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Abstract

BACKGROUND:

The ability to train unilaterally and experience contralateral strength gains, a phenomenon known as cross-education, has been well documented in the lower extremity but not the upper. Additionally, short-term training of one to two weeks has shown neural adaptations in strength.

OBJECTIVE:

To examine the effects of a short-term cross-education strength-training program on contralateral shoulder strength.

METHODS:

Twenty-seven healthy college students (age: 21.37 $\pm$ 2.02 years; height: 167.85 $\pm$ 7.63 cm; mass: 74.42 $\pm$ 16.73 kg) were randomly assigned to either an experimental ( $n=$ 13) or control group ( $n=$ 14). All participants completed a pre and post-test separated by two weeks. Testing consisted of 6 maximal repetitions of shoulder internal/external rotation, at each of three speeds (60 and 180 ${}^{\circ}$ /s). The experimental group underwent right arm unilateral training (3 $\times$ 10 repetitions at all 3 speeds) two times a week for the next two weeks on the same dynamometer. Control group did not train.

RESULTS:

A 2 $\times$ 2 $\times$ 2 $\times$ 2 (time $\times$ arm $\times$ speed $\times$ group) ANOVA revealed no significant interactions by group, however there main effects of time and arm. For internal rotation, peak moment was significantly greater in the right arm and at 60 ${}^{\circ}$ /s, and average power was significantly greater in the right arm and at 180 ${}^{\circ}$ /s. For external rotation; peak moment, average power and total work, were significantly greater in the right arm and at 60 ${}^{\circ}$ /s.

CONCLUSIONS:

Short-term cross education training showed no increase in contralateral shoulder strength. Therefore, short-term cross education strength training should not be considered a useful tool in shoulder rehabilitation at this time. However, future research should investigate contrasting training programs.

Keywords

Muscle neural strength

1. Introduction

Injury to the rotator cuff is one of the most common upper body pathologies with an incident rate of between 30–70% of all shoulder injuries [1]. These become more prevalent as one ages, with imaging showing rotator cuff tears in approximately 40% of people 50 years of age or older [1]. The rotator cuff muscles act as the major muscles in shoulder rotation [2]. The rotator cuff is comprised of four muscles, but the two most commonly torn are the supraspinatus and the infraspinatus with a tear of both occurring in 18.2% of the cases [3]. Following surgery a patient’s shoulder is immobilized, therefore, unilaterally training the uninjured shoulder can minimize the effects on musculature that occur through immobilization [4].

Cross-education is a neural adaptation to strength training in which there is an increase in strength in the untrained, contralateral limb after unilateral training [5]. Cross education has been shown to improve quadriceps strength recovery in patients following anterior cruciate ligament reconstruction [6]. Short-term resistance training programs have also been shown to increase strength due to neural adaptations [7]. Isokinetic testing takes place in a constant, pre-set speed, through a defined range of motion, making it is an easy way to strengthen the muscles in a controlled environment [8, 9], and allows the clinician to compare the progress of the injured shoulder bilaterally. This can be done with normal rehabilitation exercises on the injured limb, as protocol allows. Due to the important role of the rotator cuff it is critical patients experience a thorough but also effective rehabilitative process.

The majority of the research focuses on the wrist flexors and knee extensors, thus studying the effects of cross education on the shoulder could lead to decreased effects of immobilization and increase the rate of rehabilitation of these muscles. This study aimed to answer the following question: will there be a significant muscle strength difference in the untrained limb after two weeks of unilateral training? The purpose of this study was to examine the effects of a two-week unilateral strength-training program on moment of the ipsilateral and contralateral shoulders. We hypothesize that the experimental group will see a significant increase in peak moment, average power, and total work in the ipsilateral and contralateral shoulders.

2. Methods

2.1 Subjects

A total of 27 participants (age: 21.37 $\pm$ 2.02 years; height: 167.85 $\pm$ 7.63 cm; mass: 74.42 $\pm$ 16.73 kg) were randomly assigned to two groups, one experimental ( $n=$ 13) and one control ( $n=$ 14). In the past 3 months prior to the study, participants completed resistance training an average of 2.18 $\pm$ 1.81 times per week. All participants were right hand dominant and had no shoulder pathology within the last year. Participants were asked to refrain from any upper body resistance training for the duration of the study. This study was approved by the University of West Florida IRB and all participants read and signed an informed consent document.

