(2) NEUROMUSCULAR ELECTRICAL STIMULATION DOES NOT AFFECT ECHO INTENSITY DURING SHORT-TERM KNEE JOINT IMMOBILIZATION

Purpose: Knee joint immobilization is a common clinical intervention that can lead to rapid changes in muscle quality, which can be an indication of function and strength. Neuromuscular electrical stimulation (NMES) has been suggested as a potential strategy to preserve muscle function. While echo intensity (EI) measured by ultrasound may offer valuable insights into muscle quality changes, the effectiveness of NMES in preventing EI alterations due to short-term immobilization remains unclear. The purpose of this study was to investigate whether NMES preserves echo intensity of the quadriceps muscles during one week of knee joint immobilization. We hypothesized that NMES would preserve muscle quality, as measured by EI. Methods: Twenty-one healthy, college-aged adults (11 males: mean±SD age=22±4 years; height=182.40±7.17 cm; mass=82.60±16.27 kg; 10 females: age=20±1 years, height=160.55±5.43 cm; mass=60.82±9.67 kg) were randomized into two groups: Immobilization only (IMM; n=9), and Immobilization + NMES (NMES; n=12). All participants wore a knee brace on the left leg locked at 90° flexion for seven consecutive days and ambulated with crutches. Accelerometers on both ankles confirmed compliance. The NMES group performed twice-daily NMES of the knee extensors, resulting in visible contraction of the quadriceps muscles. Transverse echo intensity of the vastus lateralis (VL EI) and rectus femoris (RF EI) muscles was measured with B-mode ultrasonography before and after immobilization. Separate ANCOVA analyses were conducted for each muscle, with Bonferroni post hoc comparisons when appropriate. An alpha level of 0.05 used to assess statistical significance. Within group changes were determined with paired samples t-tests. Results: The covariates PRE VL EI and PRE RF EI were significantly related to POST VL EI [F(1,18)=191.875, p < .001, n_p²=0.914] and POST RF EI [F(1,18)=50.782, p < .001, n_p²=0.738], respectively. After controlling for PRE values, there were no significant differences between groups for POST VL EI (F(1,18)=2.748, p=0.115) or RF EI [F(1,18)=2.759, p=0.114]. Both groups exhibited increased EI after immobilization, indicating a decline in muscle quality (within-group percent increases: IMM: VL EI= +1.1%, RF EI= +5.0%; NMES: VL EI= +5.8%, RF EI= +1.6%). Conclusions: Daily NMES sessions did not appear to mitigate immobilization-related increases in EI during short-term knee joint immobilization. We suggest this may be due to several factors. First, the predominant recruitment of Type II fibers by NMES, combined with immobilization-induced shifts from Type I to Type II fibers, may have contributed to elevated EI values. Second, participants may have applied varying NMES intensities (despite instructions and daily monitoring), with some possibly using high intensities that led to muscle microtrauma or inflammation, which, along with transient fluid shifts from NMES and immobilization, may have impacted EI measurements. Third, the short study duration may not have been sufficient to capture significant changes in muscle quality, as chronic adaptations typically occur gradually. Practical Applications: Rehabilitation protocols relying solely on NMES may not sufficiently counteract muscle quality deterioration during short periods of immobilization. Future work can focus on optimizing NMES parameters and combining NMES with other interventions to better sustain muscle quality when immobilization is unavoidable.
Acknowledgements: None