REF-1179 · Pignanelli et al., 2020 · American Journal of Physiology - Regulatory, Integrative and Comparative Physiology · Paper · PubMed · Free full text

Does light lifting to failure add capillaries and mitochondria?

In 10 men, 6 wk of 30% 1RM squats to failure raised type I capillary contacts ~18% and respiration ~30%, not mitochondrial proteins. Normal tempo; no non-training control.

Blimited evidenceMeasured on recreational people

Pignanelli, Burr and colleagues trained 10 young men for 6 weeks, three times a week, with three sets of single-leg squats at 30% of 1RM to repetition failure on each leg; one leg trained with free blood flow (LL-RE), the other with a thigh cuff at 60-70% of arterial occlusion pressure (LL-BFR).

Strength rose about 25% in both legs, and vastus lateralis thickness rose 4.9% and 7.4% (time effect P = 0.043), although the BFR leg did about 33% less cumulative work. Both legs increased capillary contacts around type I fibres (from 4.53 to 5.33 with LL-RE and 5.17 with LL-BFR).

Maximal mitochondrial respiration rose about 30% only in the LL-RE leg, with no change in any measured mitochondrial protein (complexes I-V, COXIV, VDAC, mitochondrial creatine kinase). Fibre size, fibre type, myonuclei and sharing factor did not change significantly.

Only the BFR leg sustained more power through the middle of a ~60 s, 30-repetition endurance test (+19 vs +3 W). The authors conclude that low-load training to failure increased indexes of oxidative capacity and strength, BFR did not augment them, and BFR's endurance-test gain was not explained by the oxidative changes.

How this bears on the Russian static-dynamic method (Seluyanov): indirect, and normal tempo. The method's claims (practice descriptions, not this paper) are that slow, continuous-tension sets of roughly 30-60 s at a light load, taken to a burning sensation with short rests, grow type I fibres and their mitochondria and so improve endurance.

This study shows that light sets to failure, of a similar length (about 50-105 s by our calculation), can add capillaries around slow fibres and raise mitochondrial respiratory capacity within 6 weeks.

But the repetitions were at a normal tempo, relaxation was not prevented, rest was 100 s, type I fibres did not grow significantly, mitochondrial protein content did not rise, and the leg with the oxidative gains did not sustain more power through the middle of the endurance test (+3 W; whole-test mean power rose in both legs, along with strength).

The partial occlusion that the method attributes to continuous tension was imposed here with a cuff, and the cuff leg did not gain more oxidative capacity. It is the nearest chronic data on oxidative adaptation to light sets, not a test of the method.

Protocol

Participants: 10 healthy young men, 24 years (95% CI 22-27), 78 kg; no major injury in 6 months; no lower-body resistance training at least once a week for at least 6 months and no periodised programme for at least 1 month (Methods). All had done an earlier single-session study by the same group.
Design: legs randomly assigned, matched for dominance; daily order of the legs randomised; 5-10 min rest between legs.
Exercise: single-leg squats in a robotically resisted Smith machine; 3 sets at 30% 1RM to task failure; 100 s rest between sets; 3 sessions per week for 6 weeks (1RM testing every two weeks counted as a session).
Tempo: 3.5 s per repetition, 1.75 s eccentric and 1.75 s concentric, paced by an audible signal. Relaxation between repetitions was not addressed; range was the full squat repetition.
Set duration (our calculation from repetitions × 3.5 s): first session about 88, 60 and 49 s for sets 1-3 in the LL-RE leg; last session about 105, 60 and 53 s.
Progression: +2 kg for a leg that did more than 30 repetitions in set 1.
BFR leg: 11-cm cuff high on the thigh at 60-70% of the lowest occlusive pressure (153 ± 7 and 156 ± 6 mmHg on the first and last days), inflated until the end of set 3.
Nutrition: about 25 g whey protein after each session.
Biopsies: one resting baseline biopsy from one leg (half dominant, half non-dominant); after training one biopsy from each leg within 5 days of the last session.
Outcomes: 1RM; thigh lean mass (DEXA); vastus lateralis and rectus femoris thickness (ultrasound); 30 maximal concentric knee extensions at 30% 1RM and 180°/s (~60 s); fibre type, CSA, capillarisation and myonuclei (immunohistochemistry); mitochondrial respiration (permeabilised fibres) and mitochondrial proteins (Western blot).

