Table 1.
Evolution of the main characteristics of the experimental designs of investigations on caffeine effects on strength performance.
| Variables | <1980 | 1980–1999 | 2000–2009 | 2010–2014 | 2015–2022 | |
|---|---|---|---|---|---|---|
| n papers | 3 (1.6%) | 8 (4.2%) | 18 (9.5%) | 29 (15.3%) | 131 (69.3%) | |
| Sample size | n | 2.3 ± 0.6 | 15.5 ± 10.7 | 15.7 ± 6.9 | 16.0 ± 7.8 * | 19.7 ± 14.4 * |
| n Male | 2.3 ± 0.6 | 14.7 ± 7.0 | 13.8 ± 5.6 | 13.8 ± 6.7 | 14.6 ± 13.6 | |
| n Female | 0 | 2.3 ± 6.0 | 1.9 ± 4.4 | 1.3 ± 3.4 | 4.6 ± 7.8 | |
| Adjusted dose | Yes | 0 (0%) | 5 (62.5%) | 14 (77.8%) | 20 (69.0%) | 96 (73.3%) |
| No | 3 (100%) | 3 (37.5%) | 4 (22.2%) | 9 (31.0%) | 35 (26.7%) | |
| Dose–response | Yes | 0 (0%) | 2 (25.0%) | 0 (0%) | 2 (6.9%) | 20 (15.3%) |
| No | 3 (100%) | 6 (75.0%) | 18 (100%) | 27 (93.1%) | 111 (84.7%) | |
| Dose | Adjusted (mg/kg) | – | 5.8 ± 0.8 | 5.2 ± 1.3 | 4.6 ± 1.5 | 4.8 ± 1.6 |
| Absolute (mg) | 300 | 437.9 ± 69.0 | 180.3 ± 90.6 | 226.1 ± 95.9 | 298.3 ± 142.4 | |
|
Mix with other
substances |
Yes | 1 (33.3%) | 1 (12.5%) | 6 (33.3%) | 16 (55.2%) | 27 (20.6%) |
| No | 2 (66.7%) | 7 (87.5%) | 12 (66.7%) | 13 (44.8%) | 104 (79.4%) | |
|
Relationship with other
co-ingested substances |
Yes | 0 (0%) | 0 (0%) | 2 (11.1%) | 2 (6.9%) | 15 (11.5%) |
| No | 3 (100%) | 8 (100%) | 16 (88.9%) | 27 (93.1%) | 116 (88.5%) | |
| Placebo-controlled design | Yes | 2 (66.7%) | 8 (100%) | 18 (100%) | 28 (96.6%) | 130 (99.2%) |
| No | 1 (33.3%) | 0 (0%) | 0 (0%) | 1 (3.4%) | 1 (0.8%) | |
|
Caffeine consumption
reported |
Yes | 0 (0%) | 5 (62.5%) | 13 (72.2%) | 19 (65.5%) | 92 (70.2%) |
| No | 3 (100%) | 3 (37.5%) | 5 (27.8%) | 10 (34.5%) | 39 (29.8%) | |
| Reported side effects | Yes | 0 (0%) | 1 (12.5%) | 6 (33.3%) | 10 (34.5%) | 33 (25.2%) |
| No | 3 (100%) | 7 (87.5%) | 12 (66.7%) | 19 (65.5%) | 98 (74.8%) | |
| Caffeine form | Capsule | 1 (33.3%) | 4 (50.0%) | 11 (61.1%) | 9 (31.0%) | 73 (55.7%) |
| Beverage | 1 (33.3%) | 4 (50.0%) | 7 (38.9%) | 20 (69.0%) | 46 (35.1%) | |
| Gum | 0 (0%) | 0 (0%) | 0 (0%) | 0 (0%) | 3 (2.3%) | |
| Mouth rinse | 0 (0%) | 0 (0%) | 0 (0%) | 0 (0%) | 5 (3.8%) | |
| Gel | 0 (0%) | 0 (0%) | 0 (0%) | 0 (0%) | 1 (0.8%) | |
| Various forms | 0 (0%) | 0 (0%) | 0 (0%) | 0 (0%) | 1 (0.8%) | |
| N/A | 1 (33.3%) | 0 (0%) | 0 (0%) | 0 (0%) | 2 (1.5%) | |
| Exercise test | Upper | 2 (66.7%) | 1 (12.5%) | 5 (27.8%) | 8 (27.6%) | 31 (23.7%) |
| Lower | 1 (33.3%) | 7 (87.5%) | 8 (44.4%) | 7 (24.1%) | 48 (36.6%) | |
| Both | 0 (0%) | 0 (0%) | 5 (27.8%) | 14 (48.3%) | 52 (39.7%) | |
Data indicate the number of studies that fulfilled (or did not) each methodological criterion, with percentages between parentheses. * indicates a significant differences with data <1980 (p < 0.05).