Thirty athletes between 18–35 years (22 males and 8 females) who received arthroscopic partial medial meniscectomy from January 2015 to December 2015 were enrolled in this study which conducted in training facility of the laboratory, comprising of 15 cases of left knee injury and 15 cases of right knee injury (Table 1). After obtaining their informed consent, subjects were randomly assigned to two groups in a cross-matched manner to receive either NT or strength training (ST), depicted as the NT group and the ST group, respectively.

The exclusion criteria were as follows: previous rehabilitation training, anterior/posterior cruciate ligament injury grade II and above; knee joint fracture or dislocation; presence of osteochondritis dissecans in the knee joint; severe OA or traumatic arthritis; and/or existence of rheumatic or metabolic diseases.

The general clinical data of the two groups were compared by statistical analysis and any of the differences observed were not statistically significant (P>0.05), indicating that the two groups of athletes were comparable.

This study followed the Declaration of Helsinki on medical protocol and ethics, and was approved by the regional Ethical Committee. All participants gave written informed consent before their participation.

After medical discharge, the athletes enrolled in this study underwent adaptive rehabilitation training for one week to recover the flexibility of the joint and reduce further swelling. The training program started at the beginning of the second week and consisted of three sessions per week of at least 50 min for a total of 8 weeks. Muscle strengthening is an integral part of rehabilitation programs and has been previously demonstrated compellingly to be crucial for recovery after arthroscopic meniscectomy procedures (Goodwin and Morrissey, 2003; Koutras et al., 2012).

The NT program method used in this study has been previously reported as a biomechanical method (Abdominal-crunches; Lunge; Incomplete circle; Get tapping; kNee bends) aimed to improve sensory motor function and skeletal muscle strength of the lower limbs in athletes (Ageberg et al., 2010; Bennell et al., 2011). In addition, a number of studies have also suggested that early moderate exercise after meniscectomy may reduce the loading pressure on the medial knee joint, improve the quality of the cartilage, and reduce the risk factors of future OA development (Ericsson et al., 2009; Roos and Dahlberg, 2005; Thorstensson et al., 2007). In brief, details of the training programs for each group are as follows: (a) postoperative week 1 for ST group: athletes in this group performed exercises for 30 min, 7 days per week to maintain and restore muscle strength and joint flexibility, such as ankle pumps, quadriceps isometric contraction, and straight leg raises. Weeks 2–4: training exercises were carried out for 50 min at least 5 times per week incorporating sandbags or elastic bands in resistance training and including hamstring curls, wall squats, and gait training. Weeks 4–8: resistance exercises were carried out for 50 min at least 5 times per week focusing on the hip abductor, hip adductor, the quadriceps, hamstrings, and calf muscles. (b) postoperative week 1 for NT group: Ankle pumps and proprioception exercises were performed for 30 min, 7 days per week from a fixed position with eyes closed, such as straight leg raised in the supine position or leg bent in the seated position. Weeks 2–4: subjects were given sit-up exercises that required lying on a bobath ball with the knees bent to stabilize the ball, hips-up exercises with the subject lying on the bobath ball and the knees bent to stabilize the ball while performing hips-up movements with both knees in a fixed position; forward lunges in which the subject stood straight, lunged forward, an incomplete circle that require the subject to stand on one leg and use the other leg to draw a semicircle, toe target movements, and knee touch movements. Other exercises included standing and static balancing training, training on a balancing board, ball catching in the squat position, and step training for at least 50 min per day, a minimum of 5 days per week. Weeks 4–8: Subjects were given training exercises for 50 min for at least 5 days per week focusing on balance and walking abilities, flexibility, and dynamic stability of the knee.