Key Results

Outcome (n)BaselineLL-RELL-BFRStatistics
Cumulative 6-week volume (10)28,544 ± 1,771 kg18,949 ± 1,541 kg~33% lower with BFR (abstract P = 0.004)
1RM (10)77 / 74 kg96 kg92 kgboth P = 0.001; no difference between legs
Vastus lateralis thickness (10)2.80 / 2.72 cm2.92 cm (+4.9%)2.92 cm (+7.4%)time P = 0.043; rectus femoris and thigh lean mass unchanged
Type I capillary contacts (9)4.53 ± 0.235.33 ± 0.27 (~18%, P = 0.018)5.17 ± 0.25 (~14%, P = 0.006)type II: trend only (P = 0.067 and 0.1)
Type I capillary-to-fibre ratio (9)1.762.092.01both P < 0.05
CFPE index, type I / type II (9)6.38 / 5.517.49 / 6.307.39 / not changedtype I both P < 0.05; type II LL-RE P = 0.048
Type I fibre CSA (9)4,393 µm²5,088 µm²4,838 µm²P = 0.116 (not significant)
Fibre type, % type I (9)575752P = 0.6
Maximal mitochondrial respiration (10)~30% higher (P = 0.006)~18% (not significant)no significant difference between legs
Mitochondrial proteins: complexes I-V, VDAC, MiCK (7-10)no changeno changeSOD2 rose in LL-RE (P = 0.02)
ADP sensitivity, apparent Km (10)1,030 µM1,122 µM960 µMP = 0.2
Endurance test, reps 11-20, change in power (10)+3 W (95% CI −6 to 11)+19 W (13 to 24)P = 0.008 between legs
Endurance test, mean power reps 1-30 (10)119 / 116 W128 W130 Wtime P = 0.001

Mechanisms

Measured: capillaries in contact with each type I fibre rose with both light-load protocols; the sharing factor did not change, so the gain came from new capillary contacts rather than redistribution.
Measured: maximal respiration per mg of fibre rose about 30% in the free-flow leg while every measured mitochondrial protein (respiratory complexes, COXIV, VDAC, MiCK) stayed flat. Authors' interpretation: intrinsic mitochondrial changes (for example post-translational modification or supercomplex assembly) rather than more mitochondria; "the brief, intermittent nature of resistance exercise seems insufficient to increase mitochondrial content after 6 wk."
Measured: the BFR leg sustained more power through repetitions 11-20 of a ~60 s test despite smaller oxidative gains. Authors' interpretation: the improvement "cannot be explained" by strength, size, capillaries or respiratory capacity; perhaps better anaerobic ATP maintenance or ion handling (speculative).
Inference: the oxidative markers that rose with light-load training did not show up as extra mid-test power on the one endurance-type test used (about 60 s): the free-flow leg's whole-test mean power rose, as did the cuff leg's, in line with the strength gain, but its repetitions 11-20 did not (+3 W); whether the oxidative gains help longer, lower-intensity efforts is untested (the authors say so).
Inference: type I fibre area did not rise significantly (P = 0.116; the authors cite low power, d = 0.65), so this study does not reproduce the type I growth seen with 30% 1RM to failure in REF-1178.

