Study design:
At baseline (T0), 6 h (T1), and 48 h (T2) after a standardized exercise protocol, MRI, creatine kinase (CK) levels, jump height, calf circumference, ankle dorsiflexion (DF), and muscle soreness were assessed. After the baseline measurements, an exercise intervention (described below) was performed with participants wearing a compression garment (21–22 mmHg, 75% polyamide, 25% elastane) on a randomly selected calf. Compression garments were worn continuously during the exercise intervention and for the following 6 h. The garments were removed immediately before the beginning of the second measurements (T1).
Participant recruitment and study population:
Healthy individuals between 20 and 30 years of age were recruited from medical and sports facilities. Inclusion criteria were lack of signs, symptoms, or history of chronic disease; no current acute or overuse injuries of the lower limb; and no history of muscle injury. All volunteers were asked to forego alcohol and sports for 72 h before and 48 h after the first MRI acquisition. Exclusion criteria were any symptoms of lower-limb muscle soreness in the 3 months prior to the study and regular training habits that included eccentric or plyometric exercises.
Ethical approval:
The study was approved by the local institutional Ethics Committee and written informed consent was obtained from all volunteers (Ref. No. 33_16 B; University of Erlangen-Nürnberg).
Exercise Intervention:
All volunteers performed an established standardized training that included plyometric and eccentric exercises of both lower legs while wearing a compression garment on a randomly selected calf (left or right side). All exercises were monitored by an experienced strength and conditioning coach. Each participant warmed up using two sets of heel raises (15 repetitions per set), with a 20 s break between sets. Immediately after the warm-up, the plyometric exercise was performed. Volunteers completed 5 sets of 10 repetitions each of drop jumps from a 0.25 m box (5 s between jumps, 1 min between sets). Volunteers were asked to hop off from the box with both feet, land with both feet together, and perform a maximal vertical jump with the shortest possible contact time to the ground. After a 2 min break, a previously described eccentric exercise was performed on a specifically manufactured slant plate (-35°) (14, 15). To increase the load, each volunteer wore a weighted vest bearing approximately 40% of his/her body weight during the entire eccentric exercise (Figure 1). All participants performed 5 sets of 50 repetitions each and rested 60 s between sets, with the last set being performed until muscle fatigue, so that no further repetition of eccentric exercise was possible.
Imaging:
MRI was performed on a 3T scanner (Magnetom Prisma; Siemens Healthcare; Erlangen, Germany) with an 18-channel body array coil. An axial T2-weighted turbo inversion recovery magnitude (TIRM) sequence (total acquisition time (AT), 4:53 min; inversion time (IT), 260 ms; echo time (TE), 69 ms; repetition time (TR), 5120 ms; resolution, 0.8 x 0.8 x 4.0 mm) and a coronal T2-weighted TIRM sequence (AT, 4:14 min; IT, 260 ms; TE, 68 ms; TR, 5870 ms; resolution, 0.9 x 0.9 x 4.0 mm) of both lower legs were acquired. An additional axial T2 mapping (AT, 9:35 min; TE, 10.6–190.8 ms; TR, 3170 ms; resolution, 0.7 × 0.7 × 8.0 mm) was applied to quantify the tissue water content. In addition, we performed a T1-weighted turbo spin-echo sequence to depict the anatomy and morphology of the lower leg.
To analyze the extent of edema, the T2 signal intensity and T2 time value (ms) were assessed according to previously published investigations (16). The compartments of the lower leg were differentiated by defining specific regions of interest in the T1-weighted images corresponding to the anatomic margin of the medial (MGM) and lateral (LGM) head of the gastrocnemius muscle, the soleus muscle (SM), and the tibialis anterior (TA) muscle and copying these over the T2-weighted TIRM and T2 mapping sequence images. In addition, the volume of total intramuscular edema in the MGM was quantified using a threshold-based manual segmentation (16).
For grading of the exercise-induced muscle damage, a modification of the Peetrons classification (17) was used: grade 0 indicated a negative MRI without any visible pathology, grade I indicated edema but no architectural disorganization, grade II indicated architectural disruption indicating partial tear, and grade III indicated total muscle rupture. All subjects with grade I lesions in axial TIRM images were considered to have induced DOMS. The MRI images were analysed blindly by an experienced radiologist using a professional image-processing program (syngo.via VB10; Siemens AG).
Jump height:
The power of both lower legs was assessed separately by jumps with full knee extension after familiarization with the movement pattern. Starting in an upright position with both arms fixed to the upper body and 90° flexion in the hip and knee of the contralateral leg, avoiding additional impulse movements, volunteers were asked to jump as quickly and explosively as possible in order to perform the highest possible jump. Tests were performed on a force platform (KMP Newton GmbH; Stein, Germany). The software provided by the manufacturer automatically calculates jumping height based on ground reaction forces. Mean jump height was determined using three repeated measurements.
Calf circumference:
The maximum calf circumference was measured using a tape measure while the participant was sitting on the edge of a table with free-hanging lower legs (14). The location of measurement was labeled with a permanent marker at baseline to ensure the same position for follow-up measurements.
Ankle dorsiflexion:
Passive DF of both ankle joints was assessed manually with a goniometer while the participant was sitting on the edge of a table with free-hanging lower legs (18).
Muscle soreness:
Self-reported muscle soreness was evaluated using a 10 cm visual analogue scale for pain (0, no pain; 10, worst pain). Pain scores were reported for each leg individually at rest and during activity (going down stairs) (14).
Creatine kinase (CK) levels:
Blood CK levels were measured at baseline and at 6 h and 48 h after the eccentric exercise. Approximately 5 mL blood was collected by vein puncture from an antecubital vein into serum tubes. CK measurement was conducted using the UV test according to the IFCC method (37°C; Cobas 6000; Roche Diagnostics; Mannheim, Germany).
Blinding:
Test assistants and outcome assessors were kept unaware of which of the participants' legs were supplied with compression garments and were not allowed to ask about this.
Statistical analysis:
All assessed parameters were checked for normality with the Shapiro–Wilk test. In cases in which the data were normal, a repeated-measures analysis of variance, with time and treatment (compression versus non-compression) as the repeated independent variables, was performed for each of the dependent variables of interest: CK, T2 signal intensity, T2 time, calf circumference, jump height, and ankle DF. When the data were not normally distributed, a Friedmann test with a post hoc Dunn–Bonferroni test was performed. Muscle soreness scores were analyzed using the Friedmann test. All statistical tests were performed using SPSS Version 23 (IBM Corporation; Armonk, NY, USA), and P values of less than .05 were considered to indicate statistical significance.