Opposite effects of isometric exercise on pain sensitivity of healthy individuals

Key points

1. 0160% of healthy participants showed exercise-induced pain reduction; 40% showed increased pain sensitivity after the same submaximal isometric exercise
2. 02Baseline offset analgesia efficacy was the sole predictor of post-exercise pain sensitivity change
3. 03Participants who became more pain-sensitive after exercise showed a significant decrease in offset analgesia efficacy post-exercise
4. 04Exercise-induced hyperalgesia correlated with higher perceived stress and body awareness during exercise
5. 05Exercise-induced hypoalgesia correlated with greater daily physical activity habits

How it was conducted

Design

Single-session experimental study with exercise group and passive control group

Participants

57 healthy adults (mean age 29.20 years); exercise group n=40, control group n=17

Intervention

Submaximal isometric handgrip at 30% maximal voluntary contraction for consecutive minutes

Pain outcome

Pressure pain threshold (PPT) measured pre- and immediately post-exercise as index of hypoalgesia or hyperalgesia

Pain modulation measures

Offset analgesia (OA), conditioned pain modulation (CPM), and pain adaptation tested pre- and post-exercise

Additional measures

Body awareness questionnaire, international physical activity questionnaire, perceived effort and stress (NRS 0-10)

What they found

• At group level, PPT significantly increased after exercise (p<0.05, Cohen d=0.31), but the delta (14.25 kPa) was smaller than the standard error of measurement (26.50 kPa), so the group-level effect may not represent a true change
• EIHypo subgroup (n=24, 60%): PPT increased by 41.6 kPa post-exercise (p<0.001, d=1.26), above the SEM
• EIHyper subgroup (n=16, 40%): PPT decreased by 27.9 kPa post-exercise (p<0.001, d=1.52), above the SEM
• PPT did not change in the control (rest) group
• Baseline OA was the only significant predictor of PPT change: for every 1-unit increase in OA (less efficient OA), PPT increased by 11.34 kPa post-exercise (95% CI 1.98 to 20.69)
• Pre-exercise OA was significantly greater (more efficient) in the EIHyper subgroup than in the EIHypo subgroup (p<0.05)
• OA efficacy significantly decreased post-exercise in the EIHyper subgroup (p<0.05, d=0.51, above SEM) and showed a non-significant trend toward increase in the EIHypo subgroup (p=0.15)
• In the EIHyper subgroup, greater perceived stress correlated with greater PPT decrease (r=-0.57, p<0.05); greater body awareness correlated with greater decrease in OA efficacy (r=-0.52, p<0.05)
• In the EIHypo subgroup, greater daily activity (IPAQ) correlated with smaller PPT increase (r=-0.45, p<0.05) and greater improvement in OA efficacy (r=0.50)
• CPM and pain adaptation did not significantly change after exercise in either subgroup

Limitations

• Small sample size (n=40 exercise, n=17 control), limiting generalizability
• Single-session design prevents assessment of cumulative or long-term exercise effects on pain modulation
• Healthy young adults only; findings may not apply to older adults, clinical populations, or those with chronic pain
• The isometric handgrip task targets forearm muscles, so results may not generalise to other exercise types or body regions

Why it matters

For patients

Isometric exercise commonly used early in rehabilitation may relieve pain for most healthy people but can increase pain sensitivity in a substantial minority, so individuals who feel worse after exercise are not imagining it.

For clinicians

Pre-exercise offset analgesia testing may help identify patients likely to respond to isometric exercise with pain sensitisation rather than relief, enabling more targeted and safer rehabilitation planning.

For readers

This study challenges the assumption that submaximal isometric exercise always reduces pain, showing that pain modulation capacity before exercise shapes whether exercise helps or harms pain sensitivity.