Practical Application

Does light lifting to failure add capillaries and mitochondria? In 10 men, 6 wk of 30% 1RM squats to failure raised type I capillary contacts ~18% and respiration ~30%, not mitochondrial proteins. Normal tempo; no non-training control.
Do not expect a cuff to add oxidative gains: BFR reached the same strength and size with a third less work and helped a ~60 s test, but did not raise capillaries or respiration more than light lifting to failure without it.
Static-dynamic method: an indirect test. Light, hard sets lasting about a minute can nudge slow-fibre capillaries and mitochondrial function in young men (training status reported inconsistently), which is in the direction of the method's claims; this study did not use slow no-relaxation repetitions, did not show slow-fibre growth or more mitochondria, and the leg with the oxidative gains did not sustain more mid-test power on the endurance test.
For endurance athletes, endurance training itself raises mitochondrial content and capillaries far more reliably (REF-465, REF-466); treat light lifting as strength or rehabilitation work, not a substitute aerobic stimulus.

Limitations

Ten men; one shared baseline biopsy for both legs; no non-training or heavy-load control, so the oxidative changes are pre-post within a small group.
Analyses were not blinded; post hoc comparisons against baseline were one-tailed.
Respiratory capacity rose without any change in protein markers of mitochondrial content; the authors caution against reading this as more mitochondria.
Training status is described inconsistently (no regular lower-body resistance training for 6 months in the Methods, "resistance-trained" in the Discussion).
Fibre-level analyses n = 9 (myonuclei n = 7); for two participants the baseline histology sample was taken 2 h after an acute bout.
The endurance test lasted about 60 s at high intensity; nothing longer or sport-specific was tested.
Funding: Natural Sciences and Engineering Research Council of Canada, Canadian Foundation for Innovation, and the Ontario Ministry of Research, Innovation and Science (grants to J. F. Burr). Conflicts of interest: none declared.
How this entry was checked and changed
Why this level

n=10 men; the leg-versus-leg comparison is controlled, but the oxidative changes are pre-post against a single shared baseline biopsy with no untrained or heavy-load control, analyses were not blinded and post hoc tests against baseline were one-tailed; read from the full text

Full text (checked 2026-10-16)
Ten healthy, young men, 24 (22, 27) yr, 78 (72, 85) kg; no previous lower-body resistance training experience (≥1 day/wk) for ≥6 mo (full text).
Three sets of single-leg squats at 30% 1-RM to task failure, rest periods set at 100 s, three times per week for 6 wk (full text).
Repetitions at a 3.5-s duty cycle (1.75-s eccentric and 1.75-s concentric contraction) (full text).
Tourniquet at 60–70% of the lowest effective occlusive pressure; 153 ± 7 and 156 ± 6 mmHg on the first and last training day (full text).
First session repetitions LL-RE vs LL-BFR: 25 vs 20, 17 vs 9, 14 vs 5; last session 30 vs 27, 17 vs 12, 15 vs 9 (full text).
Cumulative volume 28,544 ± 1,771 vs. 18,949 ± 1,541 kg; 1-RM 96 and 92 kg after training, both P = 0.001 (full text).
Vastus lateralis thickness increased 4.9% and 7.4%, time effect P = 0.043 (full text).
Type I capillary contacts 4.53 ± 0.23 before vs. 5.33 ± 0.27 and 5.17 ± 0.25 after; ~18% (P = 0.018) and ~14% (P = 0.006) (full text).
Type I fibre CSA 4,393, 5,088 and 4,838 µm2, P = 0.116 (full text).
Mitochondrial respiratory capacity increased only in the LL-RE leg by 30% (P = 0.006); LL-BFR a nonsignificant 18% change (full text).
Endurance test repetitions 11–20: 3 (95% CI: −6, 11) and 19 (95% CI: 13, 24) W, P = 0.008 (full text).
"The brief, intermittent nature of resistance exercise seems insufficient to increase mitochondrial content after 6 wk of training" (full text).
Grants: NSERC Discovery Grant 03974, Canadian Foundation for Innovation Grant 35460, Ontario Ministry Grant 460597; no conflicts of interest declared (full text).