Explosive or slow isometric reps?
In untrained men over 12 weeks, 3-s holds at 75% raised max strength 23% and quad size 8.1%; 1-s fast-and-hard reps gave 17% and alone improved force in the first 100 ms.
Balshaw, Massey, Maden-Wilkinson, Tillin and Folland randomised 48 untrained young men (43 finished) to explosive-contraction training, sustained-contraction training or no training for 12 weeks. Both training groups did 40 isometric knee extensions per leg, three times a week.
Explosive training meant contracting "as fast and hard as possible" to at least 80% of maximum for about 1 s; sustained training meant ramping to 75% of maximum over 1 s and holding it for 3 s. Sustained training raised maximal torque more (+23% vs +17%) and grew the quadriceps (+8.1% by MRI) while explosive training did not significantly (+2.6%).
Only explosive training improved force in the first 50-100 ms (+17 to 34%), through greater early neural drive. Explosive training involved only 7% of the time above 65% of maximum that sustained training did. Both kinds of training slowed the muscle's evoked contraction.
The authors conclude that peak loading magnitude drives maximal strength and neural drive at maximum, loading duration drives hypertrophy, and a high rate of force development drives early rapid force.
Protocol
Participants: 48 healthy young men randomised, groups matched for maximal torque and body mass; 5 withdrew (4 personal, 1 non-compliance). Completers: explosive n=13, sustained n=16, control n=14; age 25 y, body mass 70-72 kg in all groups.
Kind of isometric: overcoming (pushing against a fixed strap), unilateral knee extension of both legs (sets alternated between legs), knee 115 degrees and hip 126 degrees with 180 = full extension (about 65 degrees of knee flexion).
Explosive (ECT): 4 sets x 10 contractions per leg, "as fast and hard as possible" to at least 80% of maximal voluntary torque (MVT), about 1 s, 5 s relaxation between; rate-of-torque-development biofeedback. Achieved peak load 81 ± 4% MVT.
Sustained (SCT): 4 sets x 10 contractions per leg, torque rising linearly to 75% MVT over 1 s, then held 3 s, 2 s rest between; target-trace feedback. Achieved peak load 75 ± 2% MVT.
Both: 60-s sets, 2 min between sets on the same leg; 3 sessions a week for 12 weeks (36 sessions); MVT re-tested at the start of each week to reset loads.
Tests: duplicate sessions before and after; maximal and explosive voluntary torque (T50, T100, T150 = torque 50, 100, 150 ms after onset), quadriceps EMG, evoked twitch and octet contractions; quadriceps volume by MRI.
Key Results
| Outcome | Explosive (ECT) | Sustained (SCT) | Control | Between groups |
|---|---|---|---|---|
| Maximal voluntary torque | +17% (P < 0.001) | +23% (P < 0.001) | no change (P = 0.739) | SCT vs ECT ES = 0.69, P = 0.052; both > control (ES ≥ 2.06) |
| Explosive torque at 50 ms | +34% (P = 0.047) | no change | no change | group difference reported only from 100 ms on |
| Explosive torque at 100 ms | +17% (P = 0.008) | no change | no change | ECT > SCT ES = 0.72, P = 0.092; ECT > control ES = 0.98, P = 0.042 |
| Explosive torque at 150 ms | +18% (P < 0.001) | +12% (P < 0.001) | no change | SCT > control ES = 1.48, P = 0.008 |
| Explosive torque relative to maximum | unchanged | fell at all time points | unchanged | SCT decrease > ECT at 100 and 150 ms (ES 0.88, 0.91) |
| EMG at maximal torque | rose (P = 0.099) | rose (P < 0.001) | no change | ECT vs SCT ES = 0.36, P = 0.370 |
| Early EMG (0-50, 0-100 ms) | rose | no change | no change | ECT > control at all epochs |
| Quadriceps volume (MRI) | 1770 to 1816 cm³ (+2.6%, P = 0.247) | 1820 to 1967 cm³ (+8.1%, P = 0.001) | 1891 to 1906 cm³ (P = 0.550) | interaction P = 0.018; SCT vs ECT ES = 0.74, P = 0.074 |
| Loading time above 65% MVT | 7% of SCT's | reference | P < 0.001 | |
| Peak rate of torque development per rep | ~6-fold SCT's | reference | P < 0.001 | |
| Evoked twitch and octet time course | slowed | slowed | unchanged |
Mechanisms
Measured: EMG at maximal torque rose similarly after both types of training while quadriceps volume rose about 3-fold more after sustained training.
