Isometric and plyometric training, added twice a week for 6 weeks, both improved 2.4-km time-trial time, maximal aerobic speed, jump height and isometric mid-thigh pull force more than a bodyweight circuit in endurance runners; the authors' claim that isometrics improved running economy more rests on a time effect, not a significant group difference
Lum and colleagues randomised 26 endurance runners to add isometric strength training, plyometric training or a circuit (control) twice a week for 6 weeks while they kept their planned running. Before and after, the runners did a countermovement jump (CMJ), an isometric mid-thigh pull (IMTP), a 2.4-km time trial, a running-economy test and a graded exercise test.
Time × group interactions favoured both the isometric and the plyometric group over the control for 2.4-km time (P = .002, ηp² = .45), maximal aerobic speed (P = .006), CMJ height (P < .001) and relative IMTP peak force (P = .001). Running economy and VO2max showed only main effects of time (P = .048 and P = .047), with no group interaction reported.
The authors conclude that the two methods were similarly effective for running performance and that isometric training gave the larger improvement in economy; the abstract's statistics do not show a significant group difference for economy.
Protocol
Participants: 26 endurance runners (18 men, 8 women), 36 ± 6 y, 61.6 ± 8.0 kg, VO2max 50.4 ± 5.8 mL/kg/min. Arm sizes from REF-801's table: isometric 9 (6 men, 3 women), plyometric 9 (6, 3), control 8 (6, 2).
Training (abstract): circuit, plyometric or isometric training twice a week for 6 weeks, on top of each runner's planned running.
Exercises (from [REF-801]'s full-text table; the abstract names none):
Isometric: isometric ankle plantarflexion at maximal isometric force, 3 sets × 3 repetitions × 3 s, 3 min rest (overcoming, maximal intent).
Plyometric: depth jump, single-leg bounding, split jump, 2-4 × 4-5, 3 min rest.
Control: bodyweight squat, lunge with knee lift, arabesque, 3 × 30 s.
The abstract reports an IMTP gain in the isometric arm; whether the isometric programme included multi-joint positions as well as the calf could not be checked without the full text.
Tests: CMJ, IMTP, 2.4-km time trial, running economy (at 10 and 12 km/h for women and 12 and 14 km/h for men, per REF-801), graded exercise test for VO2max and maximal aerobic speed (MAS).
Key Results
| Outcome | Time × group interaction | Main effect of time | Reading |
|---|---|---|---|
| 2.4-km time trial | P = .002, ηp² = .45 | P < .001, ηp² = .72 | isometric and plyometric > control |
| Maximal aerobic speed | P = .006, ηp² = .39 | P < .001, ηp² = .63 | isometric and plyometric > control |
| CMJ height | P < .001, ηp² = .55 | P < .001, ηp² = .51 | isometric and plyometric > control |
| IMTP relative peak force | P = .001, ηp² = .50 | P < .001, ηp² = .58 | isometric and plyometric > control |
| Running economy | not reported as significant | P = .048, ηp² = .17 | all groups changed over time |
| VO2max | not reported as significant | P = .047, ηp² = .18 | all groups changed over time |
Economy values extracted by [REF-801] (energy cost; women's/men's slower and faster speeds): isometric 1.06 ± 0.06 → 1.04 ± 0.08 and 1.08 ± 0.10 → 1.04 ± 0.09; control 1.13 ± 0.12 → 1.11 ± 0.07 and 1.12 ± 0.12 → 1.10 ± 0.06; plyometric 1.08 → 1.07 and 1.09 → 1.08. That is about -2% and -4% for isometric against -2% and -2% for control (our arithmetic).
2.4-km time trial, plyometric and control arms only (from [REF-865]'s table): plyometric 598 → 584 s, control 616 → 614 s. The isometric arm's times are not in either meta-analysis.
n = 26 (9 / 9 / 8). The abstract gives no between-group post-hoc tests or effect sizes for the isometric vs plyometric comparison.
Mechanisms
Measured: isometric and plyometric training both improved IMTP force and CMJ height more than the circuit (interactions in favour of both), alongside the time-trial gain; the abstract gives no isometric-vs-plyometric comparison.
Authors' interpretation: both methods improve the neuromuscular side of running performance; isometric training improved economy more.
Inference: with economy changing in every group and no significant interaction, the time-trial gain cannot be attributed to economy from the abstract; maximal aerobic speed improved with it.
Inference: if the isometric arm trained only the calf (as REF-801 tabulates), the gain in a multi-joint pull would point to a learning or neural effect on the test.
Practical Application
Isometrics or plyometrics for a runner? One small RCT (26 runners, 8-9 per arm, abstract only): 6 weeks of either, twice a week, cut 2.4-km time more than a circuit (P = .002); the claimed economy edge for isometrics was not significant.
Inference, not tested: if the runner dislikes jumping or is returning from impact-related pain, short maximal isometric work may be an alternative to plyometrics for a 6-week block. TRANSFER: tested only over 6 weeks, in 9 runners per arm, with a 2.4-km outcome.
Limitations
Very small arms (8-9); no sample-size calculation, blinding or allocation concealment reported in the abstract.
The economy conclusion is not supported by a reported group × time interaction.
6 weeks only; one short time trial; running not standardised across arms beyond "planned running training".
Funding: not stated in the abstract. The Europe PMC record lists a US Defense Advanced Research Projects Agency / Army Research Office grant, which does not fit the Singapore authors and looks like a metadata error (unverified).
How this entry was checked and changed
Why this level
parallel RCT with n=26 (rubric B+), held at B because three arms leave 8-9 per group and only the abstract was read; the repository copy is behind a bot check