Clinical measures of foot posture and ankle joint dorsiflexion do not differ in adults with and without plantar heel pain

Key points

1. 01No statistically significant difference in foot posture (Foot Posture Index or Arch Index) between the plantar heel pain group and matched controls
2. 02No clinically important difference in ankle joint dorsiflexion on either the knee-extended or knee-flexed lunge test
3. 03All group differences were categorised as small or very small using Cohen's d effect sizes
4. 04Matching participants for BMI was critical - prior studies that found associations may have been confounded by body mass
5. 05Findings apply to middle-aged, overweight community-dwelling adults, not necessarily to younger or athletic populations

How it was conducted

Design

Cross-sectional observational study (reported per STROBE)

Participants

75 community-dwelling adults from Victoria, Australia: 50 with plantar heel pain, 25 age-, sex-, and BMI-matched controls (ratio 2:1)

Age and BMI

Mean age 49.1 years (PHP group) and 48.9 years (control); mean BMI 30.6 and 30.2 kg/m2 respectively

Foot posture measures

Foot Posture Index-6 (FPI) and Arch Index (AI) from static footprints

Ankle dorsiflexion measures

Weight-bearing lunge test with knee extended and knee flexed, measured in degrees with an inclinometer

Analysis

Linear Mixed Models pooling bilateral data; Cohen's d effect sizes calculated; unblinded assessors

What they found

• Foot Posture Index: PHP group mean 4.3 (SD 2.8) vs control 3.4 (SD 3.0); mean difference -0.9 (95% CI -2.3 to 0.5); p=0.198; Cohen's d=0.32 (small)
• Arch Index: PHP group mean 0.20 (SD 0.10) vs control 0.21 (SD 0.05); mean difference 0.01 (95% CI -0.02 to 0.04); p=0.554; Cohen's d=0.05 (very small)
• Lunge test knee extended: PHP group mean 32.5 degrees (SD 6.4) vs control 35.5 degrees (SD 6.5); mean difference -3.0 degrees (95% CI -0.1 to 6.1); p=0.054; Cohen's d=0.14 (very small)
• Lunge test knee flexed: PHP group mean 40.1 degrees (SD 7.8) vs control 41.9 degrees (SD 7.5); mean difference -1.8 degrees (95% CI -1.8 to 5.5); p=0.321; Cohen's d=0.09 (very small)
• Mean FPI values in both groups (4.3 and 3.4) were within the normative range for normal adult foot posture (1 to 7)
• Median duration of symptoms in the PHP group was 6.5 months (range 1.5 to 80.0 months); mean first-step pain on 100 mm VAS was 53 mm

Limitations

• Convenience sample with no prospective power calculation, so the study may be underpowered to detect small differences
• Cross-sectional design cannot establish causality - foot posture changes could be a consequence, not a cause, of plantar heel pain
• Assessors were not blinded to group allocation, introducing potential assessment bias
• Results generalise to middle-aged, overweight, sedentary adults and may not apply to younger or athletic subgroups with plantar heel pain

Why it matters

For patients

If you have plantar heel pain, your arch shape and ankle flexibility are likely similar to people without it once body weight is considered, so targeting these features alone may not address the root cause.

For clinicians

Clinicians should not focus exclusively on correcting foot posture or improving ankle dorsiflexion in plantar heel pain patients, and should prioritise addressing overweight or obesity as a potentially more important contributing factor.

For readers

This study challenges the assumption that pronated feet or limited ankle mobility cause plantar heel pain in middle-aged adults, and highlights BMI as a key confounders in earlier studies that reported such links.