On their first visit, participants had their height and mass recorded using a Detecto scale and stadiometer (Webb City, MO, USA). The experimental group completed six total visits consisting of: pre-testing, two training sessions per week [10, 11] with their right arm only for two weeks with at least 48 hours between sessions and a post-test at least 48 hours after the last visit. The control group participated in only pre and post-testing. Pre and Post testing was performed bilaterally.

2.2 Isokinetic testing

Testing was performed on a Biodex System 4 isokinetic dynamometer. Participants performed a general warm-up using a resistance band, completing 10 repetitions of shoulder internal/external rotation bilaterally [12]. They were then seated on the dynamometer and their glenohumeral joint was placed in 30 ${}^{\circ}$ of abduction, 30 ${}^{\circ}$ of flexion, with a dynamometer tilt of 30 ${}^{\circ}$ , known as the modified neutral position [9]. The chair was reclined to 85 ${}^{\circ}$ with straps across their waist and torso coming from the shoulder to the opposite hip. To minimize muscular substitution, participants were instructed to place the contralateral arm across the chest and grasp the strap. Participants performed a specific warm-up on the dynamometer at submaximal level performing 5 repetitions at each speed (60, 180, and 300 ${}^{\circ}$ /s), which familiarized them with the movement [8]. Testing began with the shoulder in full external rotation. They then performed 6 maximal reciprocal concentric/concentric repetitions at 60 and 180 ${}^{\circ}$ /s, with one-minute rest between speeds. Both shoulders were tested in a counterbalanced fashion. Peak moment, average power, and total work were collected at each speed, in each direction. Post-testing was identical to pre-testing.

2.3 Training

The experimental group underwent right arm unilateral training two times a week for the next two weeks on the same dynamometer. First, they performed the same warm-up used during testing, on the right arm only. They then performed three sets of 10 maximal concentric, concentric repetitions of internal/external shoulder rotation at speeds of 60, 180, and 300 ${}^{\circ}$ /s, with one minute rest between sets and two min rest between speeds [8].

2.4 Statistical analyses

Separate 2 $\times$ 2 $\times$ 2 $\times$ 2 (time $\times$ arm $\times$ speed $\times$ group) ANOVAs analyzed peak moment (PM), average power, and total work for both internal and external shoulder rotation. Data represented as mean $\pm$ standard deviation (SD).

Table 1
Peak moment (Nm) at 60 ${}^{\circ}$ /s of shoulder internal rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	23.07 $\pm$ 7.61	21.67 $\pm$ 8.80	24.95 $\pm$ 12.53	27.57 $\pm$ 15.36
Right	23.79 $\pm$ 10.00	24.62 $\pm$ 7.42	28.75 $\pm$ 15.35	29.59 $\pm$ 16.11

Table 2

Peak moment (Nm) at 180 ${}^{\circ}$ /s of shoulder internal rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	19.00 $\pm$ 7.72	18.80 $\pm$ 8.41	19.64 $\pm$ 12.44	21.82 $\pm$ 11.59
Right	19.86 $\pm$ 8.15	20.79 $\pm$ 8.48	19.12 $\pm$ 12.86	24.11 $\pm$ 14.05

Table 3

Peak moment (Nm) at 60 ${}^{\circ}$ /s of shoulder external rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	19.59 $\pm$ 4.77	20.52 $\pm$ 5.68	23.12 $\pm$ 8.63	23.00 $\pm$ 7.46
Right	21.41 $\pm$ 4.97	22.70 $\pm$ 5.00	23.84 $\pm$ 8.65	24.45 $\pm$ 8.30

Table 4

Peak moment (Nm) at 180 ${}^{\circ}$ /s of shoulder external rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	19.59 $\pm$ 4.77	16.40 $\pm$ 5.43	23.13 $\pm$ 8.63	18.83 $\pm$ 7.48
Right	16.65 $\pm$ 4.64	18.62 $\pm$ 4.71	17.30 $\pm$ 7.51	19.66 $\pm$ 8.79

Table 5

Average power (W) at 60 ${}^{\circ}$ /s of shoulder internal rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	13.93 $\pm$ 5.62	13.56 $\pm$ 6.07	15.09 $\pm$ 10.11	17.19 $\pm$ 11.51
Right	14.42 $\pm$ 7.39	15.64 $\pm$ 4.95	16.35 $\pm$ 9.18	19.25 $\pm$ 12.55

Table 6

Average power (W) at 180 ${}^{\circ}$ /s of shoulder internal rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	24.26 $\pm$ 14.63	25.56 $\pm$ 17.55	23.47 $\pm$ 21.18	30.47 $\pm$ 21.26
Rightt	25.18 $\pm$ 15.24	27.62 $\pm$ 15.02	22.23 $\pm$ 19.18	32.67 $\pm$ 24.95

