Examination of Muscle Oxygenation During Isometric Exercise in Different Positions in Individuals with Unilateral Transtibial Amputation

Technical Summary: Examination of Muscle Oxygenation During Isometric Exercise in Different Positions in Individuals with Unilateral Transtibial Amputation

Problem Statement
Unilateral transtibial amputation induces significant structural and microvascular adaptations in the residual limb, including muscle atrophy, increased intramuscular fat, and altered arterial hemodynamics. These changes potentially impair muscle force production and oxygen delivery during functional activities. While the general mechanisms of muscle oxygenation are understood, there is a lack of extensive research regarding how specific body positions (sitting vs. standing) and weight-bearing loads differentially influence local muscle perfusion and oxygenation dynamics between the residual and sound limbs. Understanding these asymmetries is critical for optimizing rehabilitation protocols and assistive device usage.

Methodology
This analytical cross-sectional observational study employed a within-subject repeated-measures design to assess 41 adults with unilateral transtibial amputation (mean age 43.3 ± 12.6 years). Participants were at least 18 years old, had sustained their amputation due to an earthquake within the preceding three years, and had used a prosthesis for at least 1.5 years.

• Instrumentation: Muscle oxygenation was measured using a continuous-wave near-infrared spectroscopy (NIRS) device (Train.Red FYER) operating at 760 nm and 850 nm wavelengths. The sensor was placed on the vastus lateralis of both the residual and sound limbs. To ensure signal penetration to the muscle tissue, only participants with subcutaneous fat thickness <17.5 mm were included.
• Parameters: The study analyzed Muscle Oxygen Saturation (SmO₂), Oxygenated Hemoglobin (O₂Hb), Deoxygenated Hemoglobin (HHb), Total Hemoglobin (tHb), and Hemoglobin Difference (HbDiff).
• Protocol: Participants underwent four standardized conditions for each limb:
  1. Sitting rest
  2. Sitting isometric contraction (submaximal knee extension against a towel, ~10 seconds)
  3. Standing rest
  4. Standing isometric contraction (submaximal knee extension against manual resistance, ~10 seconds)
• Statistical Analysis: Data were analyzed using a two-way repeated measures ANOVA (Limb × Condition) with post-hoc comparisons. Significance was set at p < 0.05.

Key Results
The study identified significant main effects for both limb status and condition, as well as significant limb × condition interactions across all NIRS parameters.

• Interlimb Asymmetry: The sound limb consistently demonstrated higher oxygenation and perfusion values than the residual limb. The most pronounced differences occurred during standing conditions.
  • SmO₂: During standing rest, the sound limb showed a mean SmO₂ of 60.66% compared to 46.77% in the residual limb (difference of 13.88%, d = 1.44).
  • HHb: The sound limb exhibited significantly higher deoxygenated hemoglobin during standing rest (50.41 µM vs. 43.22 µM), suggesting greater oxygen extraction under gravitational loading in the intact extremity.
• Effect of Posture and Load: Both standing and isometric contraction elicited greater physiological responses compared to sitting.
  • SmO₂ Trends: In the residual limb, SmO₂ increased progressively from sitting rest (41.32%) to standing isometric (61.98%). However, the magnitude of change and the absolute values remained lower than the sound limb in most resting states.
  • Convergence under Load: Notably, during standing isometric contraction, the difference in Total Hemoglobin (tHb) between limbs was not statistically significant (p = 0.065), suggesting a convergence of local blood volume under maximal postural load, though oxygenation saturation remained distinct.
• Interaction Effects: Significant limb × condition interactions for SmO₂, tHb, O₂Hb, and HHb indicated that the microvascular response to mechanical loading differs fundamentally between the two limbs. The residual limb exhibited an attenuated oxygenation response compared to the sound limb during weight-bearing tasks.

Key Contributions

• Quantification of Postural Impact: The study provides empirical evidence that body position significantly modulates muscle oxygenation in amputees, with standing imposing disproportionately greater physiological demands on the residual limb compared to sitting.
• Microvascular Characterization: It characterizes the specific hemodynamic response of the residual limb, highlighting a supply-demand mismatch where the limb relies more heavily on oxygen extraction (elevated HHb) rather than increased delivery, particularly during weight-bearing.
• Methodological Application: The research demonstrates the utility of NIRS in objectively monitoring real-time muscle oxygenation asymmetries in a clinical rehabilitation setting, moving beyond gait analysis to include static postural loading.

Significance and Claims
The authors claim that these findings highlight a persistent imbalance between oxygen delivery and utilization in the residual limb, which is exacerbated by weight-bearing activities. The study asserts that:

1. Rehabilitation Planning: Rehabilitation strategies must account for the residual limb's constrained local oxygen supply. The authors suggest that graded loading strategies are necessary to prevent overuse and optimize adaptation, given the amplified desaturation response in the residual limb.
2. Clinical Monitoring: NIRS-based monitoring can serve as an objective tool for individualizing exercise prescription and assessing rehabilitation progression, ensuring that load management is tailored to the specific physiological capacity of the residual limb.
3. Prosthetic Considerations: The findings imply that prosthetic socket design should minimize external compression of the stump's vessels to avoid further impairing perfusion, as mechanical occlusion may exacerbate the observed microcirculatory deficits.

The paper concludes that while the residual limb demonstrates reduced oxygen saturation and local blood volume compared to the sound limb, these differences are dynamically influenced by posture. Therefore, integrating position-specific strategies into rehabilitation is essential to enhance local perfusion and oxidative capacity in individuals with lower-limb amputation.