https://www.sciencedirect.com/science/article/pii/S0966636226001128
I think this is a chicken and egg situation. If the body becomes more inflexible then it is harder to maintain balance as well, but:
chatGPT(5.6maxpaid):
Paper reviewed
Lindsay C, Radcliffe CR, Immink MA. “Ageing alters ankle mechanics and muscle co-contraction patterns across the gait cycle.” Gait and Posture, 2026. Paper and DOI
Summary
Research question
The study asked how increasing age is associated with:
- Tibialis anterior, or TA, activity
- Gastrocnemius, or GAS, activity
- Simultaneous TA-GAS co-contraction
- Ankle angle and joint moment
- Braking and propulsive ground reaction forces
Importantly, it examined changes throughout the entire gait cycle rather than comparing only isolated peaks.
Methods
This was a secondary analysis of an open-access gait dataset.
- 107 healthy, able-bodied adults
- Ages 26 to 86 years
- 58 women and 49 men
- Barefoot walking along a 12-metre walkway
- Self-selected walking speed
- At least six walking trials where possible
- Three-dimensional motion capture, force plates and surface EMG
- Left and right sides and repeated strides were averaged
- Age was treated as a continuous variable
- One-dimensional Statistical Parametric Mapping was used to find periods of the gait cycle associated with age
EMG was divided by each participant’s maximum recorded value for that muscle. It therefore describes the shape and timing of activity relative to that person’s own maximum, not absolute muscle activation.
Main findings
| Measure | Association with increasing age | Approximate gait-cycle period |
|---|---|---|
| TA normalized EMG | Higher | 9% to 17% |
| TA normalized EMG | Lower | 38% to 46% and 81% to 85% |
| GAS normalized EMG | Higher | 1% to 2% and 14% to 33% |
| TA-GAS co-contraction index | Higher | 1% to 4% and 16% to 21% |
| TA-GAS co-contraction index | Lower | 38% to 48% |
| Ankle angle | More dorsiflexed and less plantarflexed | 48% to 70% |
| Plantarflexion moment | Lower | 12% to 16% |
| Plantarflexion moment | Higher or prolonged later in the cycle | 57% to 63% |
| Braking force | Smaller braking magnitude | 7% to 26% |
| Propulsive force | Lower | 42% to 54% |
Small additional ground-reaction-force differences appeared around or after toe-off, but the authors correctly regard these as unlikely to represent meaningful propulsion.
Authors’ interpretation
The authors propose that ageing shifts ankle control toward stability:
- Greater relative TA-GAS overlap during loading and mid-stance may stiffen the ankle.
- This may compensate for less reliable proprioceptive information.
- Greater relative gastrocnemius activity occurs despite reduced or unchanged mechanical output.
- During late stance, propulsion falls and the ankle remains more dorsiflexed.
- The resulting gait may be more stable but mechanically less economical.
The most interesting pattern is therefore not simply “more co-contraction with age.” Co-contraction increases early in stance but decreases during terminal stance. The adaptation is phase-specific.
Novelty
What is genuinely new
The principal novelty is analytical and integrative rather than a new biological discovery.
- Age is modeled continuously. This avoids arbitrary decade boundaries and identifies gradual associations from early to late adulthood.
- The entire waveform is analyzed. The study identifies precisely where in the gait cycle age associations occur instead of examining only peak values.
- Neuromuscular and mechanical measurements are considered together. Relative EMG, co-contraction, ankle mechanics and ground reaction forces are aligned within the same gait cycle.
- It identifies a phase-dependent reversal in co-contraction. Co-contraction rises during loading and mid-stance but falls during terminal stance.
- It highlights an apparent EMG-mechanics mismatch. Relative gastrocnemius activity increases early in stance while ankle moment and braking force decline.
What is not new
Several underlying observations were already established:
- Older adults commonly exhibit altered or prolonged lower-limb muscle activation.
- Antagonist co-contraction often increases with age.
- Ankle propulsion and push-off power decline in later life.
- Reduced tendon and muscle function can impair mechanical output.
A 2018 study had already used full-cycle Statistical Parametric Mapping to compare EMG patterns across age decades. A 2021 analysis of the parent cohort had already examined age-related propulsion decline.
