# Acute molecular responses to concurrent resistance and high-intensity interval exercise in untrained skeletal muscle

PMID: 25902785
Journal: Physiological Reports
Published: 2015-04-22
Authors: Pugh JK, Faulkner SH, Jackson AP, King JA, Nimmo MA

## Question

Does adding acute HIIT after resistance exercise interfere with skeletal muscle molecular responses in untrained men?

## Summary

Ten untrained men completed resistance exercise alone and resistance exercise followed by cycling HIIT in a counterbalanced crossover design. Adding HIIT did not show an acute molecular interference effect, but outcomes were muscle signaling and mRNA markers rather than real training gains.

## Population

- Healthy untrained young men.
- Sample size: 10
- Age: 21.3 +/- 1.0 years
- Sex: Men only
- Fitness level: No structured exercise training for 12 months.
- Health status: Healthy, nonsmokers, injury-free, no medication or supplements.

## Methodology

- Counterbalanced crossover acute exercise study
- Two acute trials separated by at least 7 days
- Laboratory exercise and muscle biopsy setting
- Controlled study design.

## Protocol

- RE plus cycling HIIT.
- Modality: Leg extension plus cycle ergometer.
- Work intervals: 10 x 1-min cycling intervals at 90% HRmax after 4 x 8 leg extensions at 70% 1RM.
- Sets or repetitions: 4 x 8 resistance reps plus 10 HIIT intervals.
- Intensity: 70% 1RM and 90% HRmax.
- Frequency: Single crossover trial.
- Program length: Acute.
- Resistance exercise only.
- Modality: Leg extension.
- Work intervals: 4 x 8 leg extensions.
- Sets or repetitions: 4 sets of 8 reps.
- Intensity: 70% 1RM.
- Frequency: Single crossover trial.
- Program length: Acute.

## Outcomes

### Interference
Status: no clear change
Results did not support an acute molecular interference effect.

### PGC-1alpha
Status: improved
PGC-1alpha responses were larger after RE+HIIT.

Total PGC-1alpha increased 8.2-fold at 2 h and 4.5-fold at 6 h in RE+HIIT.

### MuRF-1
Status: worse/safety concern
MuRF-1 was higher after RE+HIIT.

Elevated 4.6-fold at 2 h and 1.6-fold at 6 h in RE+HIIT.

### Completion
Status: improved
All participants completed both exercise trials.

10/10 completed.

## Practical Insights

- Same-session resistance exercise followed by short cycling HIIT is mechanistically plausible but not proven for long-term outcomes here.
- Do not overstate acute molecular signals as actual hypertrophy or endurance gains.

## Limitations

- Very small sample.
- Men only.
- Acute molecular outcomes only.
- No long-term training data.
- Exact HIIT recovery interval was not clear in the extracted text.

## Safety And Adherence

- No adverse events were reported.
- Participants were healthy, screened, and supervised; the study included invasive muscle biopsies.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/25902785/) (pubmed)
- [DOI](https://doi.org/10.14814/phy2.12364) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4425969/) (full text)

## Agent Guidance

Preserve the paper-level scope of this note. Do not generalize beyond the population, protocol, measured outcomes, and limitations above.