Table 7

Average power (W) at 60 ${}^{\circ}$ /s of shoulder external rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	11.67 $\pm$ 3.68	11.53 $\pm$ 3.67	13.02 $\pm$ 4.96	13.73 $\pm$ 4.64
Right	12.94 $\pm$ 3.47	13.46 $\pm$ 3.31	14.07 $\pm$ 5.31	14.93 $\pm$ 5.56

Table 8

Average power (W) at 180 ${}^{\circ}$ /s of shoulder external rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	18.27 $\pm$ 9.10	19.65 $\pm$ 9.44	16.28 $\pm$ 7.28	21.00 $\pm$ 9.91
Right	23.46 $\pm$ 8.82	15.64 $\pm$ 4.95	17.84 $\pm$ 11.27	23.80 $\pm$ 12.81

Table 9

Total work (J) at 60 ${}^{\circ}$ /s of shoulder internal rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	155.83 $\pm$ 59.91	142.70 $\pm$ 66.06	147.00 $\pm$ 72.88	176.04 $\pm$ 97.43
Right	156.59 $\pm$ 76.09	162.40 $\pm$ 54.11	168.24 $\pm$ 75.19	185.90 $\pm$ 91.29

Table 10

Total work (J) at 180 ${}^{\circ}$ /s of shoulder internal rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	118.83 $\pm$ 64.29	113.86 $\pm$ 72.76	105.07 $\pm$ 71.58	134.85 $\pm$ 83.04
Right	121.93 $\pm$ 67.32	123.97 $\pm$ 66.25	99.90 $\pm$ 75.70	143.22 $\pm$ 98.33

Table 11

Total work (J) at 60 ${}^{\circ}$ /s of shoulder external rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	122.98 $\pm$ 36.95	117.56 $\pm$ 37.69	125.09 $\pm$ 44.62	132.87 $\pm$ 36.09
Right	135.36 $\pm$ 31.95	135.98 $\pm$ 34.62	133.92 $\pm$ 38.66	140.87 $\pm$ 34.76

Table 12

Total work (J) at 180 ${}^{\circ}$ /s of shoulder external rotation

	Experimental		Control
	Pre	Post	Pre	Post
Left	83.87 $\pm$ 35.33	84.57 $\pm$ 37.14	71.58 $\pm$ 27.75	88.00 $\pm$ 36.29
Right	89.84 $\pm$ 30.94	99.04 $\pm$ 34.10	76.56 $\pm$ 42.23	98.46 $\pm$ 46.28

3. Results

3.1 Peak moment

For internal rotation peak moment, there were no four-way, three-way, or two-way interactions but there were main effects for arm and speed. Right arm was significantly greater than the left and 60 ${}^{\circ}$ /s was significantly greater than 180 ${}^{\circ}$ /s (Tables 1 and 2). For external rotation there were no four-way, three-way, or two-way interactions, but there were main effects for time, arm, and speed. Post-testing was significantly greater than pre-testing with the right arm being significantly greater than the left, and 60 ${}^{\circ}$ /s significantly greater than 180 ${}^{\circ}$ /s (Tables 3 and 4).

3.2 Average power

For internal rotation average power, there were no four-way, three-way, or two-way interactions but there were main effects for arm and speed. Right arm was significantly greater than the left and 180 ${}^{\circ}$ /s was significantly greater than 60 ${}^{\circ}$ /s (Tables 5 and 6). For external rotation, there were no four-way, three-way, or two-way interactions but there were main effects for time, arm and speed. Post-testing was significantly greater than pre-testing with the right arm being significantly greater than the left, and 60 ${}^{\circ}$ /s significantly greater than 180 ${}^{\circ}$ /s (Tables 6 and 7).

3.3 Total work

For internal rotation total work, there were no four-way or three-way interactions but there was a two-way interaction of time by group. The control group post-test was significantly greater than the pre-test with no change in the experimental group (Tables 9 and 10). For external rotation there were no four-way, three-way, or two-way interactions but there were main effects for arm and speed. Right arm was significantly greater than the left and 60 ${}^{\circ}$ /s significantly greater than 180 ${}^{\circ}$ /s (Tables 11 and 12).

4. Discussion

The purpose of this study was to determine the effects of short-term unilateral resistance training on strength in the contralateral untrained shoulder. The main findings of this study were that there was no significant strength increases in either the trained or untrained shoulder. Possible reasons for this could be the training program was too short, not enough volume, or heterogeneity of participants.