The paper’s novelty is therefore moderate and incremental: continuous-age waveform analysis plus the integration of co-contraction and ankle mechanics, rather than discovery of an entirely new ageing mechanism.
Strengths
- A relatively large laboratory gait sample covering six decades of adulthood.
- Synchronized EMG, kinematic, kinetic and force-platform measurements.
- Whole-curve analysis reduces dependence on investigator-selected peaks.
- Treating age continuously preserves more information than dividing participants into age groups.
- The underlying dataset is openly available and uses a consistent measurement system.
- The authors explicitly acknowledge important limitations of peak-normalized EMG and the co-contraction index.
- The results distinguish early-stance stabilization from late-stance propulsion rather than describing ageing as a uniform increase in muscle activity.
Critique
1. Walking speed is the major unresolved confounder
Participants walked at self-selected speed, but walking speed was not included in the regression.
This matters because speed directly affects:
- Braking and propulsive ground reaction forces
- Ankle moments and power
- Muscle activation timing
- Co-contraction
- Stance and swing duration
A previous analysis of the parent cohort found that reduced push-off power was much more strongly related to walking speed than to age: speed explained 54% of the variance, compared with 4% for age. This makes the present omission particularly consequential.
The results establish what older participants did while walking naturally, but they cannot distinguish:
- Direct biological effects of ageing
- Effects produced by choosing to walk more slowly
- A combination of both
2. Normalized EMG cannot demonstrate greater absolute muscle activation
Each muscle’s signal was divided by its own maximum during the walking task. An older participant with a lower absolute maximum can therefore have a higher normalized value during one phase without producing more electrical activity or force than a younger participant.
Consequently, the data support:
- A redistribution or broadening of activity across the gait cycle
- Greater relative overlap between TA and GAS signals
They do not securely demonstrate:
- Greater absolute neural drive
- Greater absolute co-contraction force
- Greater energy expenditure
- Reduced efficiency of converting neural input into force
The claim of reduced neuromuscular or mechanical efficiency is therefore stronger than the measurements justify.
3. The proposed mechanism was not measured
The study did not measure:
- Proprioceptive acuity
- Ankle stiffness or impedance
- Tendon stiffness or elastic energy return
- Muscle force
- Metabolic cost
- Balance responses to perturbation
- Fatigue
- Falls or fall risk
Thus, the proposed sequence from impaired proprioception to co-contraction, joint stiffening and maintained stability is biologically plausible but remains an interpretation. It was not directly tested.
Similarly, statements that the pattern maintains stability, increases fatigue or elevates fall risk go beyond the observed outcomes.
4. Separate age associations do not prove EMG-mechanical coupling
The authors infer a mismatch because age was associated with higher normalized GAS activity during some periods and lower mechanical output during others. However, they did not directly test:
- Whether participants with greater co-contraction had lower propulsion
- Whether EMG predicted ankle moment after controlling for age and speed
- Whether co-contraction mediated the relationship between age and mechanics
The claimed neural-mechanical dissociation is therefore suggestive rather than statistically demonstrated.
5. The cross-sectional linear model is restrictive
This study compares different people of different ages at one time point. It cannot show that individuals develop these changes as they age.
The model also assumes a linear relationship from age 26 to 86. Gait deterioration may accelerate after a threshold such as age 65 or 70. Indeed, previous work in the parent cohort suggested that propulsion decline became pronounced after approximately age 70.
Predicted curves were also shown for ages 20 and 90, slightly outside the observed range.
6. Important covariates were not considered
The analysis did not adjust for:
- Sex or age-by-sex interactions
- Height and leg length
- BMI and body composition
- Physical activity
- Muscle strength
- Health conditions or medication
- Walking speed
Moments and forces were normalized to body mass, but this does not remove all anthropometric or behavioural differences.
7. Limited statistical reporting
The paper reports significant time intervals and cluster p-values but does not provide clear:
- Regression slopes
- Confidence intervals for age effects
- Variance explained
- Standardized effect sizes
- Clinically meaningful thresholds
It is therefore difficult to judge whether some statistically significant differences are functionally important. Statistical Parametric Mapping controls comparisons across time within a waveform, but no additional correction is described across the six separate outcome families.
