = 1.005 [0.186, 1.825]). D In women, biceps brachii EMG RMS amplitude during the CON phase of ECC_max–CON_max repetitions on the CARE machine was greater than during the CON phase of the CON dumbbell 1RM (mean raw difference = − 0.000073 V [− 0.000159, 0.000012 V]; Hedges g = − 0.379 [− 0.872, 0.115])

Fig. 4

Group means (black circles) and individual participant data (white or gray circles) for root mean square (RMS) amplitudes of biceps brachii electromyographic (EMG) activity during the concentric (CON) bicep curl one repetition maximum (1RM) with a dumbbell (A, C) and during maximal eccentric (ECC), maximal CON (ECC_max–CON_max) repetitions on the connected adaptive resistance exercise (CARE) machine (B, D). A In men, biceps brachii EMG RMS amplitude was lower during the ECC than CON phase of the CON dumbbell 1RM test (mean difference [95% CIs] = − 0.000315 V [− 0.000417, − 0.000214 V]; Hedges g [95% CIs] = − 1.013 [− 1.589, 0.437,]). B In men, biceps brachii EMG RMS amplitude was similar during the ECC and CON phases of initial ECC_max–CON_max repetitions on the CARE machine (mean difference = − 0.000095 V [− 0.000201, 0.000010, V; Hedges g = − 0.249 [− 0.550, 0.051,]). C In women, biceps brachii EMG RMS amplitude was lower during the ECC than CON phase of the CON dumbbell 1RM test (mean difference = − 0.000241 V [− 0.000341, − 0.000141 V]; Hedges g = − 1.270 [− 2.173, − 0.367]). D In women, biceps brachii EMG RMS amplitude was similar during the ECC and CON phases of initial ECC_max–CON_max repetitions on the CARE machine (mean difference = − 0.000020 V [− 0.000083, 0.000043 V]; Hedges g = − 0.118 [− 0.497, 0.261])

Fig. 5

Means (black circles) and individual participant data (gray and white circles) of numbers of repetitions completed by men (A) and women (B) during one set of maximal eccentric, maximal concentric (ECC_max–CON_max) unilateral bicep curl exercise on the connected adaptive resistance exercise (CARE) machine and during the dumbbell repetitions-to-failure tests with loads equal to 60 and 80% of the one repetition maximum (1RM). A All men completed the assigned 25 ECC_max–CON_max repetitions on the CARE machine without stopping. On average, men completed 12.3 and 6.5 repetitions during the repetitions-to-failure tests with dumbbells equal to 60 and 80% of 1RM, respectively. B All women completed the assigned 25 ECC_max–CON_max repetitions on the CARE machine without stopping, except for one woman who stopped after repetition 24 due to fatigue. On average, women completed 15.6 and 6.2 repetitions during the repetitions-to-failure tests with dumbbells equal to 60 and 80% of 1RM, respectively. See Table 1 for between-sexes effect sizes

Fig. 6

Means (black circles) and individual participant data (gray circles) for root mean square (RMS) amplitudes of biceps brachii electromyographic (EMG) activity at the start (initial) and end (final) of one set of 25 maximal eccentric, maximal concentric (ECC_max–CON_max) repetitions of the unilateral bicep curl on the connected adaptive resistance exercise (CARE) machine. A By the end of exercise, men exhibited a 39% mean reduction in ECC phase biceps brachii EMG amplitude. Not displayed is the mean raw difference [95% CI] between initial and final values which was − 0.000326 V [− 0.000505, − 0.000147 V]; Hedges g [95% CI] = − 1.062 [− 1.829, − 0.294]). B By the end exercise, men exhibited a 37% mean reduction in CON phase biceps brachii EMG amplitude. Not displayed is the mean raw difference between initial and final values which was − 0.000342 V [− 0.000503, − 0.000181 V]; Hedges g = − 1.192 [− 1.848, − 0.536]). C By the end exercise, women exhibited a 26% mean reduction in ECC phase biceps brachii EMG amplitude. Not displayed is the mean raw difference between initial and final values which was − 0.000120 V [− 0.000186, − 0.000055 V]; Hedges g = − 1.112 [− 1.990, − 0.234]). D By the end exercise, women exhibited a 23% mean reduction in CON phase biceps brachii EMG amplitude. Not displayed is the mean raw difference between initial and final values which was − 0.000116 V [− 0.000184, − 0.000048 V]; Hedges g = − 0.761 [− 1.387, − 0.136]). See Table 1 for between-sexes effect sizes

Fig. 7

Means (black circles) and individual participant data (gray circles) for perceived arm strength capacity (A, D), biceps fatigue (B, E), and biceps pain (C, F) after one set of 25 consecutive maximal eccentric, maximal concentric (ECC_max–CON_max) repetitions of the unilateral bicep curl on the connected adaptive resistance exercise (CARE) machine and after repetitions-to-failure tests with dumbbells equal to 60, 80, and 100% of the one repetition maximum (1RM). A In men, perceived arm strength capacity was lowest after the set of 25 ECC_max–CON_max repetitions on the CARE machine (8% perceived capacity remaining). Not displayed in the figure are perceived arm strength capacities for men at repetitions 5 (56.7 ± 17.3%), 10 (36.0 ± 17.4%), 15 (21.5 ± 16.0%), and 20 (12.7 ± 12.2%). B In men, perceived biceps fatigue was greatest after the set of 25 ECC_max–CON_max repetitions on the CARE machine (i.e., large to extreme; 7.8 of 10). C In men, perceived biceps pain was greatest after the set of 25 ECC_max–CON_max repetitions on the CARE machine (i.e., small to moderate; 4 of 10). D In women, perceived arm strength capacity was lowest after the set of 25 ECC_max–CON_max repetitions on the CARE machine (3% perceived capacity remaining). Not displayed in the figure are perceived arm strength capacities for women at repetitions 5 (46.1 ± 12.2%), 10 (31.7 ± 6.1%), 15 (18.9 ± 6.5%), and 20 (11.3 ± 5.5%). E In women, perceived biceps fatigue was greatest after the set of 25 ECC_max–CON_max repetitions on the CARE machine (i.e., large to extreme; 8.2 of 10). F In women, perceived biceps pain was greatest after the set of 25 ECC_max–CON_max repetitions on the CARE machine (i.e., moderate to large; 5.9 of 10). See Table 1 for between-sexes effect sizes