Rehabilitation assessments, such as the Lysholm knee scoring (LKS) was employed to test knee function before surgery and again at 4 and 8 weeks following the meniscectomy procedure. Functional performance tests (FPTs) included a 30-sec one-leg raise test and a square-hop test. In the 30-sec one-leg raise test, the subject sat on a chair of 45 cm in height, stood up with the leg straight, and sat back down in the chair immediately when instructed. This process was repeated for 30 sec. The healthy leg was tested first followed by the injured leg. The arms and the body were not allowed to swing during the duration of the test and the number of times the subject stood or sat during the 30-sec test was recorded as previously reported (Hall et al., 2015). For the square-hop test evaluation, the subject was asked to stand in front of a 30 cm×35 cm square and was instructed to hop into different zones, beginning at zone 0, in a clockwise sequence while hearing the following commands: zone 1→zone 2→back to zone 0→zone 3→zone 4, as shown in Fig. 1. The healthy leg was tested first followed by the injured leg. The number of valid motions, excluding jumps with both legs, hops in a wrong direction, and hops stepping on the border marks, within a 10-sec period was recorded as previously described (Bremander et al., 2007; Ericsson et al., 2006). The third test used was the star excursion balance test (SEBT), which has been described in detailed elsewhere (Chaiwanichsiri et al., 2005; Gribble and Hertel, 2003). In brief, the SEBT method first requires to measure the length of the subjects’ legs in the prone position. The subject is then allowed to carry out the test in eight directions as follows: anterior, anterolateral, lateral, posterolateral, posterior, posteromedial, medial, and anteromedial. During the test, the subjects were told to place their hands on their hips, stand on one leg in the center of an 8-point star while bending the supporting leg, and extending the other leg as far as possible towards the eight directions at every 45° angle (Fig. 2). The subjects carried out the tests in a counter-clockwise manner when using the left leg as support and in the clockwise direction when using the right leg as support. As shown in Fig. 2, the subject proceeded from point 1 to point 8 on each leg. Before moving to the next direction, the subject was required to return the extended leg back to the central position and stand on both legs to rest for 3 sec in order to prevent discrepancies in the test results due to improper body balance. The test was repeated 5 times and the average value of the top three results was calculated. When moving in the lateral and posterolateral directions, the subject was required to swing the leg being tested back to the supporting leg to complete the action as described previously. The fourth test applied was the isokinetic muscle strength assessment of the subject’s knee extensors and flexors. For this we used the PrimusRS (BTE, Greenwood Village, CO, USA) physical therapy equipment specifically designed for multijoint isokinetic testing according to previously described methods, which was routinely calibrated prior to each test (Bonfim et al., 2003). In brief, the subjects were allowed to warm up on a cycle ergometer for 5 min before starting the test. The subject was then asked to sit on a seat placed at a 90° angle during the entirety of the test. The axis of the knee was placed in line with the axis of the power arm and the resistance pad with the rear of the power arm fixed 3 cm above the medial malleolus of the ankle. This allowed a motion range of the joint between 15° and 100°. The test was performed at two angular velocities of a 60°/sec and 120°/sec respectively. The subject performed 3 maximal contractions at these 2 angular velocities with a 60-sec rest between each contraction. Prior to the formal test at each velocity, the subject performed 3 sets of submaximal flexion-extension exercises as a warm-up. The subject’s healthy leg was tested first followed by the injured leg 2 min later. The R value was calculated as the ratio of the peak torque (PT) of the same muscle group between both knees (healthy leg/injured leg). The R value indicated an optimal recovery of maximal muscle strength and endurance when it lied close to 1. The flexors/extensors torque ratios (F/E) on either knee were also calculated and used as a balance indicator and motion control ability.

Data were expressed as mean±standard error and analyzed using SPSS ver. 17.0 (SPSS Inc., Chicago, IL, USA). LKS and BTE PrimusRS isokinetic test results were compared by two-way analysis of variance. The SEBT results were analyzed using multifactor variance analysis and the FPT results were analyzed using a chi-square test. Differences were considered statistically significant when P<0.05.

The assessment of knee function in the LKS test taken preoperative from the two groups were comparable and without significant differences (Table 2) (P>0.05). After 4 and 8 weeks of rehabilitation, the LKS scores improved significantly within both groups (P<0.05). At 8 weeks post operation, the LKS in the ST group was significantly higher than that in the NT group (P<0.05).

Both NT and ST groups performed significantly better on the injured leg in the 30-sec one-leg raise test and square-hop test as compared with their preoperative performance (Table 3) (P<0.01). After 8 weeks of training, the athletes in the ST group performed better in the 30-sec one-leg raise test than the athletes placed in the NT group (P<0.05). However, the athletes in the NT group scored significantly higher in the square-hop test than the athletes in the ST group (P<0.05). The NT and ST group differences were found to be statistically significant (P<0.01).

Both the NT and ST group performance were significantly improved in all the 8 directions taken in the plyometric after training (Table 4) (P<0.01). Moreover, the SEBT scores in the NT group were significantly higher than those in the ST group (P<0.01).

The R value and the F/E ratio of the BTE PrimusRS isokinetic test at 60°/sec and 120°/sec were significantly increased in both groups after eight weeks of training (Table 5) (P<0.01). The R value of the extensor at 60°/sec was significantly higher in the ST group than that in the NT group (P<0.05). After 8 weeks of training, the R value of the flexor at 120°/sec and the F/E ratio of the injured leg were significantly increased in the NT group compared to the R value from the ST group (P<0.05).