8. Restricted measurement scope
Only TA and lateral gastrocnemius EMG were available for ankle analysis. This excludes important contributors such as soleus and other muscles controlling inversion, eversion and ankle stiffness.
Other limitations include:
- Sagittal-plane analysis only
- Averaging left and right sides, which can conceal age-related asymmetry
- Time normalization, which can obscure differences in absolute phase duration
- Barefoot laboratory walking, which may differ from everyday shod walking
- Selection of healthy volunteers, limiting application to frail adults or people with a history of falls
- Exclusion of 31 dataset participants without a comparison of included and excluded participants
Overall assessment
This is a useful and reasonably well-executed exploratory reanalysis. It convincingly shows that age associations are not uniform across the gait cycle: relative ankle-muscle overlap increases during early support but decreases before push-off, while ankle posture and propulsive forces also change.
Its strongest contribution is mapping when these age-associated differences occur.
However, it does not establish that ageing itself causes the changes independently of slower walking speed, nor that co-contraction compensates for impaired proprioception or reduces mechanical efficiency. Those mechanistic claims should be treated as hypotheses requiring speed-adjusted, longitudinal and directly instrumented studies.
Overall novelty: moderate.
Overall evidence strength: good for descriptive, phase-specific associations; weak to moderate for the proposed causal mechanism and clinical consequences.
Q2: is the paper in conformance with the hypothesis that with age the body becomes less flexible and hence balance is harder and gait length and speed reduces
Overall assessment
Yes, the paper is broadly consistent with that hypothesis, but it does not directly test the complete causal chain.
| Hypothesis component | Evidence from the paper | Strength of support |
|---|---|---|
| The body becomes less flexible with age | Older participants showed increased ankle muscle co-contraction during early and mid-stance. This would increase functional joint stiffness. They also showed less plantarflexion around push-off. | Partial support |
| Balance becomes harder | The authors interpret increased co-contraction as a compensatory attempt to stabilize the ankle when proprioception becomes less reliable. However, balance and proprioception were not measured. | Indirect support |
| Stride length decreases | Stride length was not analyzed in this paper. | Not tested |
| Walking speed decreases | Participants walked at self-selected speeds, but speed was neither reported as an outcome nor controlled statistically. Lower propulsion is consistent with slower walking, but does not prove it. | Indirect support |
| Reduced flexibility causes shorter and slower gait | The paper shows associations between age, altered ankle control and reduced propulsion, but it does not test the causal sequence. | Plausible but unproven |
Important qualification about “flexibility”
The findings do not show a simple, universal loss of ankle range of motion. Older participants actually had greater dorsiflexion during late stance, but less plantarflexion during push-off.
A more accurate interpretation would be:
With age, the ankle becomes less capable of dynamically moving between stability and propulsion. Increased muscle co-contraction makes it functionally stiffer during weight acceptance, while reduced or delayed plantarflexion impairs push-off.
This is subtly different from saying that the tissues simply become less flexible. Ageing can produce both:
- Reduced muscle-tendon performance or elastic recoil
- Increased actively generated joint stiffness through co-contraction
A causal model consistent with the paper
- Age-related sensory and muscle-tendon deterioration occurs.
- The nervous system responds by activating opposing ankle muscles together.
- This increases ankle stiffness and may improve immediate stability.
- Increased stiffness and impaired tendon recoil reduce efficient ankle movement.
- Push-off and propulsive ground reaction force decline.
- Shorter steps and slower walking would be expected consequences.
The paper directly supports steps 2, 4 and 5. Steps 1, 3 and 6 are reasonable interpretations but were not directly measured.
Conclusion
The paper is therefore in conformance with the hypothesis, especially if “less flexible” means reduced dynamic adaptability and greater functional joint stiffness. It provides evidence for an ageing-related shift from flexible, propulsion-oriented ankle control toward a more cautious, stabilizing pattern.
It does not, however, demonstrate that reduced flexibility causes impaired balance, shorter strides or slower walking. A study measuring ankle stiffness, proprioception, balance, stride length and walking speed together would be needed to confirm that complete pathway.