4.1 Cross education

The results of the current study are similar to a previous study conducted by Farthing et al. [13] on the effects of cross-education strength training. Using an isometric dynamometer participants trained ulnar deviation in their dominant wrist, holding the contraction for 3 s, for three sets of eight, five days a week for three weeks, progressing up to six sets of eight, with results showing no difference in the untrained, contralateral wrist [13]. However, Farthing et al. [13] did show an increase in the trained wrist following the program, which differs from the current study’s findings in which demonstrated no strength change in the trained limb. This difference could be attributed to the above study having a higher frequency, of 15 total training sessions compared to just four in the current study. The National Strength and Conditioning Association suggests that training adaptations are related in part to training frequency, with three-five days per week being more effective than training two days per week [14]. The current study also differs from previous research suggesting that unilateral strength training increases strength in the untrained, contralateral limb [15, 16]. Using resistance band tubing, commonly used in rehabilitation, Magnus et al. [15] found that training three times a week for four weeks caused a significant increase in internal/external rotation shoulder strength compared to a control group. Billis et al. [16] had patients following ACL reconstruction train their uninjured quadriceps on an isotonic leg extension machine at 80% of 1RM. Following training three or five times a week for eight weeks, they found that both training groups significantly improved their quadriceps strength in comparison to the control group. The training program of the current study differs in overall volume and length when compared to Magnus et al. [15], and Billis et al. [16], therefore greater volume could have led to greater strength increases [14]. Additionally, the training program by Billis et al. [16] involved eccentric training, which has previously been shown to lead to greater strength gains when compared to concentric training alone [13, 17] due to the muscle lengthening under tension rather than shortening, thus producing more force [18].

4.2 Short-term training

The results from this study are consistent with previous research examining the effect of short-term training on muscular strength [12, 21]. Using speeds of 60 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s, Brown and Whitehurst [10] found no significant changes in PM at either velocity following two days of velocity specific training, for either the fast or slow training group, at either speed. Likewise, Prevost et al. [19] did not see a significant increase in PM at slow or fast speeds in the slow training group (30 ${}^{\circ}$ /s), following two days of velocity specific training. However, the fast velocity-training group did show an increase in PM, but only at the fast speed (270 ${}^{\circ}$ /s). The present study does differ from the results of Coburn et al. [20] who found an increase in PM at both slow and fast speeds in a slow training group and only the fast speed of a fast training group. This difference from the present study could be attributed to the population used. Coburn et al. [20] used participants that had not trained for the previous three months compared to heterogeneity of training experience in participants in the present study. Therefore, participants in the current study may have been already exposed to early adaptations to training and thus required greater stimuli to achieve new strength increases [14]. Another factor could be greater volume each day in the current study (90 repetitions per session $=$ 360 repetitions total) compared to Coburn et al. [20] (40 repetitions pre session $=$ 120 repetitions total). This increased volume could have allowed for familiarization, allowing participants to perform the action more efficiently [5]. However, when looking at the specific training speeds, the same number of total repetitions were performed over two weeks in the current study compared to just one [21]. The increased number of repetitions at the higher resistance could be great enough to achieve muscular adaptations that the current study did not see [14].

5. Conclusions

It has been suggested that the strength increases over the initial few weeks of a resistance-training program are primarily attributed to neural adaptations rather than hypertrophy [21]. Cross-education effects have also been suggested to be a result of neural factors [5, 21], with larger muscles groups exhibiting greater contralateral effects due to their ability to generate greater hormonal changes [21]. Therefore, the diversity of training experience of the participants and use of a relatively smaller muscle group in the current study could have limited the responses to the short-term training program. Thus, future studies should focus on untrained individuals to allow for the initial adaptations of neural factors to short-term training.

Footnotes

Conflict of interest

The authors declare no conflict of interest of any kind.

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No effect of short term cross-education training on concentric contralateral shoulder strength

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Subjects

2.2 Isokinetic testing

2.3 Training

2.4 Statistical analyses

Table 1 Peak moment (Nm) at 60 ∘ /s of shoulder internal rotation

3.1 Peak moment

3.2 Average power

3.3 Total work

4. Discussion

4.1 Cross education

4.2 Short-term training

5. Conclusions

Footnotes

Conflict of interest

References

Table 1
Peak moment (Nm) at 60 ${}^{\circ}$ /s of shoulder internal rotation