Authors' interpretation: the extra maximal strength after sustained training came from hypertrophy, not from extra neural drive; neural drive at maximum depends on load magnitude (81% vs 75%, similar), hypertrophy on loading duration.
Measured: only explosive training raised early-phase EMG (first 50-100 ms), and early explosive torque rose with no gain in muscle size or evoked early torque.
Authors' interpretation: early rapid force is a neural adaptation that needs a high rate of force development in training, starting from rest.
Measured: evoked twitch and octet contractions became slower after both trainings. Authors' interpretation: probably fewer type IIX fibres, which may blunt explosive gains in previously untrained people. Fibre type was not measured.
Practical Application
Explosive or slow isometric reps? In untrained men over 12 weeks, 3-s holds at 75% raised max strength 23% and quad size 8.1%; 1-s fast-and-hard reps gave 17% and alone improved force in the first 100 ms.
Pick by goal: sustained holds for size and maximal force, explosive reps for rapid force. Knee extension on a dynamometer only.
Explosive contractions are the low-fatigue option. They needed 7% of the time under high load yet delivered about 70% of the maximal-strength gain. For an athlete deep in an endurance block this is the authors' "highly efficient" option; that it suits endurance athletes is the base's inference.
Cue the start, not just the peak. Rapid force improved only when each contraction started from rest and rose as fast as possible; slow-ramp holds lowered explosive force relative to maximum.
Mixing both in one session may cancel the early-force gain (REF-1159, same lab, 4 weeks). Keep explosive work separate if it matters; that is the base's reading.
Limitations
Untrained young men, single-joint isometric knee extension; transfer to trained athletes, multi-joint or dynamic movements and endurance performance is untested.
The two training arms were not matched for peak load (81% vs 75% MVT) or loading time (7%); the design separates "explosive" from "sustained" as packages, not single variables.
Head-to-head differences were moderate and at P = 0.052-0.092 for maximal torque, volume and T100; the authors' threshold for "good evidence" was ES > 0.50 with P < 0.10.
Octet data missing in seven participants (discomfort); one MRI excluded for movement.
Funding: Arthritis Research UK Centre for Sport, Exercise and Osteoarthritis (grant 20194); no conflict-of-interest statement in the manuscript version read.
How this entry was checked and changed
Why this level
n=43 completers (13 explosive, 16 sustained, 14 control) of 48 randomised; 43 nominally reaches the A- bar, but the head-to-head comparison is 13 vs 16 and the authors accept "good evidence" at ES > 0.50 with P < 0.10; read from the accepted manuscript (full text)
Full text (checked 2026-10-16)
Forty-eight males recruited; five withdrew; forty-three completed (ECT [n=13]; SCT [n= 16]; CON [n=14]) (full text).
Four sets of ten unilateral isometric knee-extensor contractions of each leg; ECT "as fast and hard as possible" up to ≥80%MVT for ~1 s; SCT rising to 75%MVT over 1 s, then a plateau for a further 3 s (full text).
Knee and hip angles of 115° and 126° (180° = full extension); 36 sessions in total (full text).
Peak loading magnitude 81 ± 4 vs. 75 ± 2%MVT; ECT ~6-fold greater RTD per repetition (full text).
MVT increased more after SCT than ECT (23 vs. 17%; effect size [ES]=0.69); MVT 38% larger after SCT than ECT (P=0.052) (full text).
ECT improved explosive torque at all time points (17-34%; 0.54≤ES≤0.76); SCT only at 150 ms (12%; ES=1.48) (full text).
QUADSVOL increased 8.1% after SCT from 1820 ± 274 to 1967 ± 316 cm3 (P=0.001), but not following ECT (1770 ± 252 to 1816 ± 286 cm3; P=0.247) (full text).
Group x time interaction for QUADSVOL P=0.018; SCT vs ECT ES=0.74, P=0.074 (full text).
ECT involved only 7% of the loading duration (time >65%MVT) of SCT (full text).
Supported by the Arthritis Research UK Centre for Sport, Exercise, and Osteoarthritis (Grant reference 20194) (full text).